Aline Tribollet
email: aline.tribollet@ird.fr

Carbonate dissolution by reef microbial borers: A biogeological process producing alkalinity under different pCO2 conditions

Keywords: Biogenic carbonate dissolution, microborers, euendoliths, coral reefs, ocean acidification, seawater alkalinity

Rising atmospheric CO2 is acidifying the world's oceans affecting both calcification and dissolution processes in coral reefs. Among processes, carbonate dissolution by bioeroding microflora has been overlooked, and especially its impact on seawater alkalinity. To date, this biogeological process was only studied using microscopy or buoyant weight techniques. To better understand its possible effect on seawater alkalinity and thus, on reef carbonate budget, an experiment was conducted under various seawater chemistry conditions (2 ≤ Ωarag ≤ 3.5 corresponding to 440 ≤ pCO2 (µatm) ≤ 940) at 25°C under night and daylight (200 µmol photons m -2 s -1 ) with natural microboring communities colonizing dead coral blocks (New Caledonia). Both the alkalinity anomaly technique and microscopy methods were used to study the activity of those communities dominated by the chlorophyte Ostreobium sp. Results

show that (1) the amount of alkalinity released in seawater by such communities is significant and vary between 12.8 ± 0.7 at ΩArag ~ 2 and 5.6 ± 0.4 mmol CaCO3 m -2 d -1 at ΩArag ~ 3-3.5 considering a 12:12 photoperiod, (2) although dissolution is higher at night (80% vs 20% during daylight), the process can occur under a significant photosynthetic activity, and (3) the process is greatly stimulated when an acidity threshold is reached (pCO2 ≥ 920 µatm vs current conditions at constant light intensity). We show that carbonate dissolution by microborers is a major biogeochemical process that could dissolve a large part of the carbonates deposited by calcifying organisms under ocean acidification.

Introduction

Atmospheric carbon dioxide partial pressure (pCO2) is rising exponentially due to burning of fossil fuels and changing land-use, and is expected to double (~750 µatm) by 2100 (Stocker et al. 2013). Thirty to 50% of anthropogenic CO2 emissions are presently absorbed by the world's oceans, decreasing seawater pH [START_REF] Sabine | The oceanic sink for anthropogenic CO2[END_REF][START_REF] Wu | Surface ocean pH variations since 1689 CE and recent ocean acidification in the tropical South Pacific[END_REF], with a decrease estimated at 0.3-0.4 pH-units by 2100 in the worst-case scenario [START_REF] Caldeira | Oceanography: anthropogenic carbon and ocean pH[END_REF]Stocker et al. 2013). As a consequence, the saturation state of surface seawater (Ω) with respect to calcium carbonate minerals (CaCO3) will also decrease, impacting calcification rates of marine organisms [START_REF] Orr | Anthropogenic ocean acidification over the twentyfirst century and its impact on calcifying organisms[END_REF][START_REF] Guinotte | Ocean acidification and its potential effects on marine ecosystems[END_REF], and increasing rates of carbonate dissolution [START_REF] Feely | Impact of anthropogenic CO2 on the CaCO3 system in the oceans[END_REF][START_REF] Tribollet | Effects of elevated pCO2 on dissolution of coral carbonates by microbial euendoliths[END_REF][START_REF] Krumins | Dissolved inorganic carbon and alkalinity fluxes from coastal marine sediments: model estimates for different shelf environments and sensitivity to global change[END_REF][START_REF] Enochs | Elevated colonization of microborers at a volcanically acidified coral reef[END_REF]Stubler and Peterson 2016;[START_REF] Trnovsky | Antagonistic effects of ocean acidification and rising sea surface temperature on the dissolution of coral reef carbonate sediments[END_REF]Schönberg et al. 2017;[START_REF] Eyre | Coral reefs will transition to net dissolving before end of century[END_REF].

A recent model [START_REF] Cyronak | Enhanced acidification of global coral reefs driven by regional biogeochemical feedbacks[END_REF] shows that the average pCO2 may have increased faster in coral reef ecosystems than in the open ocean (3.5-fold) over the past 20 years due to additional anthropogenic disturbances (e.g. nutrient and organic matter inputs), putting these carbonate ecosystems even more at risk under ocean acidification [START_REF] Andersson | Revisiting four scientific debates in ocean acidification research[END_REF]. Recent studies highlighted the negative effect of ocean acidification (combined or not with a rise in seawater temperature) on growth, abundance and calcification rates of the main reef framebuilders, i.e. corals and calcifying algae [START_REF] Langdon | Effect of elevated pCO2 on photosynthesis and calcification of corals and interactions with seasonal change in temperature/irradiance and nutrient enrichment[END_REF]Kuffner et al. 2007;[START_REF] Pandolfi | Projecting coral reef futures under global warming and ocean acidification[END_REF][START_REF] Comeau | Ocean acidification accelerates dissolution of experimental coral reef communities[END_REF][START_REF] Johnson | Contrasting effects of ocean acidification on tropical fleshy and calcareous algae[END_REF]. Although the same trend is usually observed at the global reef scale, the relationship between community calcification and aragonite saturation state varies dramatically from one location to the next [START_REF] Yates | CO3 2concentration and pCO2 thresholds for calcification and dissolution on the Molokai reef flat, Hawaii[END_REF]Silverman et al. 2007;Shamberger et al. 2011;Chan and Connolly 2012;[START_REF] Falter | Seasonal coupling and de-coupling of net calcification rates from coral reef metabolism and carbonate chemistry at Ningaloo Reef, Western Australia[END_REF]Shaw et al. 2012). [START_REF] Pandolfi | Projecting coral reef futures under global warming and ocean acidification[END_REF] suggested that the largest variations among ecosystems could result from differences in sediment composition; some being enriched in Mg-calcite are thus the first to be dissolved (see also [START_REF] Yates | CO3 2concentration and pCO2 thresholds for calcification and dissolution on the Molokai reef flat, Hawaii[END_REF]. Dissolution however includes several processes: chemical dissolution resulting from environmental conditions (i.e. properties of the bulk of seawater), metabolic dissolution driven by bacteria through the remineralisation of organic matter, and metabolic dissolution driven by bioeroders such as microborers and sponges, also called biogenic dissolution [START_REF] Tribollet | Effects of elevated pCO2 on dissolution of coral carbonates by microbial euendoliths[END_REF]Schönberg et al. 2017). Rates of dissolution and resulting fluxes are therefore highly variable in shallow water sediments and affected by a number of factors other than aragonite saturation state, which may explain part of those variations [START_REF] Milliman | Production and accumulation of calcium carbonate in the ocean: budget of a nonsteady state[END_REF][START_REF] Pandolfi | Projecting coral reef futures under global warming and ocean acidification[END_REF][START_REF] Andersson | Ocean acidification and coral reefs: effects on breakdown, dissolution, and net ecosystem calcification[END_REF][START_REF] Eyre | Benthic coral reef calcium carbonate dissolution in an acidifying ocean[END_REF].

Biogenic dissolution due to microborers (or euendoliths; see definition [START_REF] Golubic | The lithobiontic ecological niche, with special reference to microorganisms[END_REF] received little attention as compared to calcification processes, although microborers which comprise cyanobacteria, algae and fungi, are efficient agents of carbonate dissolution, especially the chlorophyte of the genus Ostreobium which dominates mature communities (Tribollet 2008a;[START_REF] Grange | The three steps of the carbonate biogenic dissolution process by microborers in coral reefs (New Caledonia)[END_REF]. The penetration of microborers into carbonates, both in dead and live substrates such as corals, is an active process (see for instance Figure 1d in [START_REF] Tribollet | On microbiocorrosion[END_REF] and results in CaCO3 dissolution as these organisms leave traces (galleries or microborings) which conform perfectly to the shape of their filaments [START_REF] Radtke | The distribution of microborings in molluscan shells from recent reef environments at Lee Stocking Island, Bahamas[END_REF]Tribollet 2008b;[START_REF] Wisshak | Temperate bioerosion: ichnodiversity and biodiversity from intertidal to bathyal depths (Azores)[END_REF]). The known rates of biogenic dissolution by these communities at ambient pCO2 measured using microscopy or buoyant weight techniques are on average 0.1-1.8 mmol m -² h -1 in dead coral skeletons (Tribollet 2008a;[START_REF] Reyes-Nivia | Ocean acidification and warming scenarios increase microbioerosion of coral skeletons[END_REF]). These values fall within the range provided for net dissolution in coral rubble reported by [START_REF] Andersson | Ocean acidification and coral reefs: effects on breakdown, dissolution, and net ecosystem calcification[END_REF] suggesting that microborers could be the main agents of carbonate dissolution in such substrates.

To date however, it is not known whether the metabolic activity of microborers can contribute significantly to an increase of bulk alkalinity of seawater and therefore, if it can buffer the system. Indeed, the process of biogenic dissolution has always been studied using microscopy or buoyant weight techniques. Biogenic dissolution can either increase seawater alkalinity (net dissolution is measured) or not if calcium carbonate is re-precipitated inside substrates or at their surfaces (micritization process; [START_REF] Kobluk | Calcification of exposed filaments of endolithic algae, micrite envelope formation and sediment production[END_REF][START_REF] Nothdurft | Earliest diagenesis in scleractinian coral skeletons: implications for palaeoclimate-sensitive geochemical archives[END_REF].

Intuitively, it is difficult to imagine net carbonate dissolution by microborers (or euendoliths) during daylight as they are mostly photosynthetic micro-organisms rising pH [START_REF] Vooren | Photosynthetic rates of benthic algae from the deep coral reef of Curacao[END_REF][START_REF] Tribollet | Endolithic microflora are major primary producers in dead carbonate substrates of Hawaiian coral reefs[END_REF], which is thermodynamically not favourable to dissolution.

The goals of the present study were thus to determine if (1) biogenic dissolution by natural mature microboring communities dominated by Ostreobium sp. significantly increases seawater alkalinity, (2) if so, when during the daily cycle, and (3) whether ocean acidification stimulates this potential release of alkalinity as recent studies performed under controlled conditions and in situ showed that rising pCO2 increase rates of gross biogenic dissolution, i.e. rates of CaCO3 removed by microborers when measured by microscopy and buoyant weight techniques [START_REF] Tribollet | Effects of elevated pCO2 on dissolution of coral carbonates by microbial euendoliths[END_REF][START_REF] Reyes-Nivia | Ocean acidification and warming scenarios increase microbioerosion of coral skeletons[END_REF][START_REF] Enochs | Elevated colonization of microborers at a volcanically acidified coral reef[END_REF]). To achieve these objectives, we carried out an experiment with blocks of dead corals (Porites lobata) naturally colonized by mature microboring communities from New Caledonia. Blocks were maintained under fully controlled conditions to test the effects of rising pCO2 (decrease of Ωarag) on microborer metabolism. The metabolic activity of microborers (photosynthesis/respiration and carbonate dissolution/precipitation) was determined by respirometry and the alkalinity anomaly technique. Samples were also studied using microscopy techniques (gross biogenic dissolution) in order to compare rates of biogenic dissolution obtained with the alkalinity anomaly technique (net biogenic dissolution).

Methods

Experimental design

Experimental blocks of dead coral skeleton (7  5  5 cm) were cut in the center of a massive live coral colony of Porites lobata collected in the lagoon of New Caledonia at Larégnère reef. This location was chosen to collect large, healthy, massive colonies of Porites to prepare experimental blocks (with negligible initial bioerosion). Blocks were bleached (at least one week), rinsed with fresh water (one week) and then dried (two weeks). In October 2009, 6 blocks were fixed on an aluminium grid with bolts and nuts to be laid at 14 m depth on the reef of Casy (Southern lagoon of Grande Terre, New Caledonia). This reef was selected to study biogenic dissolution under nearshore conditions. Water surrounding Casy is regularly impacted by terrigeneous inputs rendering visibility underwater very limited (04 m). After 1 year of colonization by epilithic and endolithic organisms (20092010), 3 colonized blocks were randomly collected and brought back to the Institut de Recherche pour le Développement in Nouméa (2 h transport) and put in a 150 l tank. The tank was filled with filtered seawater (Whatman GF/C) pumped at Casy. A 12:12 h daylight to night photoperiod was applied with a light intensity of 200 µmol photons m² s -1 delivered by two Solar simulator Arrays (Tailored lighting). A heater-chiller provided a constant temperature in the tank (25.2 ±0.1 °C). Three different saturation states were tested, ΩArag = 2, 3 and 3.5 with a constant seawater alkalinity AT (see initial conditions in Table 1), by mixing HCl (0.1 mol l -1 ) and NaHCO3 (1 mol l -1 ) to seawater, simulating a realistic CO2 enrichment scenario [START_REF] Chauvin | Is the response of coral calcification to seawater acidification related to nutrient loading?[END_REF].

Prior incubation, colonized blocks were gently brushed to remove all epiliths, including calcifying organisms such as crustose coralline algae (Fig. 1A). Blocks were then incubated for 2 h in the morning (daylight conditions) and in the evening (night conditions) using 1.6 l incubation chambers submerged in the tank. A battery-driven magnetic stirrer homogenized seawater inside chambers. In addition, 3 controls of seawater and 3 controls of bleached uncolonized blocks were incubated in the same conditions per incubation. At the end of the incubations, seawater samples were collected to measure AT and pHT according to the methods described below. The mean value obtained for seawater controls at the beginning of the incubations was used as the initial condition in the incubation chambers. No metabolic activity was recorded on seawater controls during incubations. The mean value obtained for bleached uncolonized blocks at the end of incubations was used to correct dissolution rates of colonized blocks. Indeed slightly significant rates of CaCO3 precipitation were measured under current conditions ('nucleus' effect of bleached coral blocks in a super-saturated seawater; see [START_REF] Morse | The kinetics of calcium carbonate dissolution and precipitation[END_REF].

At the end of the experiment, the 3 colonized blocks were preserved in a seawater buffered formaldehyde solution (3%) prior microscopy analysis in the laboratory. Using microscopy techniques similar to those used by [START_REF] Tribollet | Effects of elevated pCO2 on dissolution of coral carbonates by microbial euendoliths[END_REF], gross rates of biogenic dissolution were quantified (microdensity and porosity of the coral colony skeleton were 2.58 ±0.06 g cm -3 and 53 ±3 %, respectively). Rates were expressed in kg CaCO3 m -² y -1 or mmol CaCO3 m -² d -1 for comparison with net dissolution rates obtained with the alkalinity anomaly technique. The block surface area used in the calculation was the surface area exposed to light i.e. the surface area of 5 colonized sides (0.0155 m 2 ).

As in previous studies of bioerosion, especially microbioerosion, a limited number of samples was used (see [START_REF] Chazottes | Bioerosion rates on coral reefs: interactions between macroborers, microborers and grazers (Moorea, French Polynesia)[END_REF][START_REF] Tribollet | Endolithic microflora are major primary producers in dead carbonate substrates of Hawaiian coral reefs[END_REF]Tribollet 2008a;[START_REF] Grange | The three steps of the carbonate biogenic dissolution process by microborers in coral reefs (New Caledonia)[END_REF][START_REF] Enochs | Elevated colonization of microborers at a volcanically acidified coral reef[END_REF]. The 3 large coral blocks were however representative of parts of reef framework (i.e. pavement and coral rubble colonized by epilithic and endolithic communities; [START_REF] Chazottes | Bioerosion rates on coral reefs: interactions between macroborers, microborers and grazers (Moorea, French Polynesia)[END_REF][START_REF] Adey | Review. Coral reefs: algal structured and mediated ecosystems in shallow, turbulent, alkaline waters[END_REF]) and allowed natural microboring communities to develop with their usual patchiness distribution [START_REF] Tribollet | Cross-shelf differences in the pattern and pace of bioerosion of experimental carbonate substrates exposed for 3 years on the northern Great Barrier Reef, Australia[END_REF]. To quantify gross rates of biogenic dissolution, 3 independent pieces of coral skeleton (1 cm x 0.5 cm x 1 cm in depth) were collected at the top surface of each block (i.e. surface facing light on the reef).

Each piece was then cut in half perpendicular to the top surface. One half was used for SEM observations and quantification of the bioeroded surface area by microborers (see [START_REF] Tribollet | Cross-shelf differences in the pattern and pace of bioerosion of experimental carbonate substrates exposed for 3 years on the northern Great Barrier Reef, Australia[END_REF], while the other half was used to prepare several thin sections (at least 5) to measure the mean depth of penetration of alive microboring filaments (n > 100 measurements per piece; see definition in [START_REF] Tribollet | Effects of elevated pCO2 on dissolution of coral carbonates by microbial euendoliths[END_REF]. A total of 9 independent pieces of blocks were therefore studied to quantify the average gross biogenic dissolution rate under ambient pCO2 conditions.

Seawater chemistry analysis

Saturated mercuric chloride was added to the seawater samples to prevent biological activity [46]. Potentiometric determination of pHT was performed using Tris/HCl and 2aminopyridine/HCl buffers in synthetic seawater to calibrate the electrode [START_REF] Dickson | Guide to best practices for ocean CO2 measurements[END_REF]). Measurements of pHT were performed at 25.0 ±0.1 °C using a Radiometer TIM865 titrator with combined pH electrode pHC2401-8 and a thermostat bath. AT samples were filtered through Whatman GF/F prior analysis. Potentiometric titration of AT was performed using 0.01 mol l -1 HCl in NaCl to approximate the ionic strength of seawater. Each day of AT measurements, Certified Reference Material from the laboratory of A. Dickson (Scripps Institution of Oceanography) was titrated to determinate the acid titrant concentration. AT was calculated in triplicate from the second inflection point of the titration curve obtained using an automatic titration of 25 ml seawater sample. The precision of the analysis based on triplicate measurements (±1 standard deviation) was typically ±2 µequiv l -1 .

Measured pHT and AT were used to calculate the other CO2 chemistry parameters with the software CO2SYS [START_REF] Pierrot | MS Excel program developed for CO 2 system calculations[END_REF]. In New Caledonia, phosphate and silicate concentrations were 0.03 and 3.1 µmol l -1 , respectively. Conversions between µmol l -1 and µmol kg -1 were performed based on seawater salinity (S = 35.33) and temperature.

Net dissolution and organic carbon metabolism calculation

Net dissolution rates (G), expressed in mmol CaCO3 m -² h -1 were calculated according to the following equation:

(1) G = (½ ∆AT  seawater volume)/ (substrate surface area  time)

where G is positive when net dissolution occurs, ∆AT is the difference in alkalinity between two measurements (final AT minus initial AT), seawater volume is the volume of the incubation chambers, substrate surface area is the surface of the 5 sides of blocks exposed to light, and time was the duration of the incubation.

Net organic carbon metabolism (NP), expressed in mmol C m -² h -1 was calculated according to the following equation:

(2) NP = ((∆DIC  seawater volume) / (substrate surface area  time)) -G where NP is the rate of net photosynthesis during daylight (negative value, although for simplicity it will be presented as positive in the results) or the rate of night respiration (positive value) and ∆DIC is the difference in DIC between two measurements (final DIC minus initial DIC). The other symbols are as defined in Eq. 1.

Daily net dissolution (in mmol CaCO3 m -2 d -1 ) was obtained by multiplying the average of daylight and night rates (in mmol CaCO3 m -2 h -1 ) by 24. Daily respiration (in mmol C m -2 d -1 ) was calculated as 24 times the night respiration rate (in mmol C m -2 h -1 ). Daily gross production (in mmol C m -2 d -1 ) was calculated by adding night respiration and net photosynthesis (both positive, in mmol C m -2 h -1 ) and multiplying by 12.

Statistical analysis

Statistical tests were performed using STATISTICA 7.1 (Statsoft). Data were tested for homogeneity of variances (Levene test) prior to variance analysis and met this requirement.

Differences among treatments were tested using a 2-way ANOVA. Means are reported ± SE.

Results

Observations under light microscopy confirmed that dead coral blocks were colonized by mature communities of microborers dominated by the chlorophyte of the genus Ostreobium (Fig. 1B). In addition to Ostreobium, filaments of the euendolithic cyanobacterium Plectonema terebrans were observed (in abundance) as well as rare fungal hyphae. Coral blocks were covered mainly by large thalli of the brown alga Lobophora variegata and the red alga Peyssonnelia sp. Some green turf puffs were also observed with entrapped sediments, as well as a few encrusting coralline algae. No traces of boring polychaetes were observed while a few traces of grazing were observed on block surfaces. These substrates were thus representative of dead coral skeletons recently exposed to bioerosion in shallow reefs (<15 m depth) [START_REF] Chazottes | Bioerosion rates on coral reefs: interactions between macroborers, microborers and grazers (Moorea, French Polynesia)[END_REF]Tribollet 2008a;[START_REF] Grange | The three steps of the carbonate biogenic dissolution process by microborers in coral reefs (New Caledonia)[END_REF].

At constant light and temperature (Table 1), CaCO3 dissolution measured by the alkalinity anomaly technique was simultaneously affected by light (daylight vs. night conditions: p < 0.0003) and ΩArag (p < 0.02), with no interaction between the two factors (Fig. 2). CaCO3 dissolution was higher at night than at 200 µmol photons m -2 s -1 (on average 8.4 ± 1.1 µmol CaCO3 block -1 h -1 and 1.9 ± 1.1 µmol CaCO3 block -1 h -1 , respectively) (Fig. 2). CaCO3 dissolution was the highest at ΩArag ~ 2 with an average of 8.3 ± 1.5 µmol CaCO3 block -1 h -1 (vs. 3.6 ± 1.2 µmol CaCO3 block -1 h -1 at ΩArag ~ 3-3.5). Besides, the difference in CaCO3 dissolution between night and daylight conditions did not differ significantly among treatments. Taking into account the exposed surface area of the blocks (5 sides = 155 cm 2 ), the daily CaCO3 dissolution was 12.8 ± 0.7 mmol CaCO3 m -2 d -1 (corresponding to an alkalinity production of 25.6 ± 1.4 mEq m -2 d -1 ) and 5.6 ± 0.4 mmol CaCO3 m -2 d -1 (11.2 ± 0.8 mEq m -2 d -1 ) at ΩArag ~ 2 and ΩArag ~ 3-3.5, respectively.

Under night conditions, respiration rate was not affected by ΩArag (Fig. 2) and was on average 17.7 ± 0.8 µmol C block -1 h -1 , or 27 ± 1 mmol C m -2 d -1 . Similarly, net photosynthesis was the same regardless ΩArag (8.7 ± 1.5 µmol C block -1 h -1 on average). Gross production (Pg), however, was slightly affected by ΩArag (p < 0.05), with a higher value at ΩArag ~ 2 than ΩArag ~ 3-3.5 (24 ± 2 versus 18 ± 1 mmol C m -2 d -1, respectively). The Pg/R ratio was clearly lower than 1 (0.89 ± 0.08 and 0.67 ± 0.02 at ΩArag ~ 2 and ΩArag ~ 3-3.5, respectively).

The mean depth of penetration of alive euendolithic filaments was 1.81 ± 0.05 mm and the mean surface area bioeroded at the surface of the blocks was 12.85 ± 1.1 %. The estimated gross rate of biogenic dissolution using microscopy techniques averaged 0.240 ± 0.002 kg m - ² y -1 , which converts to 7.40 ± 0.05 mmol m -² d -1 .

Discussion

Production of alkalinity in seawater by microborers under present conditions

To our knowledge, our experiment is the only study in which dissolution of experimental carbonate substrates colonized by natural microboring communities was simultaneously quantified using microscopy and alkalinity anomaly techniques. Under 200 µmol photons m -2 s -1 and ΩArag ~ 3.5 (similar to ΩArag measured at Ilot Casy during an entire day upon block collection; 3.58 ± 0.03, n = 14), the photosynthetic activity of endolithic communities was presumably limited by light and/or DIC availability as the Pg/R ratio was clearly lower than 1 (0.67 ± 0.02). Although this may have slightly limited the erosive activity of microborers under controlled conditions, the rates of gross biogenic dissolution (microscopy technique) and net dissolution (alkalinity anomaly technique) were of the same order of magnitude at ΩArag 3-3.5 (7.4 ± 0.1 mmol CaCO3 m -2 d -1 versus 5.6 ± 0.4 mmol m -² d -1 , respectively). Other processes than the microborer activity, such as denitrification, can increase alkalinity in seawater (Stumm and Morgan 1996) but are likely to be insignificant in most hard substrate situations [START_REF] Kinsey | Productivity and calcification estimates using slack-water periods and field enclosures[END_REF][START_REF] Gattuso | Photosynthesis and calcification at cellular, organismal and community levels in coral reefs: a review on interactions and control by carbonate chemistry[END_REF]). High-magnesium calcite is also rarely observed in coral skeletons [START_REF] Nothdurft | Calcite-filled borings in the most recently deposited skeleton in live-collected Porites (Scleractinia): implications for trace element archives[END_REF], therefore dissolution of this form of carbonates was most probably not involved in the measured dissolution process. Although some bacterial activity at the block surfaces cannot be ruled out, bacterial activity inside blocks was not involved in CaCO3 dissolution as we did not observe random patterns of dissolution on scanning electron microscopy pictures. We thus strongly suggest that alive microboring communities in coral blocks were responsible for the bulk of alkalinity production in our experiment.

Daily cycle of biogenic dissolution

Surprisingly 20% of the daily rate of biogenic dissolution was occurring during daylight.

Although it seems a paradox to measure simultaneously carbonate dissolution (i.e. release of seawater alkalinity) and photosynthesis, Garcia-Pichel et al. ( 2010) clearly showed that biogenic dissolution of experimental carbonate substrates by the cultured cyanobacterium M. testarum was coupled to the photosynthetic energy production. In our experiment, higher rates of net biogenic dissolution were quantified under night conditions than at 200 µmol photons.m -2 .s -1 . The decrease of the interstitial pH with increasing depth into a Porites cylindrica skeleton colonized mainly by Ostreobium was also particularly important in the dark in Reyes-Nivia et al. ( 2013)'s study, suggesting that night respiration by microborers could be an important factor involved in CaCO3 dissolution.

Effect of ocean acidification on microborer metabolism and alkalinity production

A 33% increase in gross production was recorded at ΩArag ~ 2 (pCO2 of 936 µatm) relative to ΩArag ~ 3-3.5 (pCO2 of 437-570 µatm), suggesting that endolithic communities dominated by Ostreobium were limited by DIC availability at ΩArag ~ 3-3.5. Photosynthesis limitation at ΩArag ~ 3-3.5 was also highlighted by the low Pg/R ratio (0.67 ± 0.02). Although Ostreobium presents carbonic anhydrases (CAs) that may serve as a mechanism to supply inorganic carbon at the required rate for photosynthesis (Shashar and Stambler 1992), it remains unknown if it possesses Carbon Concentrating Mechanisms (CCMs) involving active transport of HCO3 -and/or CO2. In general, photosynthetic rates of algae that have CCMs are not carbon limited, at least under most environmental conditions (e.g. [START_REF] Hurd | Testing the effects of ocean acidification on algal metabolism: considerations for experimental designs[END_REF]. Some species that have CCMs may however, be dependent on CO2 and positively respond to elevated pCO2 at sub-saturating irradiance, because light limitation of active carbon uptake increases reliance on CO2 diffusion [START_REF] Hepburn | Diversity of carbon use strategies in a kelp forest community: implications for a high CO2 ocean[END_REF].

To the contrary of photosynthesis, night respiration was not affected by acidification (Fig. 2).

Most studies to date have reported no or little effect of elevated CO2 on dark respiration rates of macroalgae (e.g. [START_REF] Zou | Short-and long-term effects of elevated CO2 on photosynthesis and respiration in the marine macroalga Hizikia fusiformis (Sargassaceae, phaeophyta) grown at low and high N supplies[END_REF][START_REF] Noisette | Effects of elevated pCO2 on the metabolism of a temperate rhodolith Lithothamnion corallioides grown under different temperatures[END_REF]. Because the responses to elevated CO2 of respiration and photosynthesis were different in our experiment, pCO2 increase resulted in an increase in the Pg/R ratio. [START_REF] Zou | Short-and long-term effects of elevated CO2 on photosynthesis and respiration in the marine macroalga Hizikia fusiformis (Sargassaceae, phaeophyta) grown at low and high N supplies[END_REF] reported that CO2 enrichment decreased the respiratory carbon lost per unit algal biomass in a low-N-grown macroalga, and suggested that CO2 enrichment may thereby enhance carbon sequestration capacity of this organism.

The amount of alkalinity released in seawater doubled at ΩArag ~ 2 compared to ΩArag ~ 3-3.5 (25.6 ± 1.4 and 11.2 ± 0.8 mEq m -2 d -1 , respectively). Net biogenic dissolution rates did not vary significantly between ΩArag ~ 3 and 3.5 (pCO2 = 570 and 437 µatm respectively) suggesting that a threshold somewhere between ΩArag ~ 2 and 3 must be reached before promoting increased CaCO3 dissolution by microboring communities from New Caledonia.

The increase in net biogenic dissolution under elevated pCO2 in New Caledonia confirms the increase of gross biogenic dissolution reported by [START_REF] Tribollet | Effects of elevated pCO2 on dissolution of coral carbonates by microbial euendoliths[END_REF] and [START_REF] Reyes-Nivia | Ocean acidification and warming scenarios increase microbioerosion of coral skeletons[END_REF] for dead coral skeletons which were exposed during a few months to high pCO2 (48% at 750 µatm and 46-89% at 1000 µatm relative to ambient pCO2 depending on coral species, respectively). A similar trend was also observed for pioneer euendolithic communities dominated by the chlorophyte Phaeophila sp. under naturally acidified conditions at Maug Island (Mariana Archipelago; [START_REF] Enochs | Elevated colonization of microborers at a volcanically acidified coral reef[END_REF].

Similarly to zooxanthellate boring sponges under high pCO2 [START_REF] Wisshak | Ocean acidification accelerates reef bioerosion[END_REF], increased biogenic dissolution by microborers could be explained by an enhancement of photosynthetic energy, and/or the lowering of the pH gradient between ambient seawater and the site of dissolution, thereby lowering the metabolic cost required for CaCO3 dissolution. Providing acidified seawater near the dissolution front of euendolithic filaments could make interstitial water thermodynamically more favourable to chemical dissolution. Changes in extracellular pH have also been shown to directly drive changes in intracellular pH of some marine microalgae and in turn affect H + gradients and cellular pH homeostasis (Taylor et al. 2011).

That such processes are used at the dissolution front in Ostreobium remains to be determined.

Basic knowledge of the process of CaCO3 dissolution is currently lacking to fully understand mechanisms behind Ostreobium response to acidification.

Significance of biogenic dissolution in coral reefs

Gross and net rates of biogenic dissolution measured as part of our experiment fall well within the range of gross and net rates of biogenic dissolution reported in the literature for hard dead reef substrates (Table 2). They are also similar to rates quantified on fringing and lagoon reefs (Tribollet 2008a;[START_REF] Tribollet | Effects of elevated pCO2 on dissolution of coral carbonates by microbial euendoliths[END_REF]). Interestingly all rates provided in Table 2 fall within the range of net dissolution rates provided for various carbonate substrates by [START_REF] Andersson | Ocean acidification and coral reefs: effects on breakdown, dissolution, and net ecosystem calcification[END_REF] suggesting that biogenic dissolution by microborers is a major process of carbonate dissolution in coral reefs.

Considering all biogenic dissolution rates for hard substrates in Table 2, the average rate is 11 mmol m -2 of reef d -1 . This average is assumed to be representative of the process occurring on reefs as it includes rates of biogenic dissolution measured under a full range of biotic and abiotic conditions (e.g. light availability and grazing). At the scale of reefs worldwide (surface area of 0.6 10 12 m 2 including 80% of sand and 20% of hard substrates, Milliman 1993) and because microborers can colonize all types of reef carbonates, gross biogenic dissolution is estimated to be 11 10 -3 × 365 × 0.2 × 0.6 10 12 = 0.5 10 12 mol CaCO3 y -1 in hard substrates (living corals being also colonized by microborers; see Tribollet 2008b). Considering an average gross rate of biogenic dissolution for sediments of 10 mmol m -2 d -1 [START_REF] Tudhope | Rate of dissolution of carbonate sediments by microboring organisms, Davies Reef, Australia[END_REF], nearly four times more CaCO3 seems to be dissolved by microborers in sediments than in hard substrates (1.8 10 12 mol CaCO 3 y -1 ). If we consider a net CaCO3 production in coral reefs of 9 10 12 mol CaCO3 y -1 [START_REF] Milliman | Production and accumulation of calcium carbonate in the ocean: budget of a nonsteady state[END_REF], at least 11.3 10 12 mol CaCO 3 y -1 (i.e. 9 10 12 + 0.5 10 12 + 1.8 10 12 ) is deposited by calcifying organisms (gross calcification) and at most, 20% of this produced CaCO3 is probably dissolved by microborers. In the worst case scenario, if we consider a doubling of gross biogenic dissolution rates (Reyes-Nivia et al.

2013

) and a 40% decrease in gross carbonate production due to ocean acidification by 2100 [START_REF] Pandolfi | Projecting coral reef futures under global warming and ocean acidification[END_REF], 2/3 of reef carbonates deposited per year may be dissolved by microborers. In the more optimistic scenario, with only a 50% increase in CaCO3 gross biogenic dissolution and a 20% decrease in gross CaCO3 production (e.g. [START_REF] Tribollet | Effects of elevated pCO2 on dissolution of coral carbonates by microbial euendoliths[END_REF][START_REF] Chauvin | Is the response of coral calcification to seawater acidification related to nutrient loading?[END_REF]Chan and Connolly 2012) , 40% of reef carbonates deposited per year may be dissolved by microborers. Those estimations are very conservative as they do not take into account sediment dissolution resulting from bacterial activity which can be important in some areas [START_REF] Andersson | Dissolution of carbonate sediments under rising pCO2 and ocean acidification: observations from Devil's Hole, Bermuda[END_REF]) and hard carbonate dissolution by boring sponges and bivalves (see review by Schönberg et al. 2017). Although more investigations on the dynamics of the biogenic dissolution process are needed, our estimations strongly suggest that biogenic dissolution of carbonates by microborers should no longer be overlooked and should be considered in reef carbonate biogeochemical models to better predict the fate of coral reefs.

Conclusions

Results show that microboring communities dominated by the phototrophic microboring chlorophyte of the genus Ostreobium, produce significant amount of alkalinity while penetrating actively into coral skeletons. Daily production is increased by a factor 2 under ocean acidification conditions (i.e. when Ω decreases from 3-3.5 to 2). Comparisons of net dissolution rates obtained in our study with those in the literature strongly suggest that (i) the biogenic dissolution process resulting from microborer metabolic activity is a major process of carbonate dissolution in coral reefs, and (ii) that it will probably dissolve a large part of reef carbonate deposited per year by the end of the century if human beings maintain their 'business as usual' (IPCC 2013). It is not known however, to which extend this process could help maintaining environmental conditions favourable for calcifying organisms ('negative feedback to ocean acidification'). This role could be significant, at least at the microscale as microborers colonize also living calcifying organisms such as corals and crustose coralline algae [START_REF] Campion-Alsumard | Microbial endoliths in skeletons of live and dead corals: Porites lobata (Moorea, French Polynesia)[END_REF][START_REF] Tribollet | Bioerosion of coralline alga Hydrolithon onkodes by microborers in the coral reefs of Moorea French Polynesia[END_REF]. [START_REF] Reyes-Nivia | Relative roles of endolithic algae and carbonate chemistry variability in the skeletal dissolution of crustose coralline algae[END_REF] showed that crustose coralline algae (CCA) heavily colonized by microborers are less sensitive to ocean acidification than CCA that are less infested, suggesting that the negative effects of ocean acidification on CCA calcification are mitigated by microborer photosynthetic activity. It remains unknown however, if this negative feedback of microborers results from their photosynthetic activity (rise of pH at the microscale) or their dissolution activity (production of alkalinity) or both. We thus urge the scientific community to study the 'potential buffering' effect of microborer metabolic activity on reefs at various spatial scales (micro-to ecosystem scale) and stress the importance to consider the dynamics of carbonate dissolution by microborers to better understand past and future effects of this factor on coral reef biogeomorphology and cycle. 3.48 ±0.01 pHT (total hydrogen ion concentration scale) and total alkalinity (AT, in units of µequiv kg -1 ) were used to calculate the other CO2 chemistry parameters with the software CO2Sys [START_REF] Pierrot | MS Excel program developed for CO 2 system calculations[END_REF]: dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO2) and aragonite saturation state (ΩArag), using [START_REF] Roy | The dissociation constants of carbonic acid in seawater at salinities 5 to 45 and temperatures 0 to 45°C[END_REF] values for carbonic acid constants K1 and K2, and KSO4 as determined by [START_REF] Dickson | Standard potential of the reaction: AgCl(s) + ½ H2(g) = Ag(s)+ HCl(aq), and the standard acidity constant of the ion HSO4in synthetic sea water from 273[END_REF] Table 2. Rates of biogenic dissolution from our study and from literature at ambient pCO2, in mmol CaCO3 dissolved per m² per day. Rates from literature where converted, when necessary, using microdensity and porosity of coral skeletons according to [START_REF] Tribollet | Effects of elevated pCO2 on dissolution of coral carbonates by microbial euendoliths[END_REF]. Here, square meters refer to m² of substrate exposed to colonization by euendoliths when coral substrates were used. * rates measured using microscopy techniques. ** rates measured using the alkalinity technique. *** rates measured using the buoyant weight technique. Cyano.: Cyanobacteria The standard error of each mean is indicated.

-

  Schönberg CH, Fang JK, Carreiro-Silva M, Tribollet A, Wisshak M (2017) Bioerosion: the other ocean acidification problem. ICES J Mar Sci 74:895-925 Shamberger KEF, Feely RA, Sabine CL et al (2011) Calcification and organic production on a Hawaiian coral reef. Mar Chem 127:64-75 Shashar N, Stambler N (1992) Endolithic algae within corals-life in an extreme environment. JEMBE 163:277-286 Shaw EC, McNeil BI, Tilbrook B (2012) Impacts of ocean acidification in naturally variable coral reef flat ecosystems. J Geophys Res 117 :C03038 Silverman J, Lazar B, Erez J (2007) Community metabolism of a coral reef exposed to naturally varying dissolved inorganic nutrient loads. Biogeochemistry 84 :67-82 Stocker TF, Qin D, Plattner GK et al (2013) Technical summary. In: Climate change 2013: the physical science basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK and New York, NY, USA: Cambridge University Press, pp 33-115 Stubler AD, Peterson BJ (2016) Ocean acidification accelerates net calcium carbonate loss in a coral rubble community. Coral Reefs 35:795-803 Stumm W, Morgan JJ (1996) Aquatic chemistry: chemical equilibria and rates in natural waters, New York, NY, USA: John Wiley & Sons Taylor AR, Chrachri A, Wheeler G, Goddard H, Brownlee C (2011) A voltage-gated H + channel underlying pH homeostasis in calcifying coccolithophores. PLoS Biology 9:e1001085 Tribollet A (2008a) Dissolution of dead corals by euendolithic microorganisms across the northern Great Barrier Reef (Australia). Microbial Ecol 55:569-580

  Figure 1. (A) Photograph of a coral block colonized by 1 year-old euendolithic communities

Figure 2 .

 2 Figure 2. Metabolic activity of microboring communities dominated by Ostreobium sp. in

Table 1 .

 1 Initial seawater chemistry conditions of the experiment (mean ±SD).

					Tables
			T in situ pHT	AT	DIC	pCO2	ΩArag
			(°C)		(µmol kg -1 ) (µmol kg -1 ) (µatm)
	ΩArag = 2	night	25.2	7.740 ±0.002 2304 ±1	2143 ±1	919 ±5	2.08 ±0.01
		daylight	25.2	7.725 ±0.002 2303 ±2	2149 ±1	954 ±4	2.02 ±0.01
	ΩArag = 3	night	25.2	7.921 ±0.003 2320 ±1	2069 ±2	568 ±4	2.97 ±0.01
		daylight	25.2	7.919 ±0.003 2322 ±1	2072 ±2	571 ±4	2.97 ±0.01
	ΩArag = 3.5	night	25.3	8.010 ±0.002 2296 ±1	1997 ±1	438 ±2	3.46 ±0.01
		daylight	25.2	8.012 ±0.001 2297 ±1	1996 ±1	436 ±1

Acknowledgments

We would like to dedicate this paper to our colleague and friend Marlin Atkinson who passed away in 2013. A.T. and P.C. conceived the experimental design. A.T., P.C. and A.C. collected samples and analysed data. A.T. and P.C. wrote the article and A.C. gave assistance on figure preparations and for formatting. We thank the Plateforme Alizes (IRD Bondy) for SEM access and S. Pyneeandy for her help with measurements of biogenic rates on coral blocks.

We thank the Center IRD in Nouméa for its support in the field. Finally, this work was supported by the French Ministry of Ecology (program 'MIDACOR', 2011-2014) and the Institut de Recherche pour le Développement. The funding sponsors had no involvement in the present study development (design, collection, analysis, etc.).

Data availability

Datasets are available from the corresponding author (aline.tribollet@ird.fr) on reasonable request, while pending to be deposited on the SEANOE repository.


Thibault Raffaillac
email: thibault.raffaillac@inria.fr

Language and System Support for Interaction

Keywords: User Interface Engineering, Software Prototyping, Hacking, Opportunistic Programming, Language Constructs ACM Classification H.5.2 [User Interfaces]: Prototyping, D.2.2 [Design Tools and Techniques]: User interfaces, D.3.3 [Language Constructs and Features]

Interaction frameworks are the norm for prototyping, implementing and sharing user interfaces and interaction techniques. However, they often lack the flexibility to easily implement new kinds of interfaces and interaction techniques, since they were basically designed for implementing standard and normalized WIMP user interfaces. This forces programmers to rely on "hacking" in order to experiment with functional prototypes, and could drastically limit the range of scenarios where these prototypes will work. In my PhD, I study the interplay between people designing interaction techniques, and their software frameworks. My goal is to identify a number of fundamental features and requirements that programming languages and systems should support, in order to improve the flexibility of interaction frameworks for programming advanced interaction techniques.

Introduction

Prototyping new interaction techniques today is still a difficult task [START_REF] Myers | How Designers Design and Program Interactive Behaviors[END_REF]5]. Most user interfaces are programmed with common interaction frameworks and platforms (e.g. Cocoa, Qt, Android, HTML), which are thus the best candidates for introducing and popularizing new interaction techniques (e.g. menu systems, pointing methods, visualization techniques). Despite being often not designed for implementing non standard techniques, they benefit from a large base of users, are based on widely understood paradigms (MVC, listeners, events), and provide many useful facilities to work on a prototype and test it (templates, logging, build scripts).

However, these frameworks are made to design interfaces, rather than interaction per se [START_REF] Beaudouin-Lafon | Designing Interaction, Not Interfaces[END_REF]. Since they adopt interactive elements at their core, the widgets, new techniques are constrained to rely on widgets. These in turn have constraints, in their drawing area, and their interaction with mouse and keyboard. Moreover, software frameworks are large code bases, with steep learning curves to transition to their internal -sometimes ill-documented -APIs [START_REF] Parnin | Measuring API Documentation on the Web[END_REF][START_REF] Endrikat | How Do API Documentation and Static Typing Affect API Usability?[END_REF]. In these conditions, "hacking" is a solution to make an idea become possible, although it could eventually reduce one's ambitions in order to get a satisfying result in time [START_REF] Hartmann | Hacking, Mashing, Gluing: Understanding Opportunistic Design[END_REF].

In my doctoral work, I defend the idea that the flexibility of interaction frameworks can be improved and their complexity reduced by: (i) identifying and extracting a set of fundamental features, or bare essentials, for expressing interaction instead of interfaces; and (ii) considering this set as part of the programming systems themselves, as 1 st class objects or constructs of the language. These are for example "receiving mouse input", "drawing things on screen", or "animating visual properties". This set should be simple enough to be reasonably acceptable by language designers, i.e. no APIs like OpenGL or SDL.

Expressing animations

The first of these features I am addressing is to code animations, and more generally things that occur in time. Most frameworks with embedded animation capabilities restrict animations to the transitions of properties values in widgets, i.e. one has to create a widget to be able to use its animation capabilities. This is inconvenient when using multiple frameworks, like Qt and OpenGL, because the animation API of one framework may not work well with the others. This is also limiting when animating an arbitrary property or object that has not been designed for this purpose in the base framework.

During the first part of my PhD I introduced and prototyped a delay operator [START_REF] Thibault Raffaillac | Turning Function Calls into Animations[END_REF], which turns an instantaneous transition myWidget.setProperty(value) into a smooth one with animation, preserving the initial syntax for readability, and being integrated at the system level (i.e. for any framework):

myWidget.setProperty(value) during 3s

The prototype runs on the Pharo Smalltalk platform [START_REF] Black | Pharo by Example[END_REF]. It works independently of any interaction frameworks on this platform, and does not have any set of predefined animatable properties. It relies on a getter-setter naming convention to retrieve the starting value for each animation. For the types of the properties, like numbers, colors and strings, it interpolates between initial and final values by relying on + and * operators, when available. Otherwise it expects an interpolate function to be provided, and fails otherwise. I tested this system successfully with three GUI and visualization frameworks, for moving windows, changing background colors, replacing text labels in buttons and titles (see figure 1), and creating a particle effect. The bottom code transitions its title from 'Gentleman' to 'Werewolf'. The snapshot was taken during this transition.

Future work

The next step of my work is to validate the usefulness of the delay operator with a showcase of diverse examples, as well as to evaluate the gain for programmers (e.g. programming speed, legibility and understandability of the code) [START_REF] Olsen | Evaluating User Interface Systems Research[END_REF].

More generally, I am also focusing on interaction devices, exploring the raise of Mouse, Keyboard, Display objects to first-class objects [START_REF] Beaudouin-Lafon | Designing Interaction, Not Interfaces[END_REF]. While this is not a novel idea -Mozilla Firefox provides a Screen interface for interacting with the display, also Microsoft Visual Basic has a Mouse class -it has seen little use and deserves more attention given my overall objective in this thesis.

Finally, I am interested in how the design of frameworks can help their hacking, how people interact with the low-level functions, and how they relate code dirtiness to the amount of hacking they put into it. I already conducted a series of 8 interviews of people who are frequently prototyping very new and advanced interaction techniques, to gather the difficulties they faced while doing so. My goal is to classify the different aspects of software frameworks, into how they prevent or help programmers for exploring and prototyping alternative ideas. I am also interested in the "hacking" strategies people use to realize their ideas. As a knowledge contribution it should help the design of tools and languages to support the development of advanced interfaces and new interaction techniques.

Figure 1 :

 1 Figure 1: A Smalltalk Inspector open over a background window.


Rémy Ul

Mai Pham-Thi

Pascal Marchet

Aging behavior in Co doped Barium Titanate ceramics

Keywords: Baryum titanate, piezoelectric, aging effect

BaTiO 3 ceramics were doped with acceptor metallic ions (Co, Mn, Fe) and co-doped with Co and Nb. Piezoelectric ceramics were prepared using conventional solidstate reaction. Addition of Li 2 O sintering aid allows obtaining dense ceramics at low sintering temperature. Doping with Co acceptor leads to significant improve of piezoelectric properties but induce distorted and shifted hysteresis loop due to the presence of internal bias field. Field cooling poled ceramics are in the aged state just after poling. Increasing acceptor dopant concentration increases the internal bias field while co-doping with donor (Nb) reduces it. The data are consistent with presence of defect dipoles formed by acceptor substituted in B site and oxygen vacancy of BaTiO 3 tetragonal structure. Alignment of defect dipoles along driven electric field was observed through low frequency switching or cycling. Aging behaviors of unpoled and poled ceramics process were investigated through the study of hysteresis loops and reveal that charge compensation is not complete

I. INTRODUCTION

The change of properties with time or aging is a wellobserved problem in piezoelectric materials. The ferroelectric aging phenomena, observed as double hysteresis loop for aged BaTiO 3 :Mn single crystal, is attributed to reversible domain switching process [START_REF] Zhang | Aging behavior in single-domain Mn-doped BaTiO3 crystals: Implication for a unified microscopic explanation of ferroelectric aging[END_REF]. This behavior is due to the creation of electric dipoles between the defects and the oxygen vacancies when Ti 4+ is substituted by Mn 3+ /Mn 2+ . The hysteresis loop is explained with the symmetry of short-range order of point defects theory [START_REF] Ren | Universal Symmetry Property of Point Defects in Crystals[END_REF]. Barium titanate has recently shown interesting performance, making it a promising alternative to the PZT. To ensure the replacement of lead-based piezoelectric materials, study of the aging of lead-free material is crucial to the application.

II. EXPERIMENTAL PROCEDURE

Baryum titanate based materials were fabricated using oxide powder as raw materials: BaCO 3 , TiO 2 , Co 3 O 4 , Nb 2 O 5 , Li 2 CO 3 . The cobalt doping was introduced in substitution to the titanium. The powders were mixed and ball-milled by attrition in water for 1 h and were then dried and calcined at 1100 °C for 2 h. Thereafter, calcined powders were ball-milled by attrition for 0.5 h, dried, then mixed with the Li 2 CO 3 powder.

The resulting powder were then pressed into a cylinder of 16.3 mm in diameter and 10-10.5 mm in thickness using polyvinyl butyral and dibutyl phthalate as a binder and a plasticizer, which was burnt out by heating at 600 °C for 0.5 h. Sintering was performed at 1100 °C for 4 h in air. The sintered cylinder was cut into several 0.5 mm thickness pellets, which were coated with silver paint on both surfaces and fired at 500 °C for 1 h. The coated samples were then poled in a silicone oil bath by Field Cooling, consisting on the applying of a DC electric field of 600 V/mm during the cooling process of the samples from 140-150 °C to 40-50 °C. Ferroelectric hysteresis curves were measured in two ways. One is by using an oscilloscope (Lecroy Waverunner 6030) in a homemade sawyer-tower circuit through a voltage measurement and the other by using electronic cards through a current measurement by using a commercial device (Aixacct aixPES with TFAnalyzer 2000).

III. RESULTS AND DISCUSSION

A. Defect dipoles -Internal bias field

It is known that acceptor doping involves aging problems [START_REF] Arlt | Internal Bias in Ferroelectric Ceramics: Origin and Time Dependence[END_REF]. The substitution of Ti 4+ in the perovskite structure by di and tri valent ions (M) which has lower valence require charge compensation to maintain charge neutrality. An oxygen vacancy is indeed created and a defect dipole (M Ti ''-V O °°) X is formed. The defect dipoles can be aligned along spontaneous polarization and deformed lattice by jumps of the oxygen vacancy towards the orientation with lowest energy and internal bias field E int was observed [START_REF] Zhang | Large recoverable electrostrain in Mn-doped (Ba,Sr) TiO3 ceramics[END_REF][START_REF] Zhang | In situ observation of reversible domain switching in aged Mn-doped BaTiO3 single crystals[END_REF]. The increase and the decrease of the internal bias proceed in BT doped with Ni or Cr were calculated and measured with different time laws [START_REF] Lohkamper | Internal bias in acceptordoped BaTiO3 ceramics: Numerical evaluation of increase and decrease[END_REF]. The internal bias time law is thermally activated and obeys the Arrhenius law. Both laws were correctly described by relaxation model of oxygen vacancy jumps between sites of oxygen octahedron.

In the ABO 3 cubic perovskite structure, apex top of the oxygen octahedron are equivalent and the oxygen vacancy V O °° can form on any of them. In the tetragonal structure, the oxygen octahedron is distorted and possesses two non-equivalent oxygen sites, which are located respectively within the ab plane and along the c axis. The c/a ratio is 1.0105 and 1.0505 for BT and PT, respectively. The quantum-mechanical calculation for Cu/ Fe doped PbTiO 3 [START_REF] Erhart | Association of oxygen vacancies with impurity metal ions in lead titanate[END_REF][START_REF] Erhart | Formation and switching of defect dipoles in acceptor-doped lead titanate: A kinetic model based on firstprinciples calculations[END_REF] show that the two sites along the c axis are non-equivalents, the most stable configuration is (MTi-Vc1), therefore the defect induced dipole is oriented in parallel to the overall spontaneous polarization Ps. Thus in equilibrium, the polarization locally increases due to the formation and alignment of local defect dipoles. In contrast, the defect dipole of the alternative c-axis configuration (MTi-Vc2) causes a local reduction of the polarization.

Co, Mn, Fe were added as acceptor doping and Nb 5+ was added as donor to substitute Ti Fig. 1 presents the classical hysteresis loops of BT and the distorted loops for BT doped ceramics. The effect of cobalt adding in BaTiO 3 on hysteresis loop is clearly observed. Pure BaTiO 3 exhibits a typical symmetric ferroelectric hysteresis loop (Fig. 1a) while Co doped BaTiO 3 shows a double hysteresis cycle for unpoled and aged ceramic. The double loop is still observed for ceramic heated above Tc (150 °C) and cooling slowly to RT whereas symmetric loop is observed for ceramics quenched from 150 °C to RT. Thermal quenching seems to freeze cubic structure in unpoled ceramic that relax with time to reach an aged state in tetragonal phase. Ceramics poled by field cooling exhibit distorted hysteresis loop and shifted to the negative electric field (Fig. 1b). Field cooling process was used to pole BT ceramics. At 150 °C (> T C ) BT ceramic is in cubic phase, when the material is cooled down up to 40 °C the structure becomes tetragonal, the oxygen octahedron is distorted. Similar to the structure of PbTiO 3 , the most stable site for the oxygen vacancy in the tetragonal structure of the BaTiO 3 would corresponds to the one allowing the defect dipole orientates along the domain polarization. Since bias field was removed, poled & doped BT ceramics are already in the aged state and hysteresis loop is asymmetric and shifted. The internal bias field is measured as 835 V/mm on the loop recorded with triangular signal of 150 mHz. The measurement frequency of the hysteresis loops at 150 mHz is probably too fast for the oxygen vacancies migration and the defect dipoles does not have time to switch with the ferroelectric domains.

The same distorted & shifted loops were obtained for Mn and Co/Fe doped BaTiO 3 ceramics. The internal bias field increases from 750 V/mm for Mn doped sample to 935 for Co doped sample and it is measured as 685 V/mm for Co/Fe co-doped sample. These observations would be related to ionic radius and electronegativity difference of substituting ions.

B. Compensation by co-doping

Donor co-doping has been made with Nb 5+ . The niobium should insert in B-site of the perovskite like Co 2+/3+ and is introduced in substitution to Ti 4+ . In addition of the Co 2+/3+ doping, the adding of Nb 5+ could compensate the charge imbalance and decrease the formation of oxygen vacancies thus decreasing dipole defect concentration [START_REF] Liu | Ferroelectric aging effect in hybrid-doped BaTiO3 ceramics and the associated large recoverable electrostrain[END_REF]. Fig. 2 presents several materials with different amount of Co/Nb co-doping. The hysteresis loop becomes less shifted when 0.75 mol% of niobium is added in BT containing 0.75 mol% of Co. The internal bias field E int decreases from 935 V/mm to 660 V/mm. When the amount of niobium increases to 1.5 mol%, the loop becomes more symmetric and centered and E int decreased to 150 V/mm. It can be observed that the higher the amount of niobium, the smaller the internal field. It can be suggested that defect dipoles concentration is reduced due to the reduction of oxygen vacancies concentration. However hysteresis loops remain shifted and the internal bias field is lower but not completely suppressed.

C. Cinetic of defect dipoles alignment

Measurement recorded at frequency of 150 mHz leads to the observation of the internal field on the hysteresis loop. Probably the defect dipoles do not have the time to switch. Measurements performed at a frequency of 10 mHz have been made in order to let the time for the defect dipole to switch. Results are presented on Fig. 3. It can be observed that the decreasing of the frequency make the shift of the hysteresis loop smaller. The Fig. 3a shows a hysteresis loop, which becomes symmetric at 10 mHz. Moreover E int decrease from 580 V/mm to 330 V/mm and the coercive field 2E C increase from 1160 V/mm to 1530 V/mm when the frequency changes from 150 to 10 mHz. It is more significant on Fig. 3b for a Co/Nb co-doped sample where the internal field is smaller: at 10 mHz, the sample recover practically a typical ferroelectric hysteresis loop with a very low value of E int (50 V/mm at 10 mHz against 150 V/mm at 150 mHz). Thus, by decreasing the frequency, at least a part of the defect dipoles can reorientate and follow the switch of ferroelectric domains during hysteresis measurement.

D. Deaging by hysteresis cycling

Deaged experiment has been design to test our polarized materials and to follow the evolution of the internal field. The TFAnalyzer 2000 E has been used to apply a triangular wave of 1Hz with amplitude of 2500 V/mm on the sample for a total of 50000 cycles. Ten measurements have been performed per decade by sending a triangular pulse of 150 mHz at 2500 V/mm. Results of the experiment are presented on Fig. 4. It can be observed that throughout cycling, the shifted hysteresis loop gradually centers on the origin. The negative remnant polarization progressively decreases to become opposed to the positive remnant polarization. It corresponds to the hysteresis loop which becomes symmetric and centered. Thus when the material is subject to the electrical cycling the internal field decreased from 880 V/mm to 200 V/mm while the coercive field 2E C increases from 900 to 1040 V/mm. This result can be explained by a creation of disorder among the defect dipoles. The alternative electric field applied during cycling redirects the defect dipoles and thus their orientation is not anymore parallel to the ferroelectric domain polarization. So these defect dipoles have less influence on the ferroelectric domain and therefore the internal field decreases. Moreover after the experiment, the material keeps a polarized state and thus is still a piezoelectric material.

IV. CONCLUSION

An addition of just 0.5 mol% of Co 2+/3+ is enough to create a strong internal field inside the BaTiO 3 . This internal field is interpreted as the creation of defect dipoles formed by acceptor substituted in B site and oxygen vacancy of BaTiO 3 tetragonal structure leading to the distorted and shifted hysteresis loops. This behavior would be explained by the fact that the polarization is locally modified due to the formation and alignment of local defect dipoles along two nonequivalents c-axis configurations. Field cooling poled ceramics are in the aged state just after poling. For a poled cobalt doped sample, the internal field leads to an important shift of the hysteresis loop. An effective way to hinder the formation of the internal field is the co-doping with donor as the Nb 5+ which significantly decreases it. But even with the niobium adding of twice the cobalt concentration, a small internal field remains in the material. Moreover depending on the frequency during hysteresis measurement, the defect dipoles will not respond in the same way. A lower frequency indeed allows the switching of a part of the defect dipoles during hysteresis measurement. Furthermore it has been shown that after aging, a way to rejuvenate the ceramic was the electrical cycling. We have seen that through the de-aged experiment, the internal field of the ceramic decrease from 900 V/mm to 200 V/mm. After cycling, an internal field also remains in the material. Thus it could be interesting to go further and try to completely suppress the internal field.

Fig. 1 .Fig. 2 .

 12 Fig. 1. P-E hysteresis loops of (a) pure BaTiO3 (typical hysteresis loop) and (b) poled Co 2+/3+ -doped BaTiO3 with a representation of the ferroelectric domains and defect dipoles orientation depending on the applied electric field.Fig. 2. P-E hysteresis loops of BaTiO3 doped with Co 2+/3+ and different amount of Nb 5+ .

  4 in BaTiO3 ceramics. The piezoelectric properties were significantly improved for codoped ceramics. Addition of Li 2 O sintering aid allows to obtain dense ceramics (d rel > 95 %) at low temperature, between 800 and 1100 °C instead of 1350 °C for BaTiO 3 (BT) conventional sintering. The piezoelectric properties were significantly improved for Co doped ceramics. Piezoelectric constant of d 33 =225 pC/N and thickness mode electromechanical coupling factor of k t =41 % are obtained for samples with x=0.75 mol% of Co.

Fig. 4 .

 4 Fig. 4. Hysteresis loops measured at 150 mHz and 10 mHz of (a) cobalt doped BaTiO3 and (b) niubium and cobalt co-doped BaTiO3.

Fig. 3 .

 3 Fig. 3. Hysteresis loops after cycling during deaged experiment on cobalt doped sample.

ACKNOWLEDGMENT

This work is supported by Thales Underwater System and the French Research Agency (ANR HECATE). R UL thanks A. Marx & A. Lordereau for materials processing and electrical measurements.


P Marchet

C Champeaux

J Aubreton

J P Mercurio

A Catherinot

In situ growth of YBaCuO superconducting thin films by excimer laser ablation: influence of deposition and cooling parameters

In situ YBaCuO thin films are deposited on MgO single-crystal by pulsed laser ablation using an excimer laser (KrF, 248 nm). Deposition and cooling conditions are investigated in order to obtain c-oriented samples. The influence of substrate temperature and oxygen pressure during both deposition and post-deposition processes on the structural (orientation, lattice parameters) and electrical (resistivity at 300 K, critical temperature, transition width) film properties is analyzed. These studies lead to reproducible high quality c-oriented thin films which exhibit a critical temperature of 90 K, a superconducting transition width less than 1 K and a good behavior in the normal state.

Introduction

Since the discovery of high-Tc superconductivity in 1987 [1], the elaboration of superconducting thin films has led to numerous investigations. Among the different deposition methods [2][3][4][5], the pulsed laser deposition (PLD) appears to be the most successful technique because of its ability to transfer the stoichiometry from the target to the substrate. However, to produce reproducible high quality YBaCuO thin films suitable for practical applications or basic physics studies, not only appropriate stoichiometry but also good crystallographic orientation is necessary. All these characteristics depend on simultaneous control of several physical parameters during both deposition and cooling steps. In this paper, we report the influence of the substrate temperature, the oxygen pressure during deposition and the environment (gas nature and pressure) during cooling on the structural and electrical properties of in situ laser-deposited YBaCuO thin films.

Experimental details

Target preparation

YBaCuO bulk targets are prepared from a corresponding mixture of 99.9% pure copper and yttrium oxides (CuO, Y2O3) and barium carbonate (BaCO3). The powders are carefully mixed, ground and then calcined in an electrical furnace as follows: rise up to 850°C at 300°C/h, first dwell at 850°C for 20 h, second dwell at 950°C for 6 h and natural cooling. After crushing and sifting, the powders are pressed to a shape of a 2 cm diameter pellets, which are sintered at 975°C for 20 h under oxygen atmosphere. The superconductivity of the samples asmade is verified at 77 K by magnet levitation.

Deposition system

Deposition experiments are performed in an ultra-high vacuum cell equipped with an introduction chamber for targets and substrates manipulation. The beam of a pulsed excimer laser (248 nm, pulse duration 14 ns, repetition rate 10 Hz, energy per pulse 20-300 mJ) is focused by a tetrasyl quartz lens onto a rotating bulk superconducting YBaCuO target. According to the 35° incidence angle, the laser spot is elliptical and its size is typically 2.3 mm 2 . The vaporized material, ejected normally to the target surface is deposited on a single-crystal (001) oriented MgO substrate placed opposite the target at 3.8 cm. The substrate can be heated up to about 820°C by a halogen lamp. Its temperature is estimated with a pyrometer in combination with a chromel alumel thermocouple mounted at the edge of substrate holder. Several gas inlets allow the control of the deposition and cooling atmospheres.

Deposition parameters

The structural properties of the deposited thin films are investigated by X-ray diffraction using a standard θ/2θ diffractometer (Cu Ka radiation) in order to determine crystalline orientation. The detection of lattice planes parallel to the substrate surface allows a computation of lattice parameters. The electrical resistivity is measured by a standard DC four probes method in the 25-300 K range using a closed cycle helium cryostat. Temperature is monitored and given by a silicon diode.

Results

Substrate temperature

It is well recognized [START_REF] Shi | [END_REF] that substrate temperature is one of the most important parameters for the growth of high quality superconducting thin films. In order to analyze its influence, several films are prepared at different substrate temperatures (Ts), as listed in table 1. All these films are mirror-like but their color evolves from brown at low temperatures (Ts < 620°C) to black at high temperature. The X-ray analysis (fig. 1) shows that the sample crystallinity changes with temperature. At low temperature (430-620°C), the films are amorphous. Then an increase of the deposition temperature improves the quality of the film crystallization and phase orientation. At 690°C, the strong diffraction peak at about 33° corresponds to the randomly oriented phase (103 / 013 / 110). At 725°C, this peak is no longer observed and the films are perfectly a-oriented. With increasing Ts, the intensity of the (h00) diffraction peaks decreases while that of (001) peaks increases. The room temperature resistivity of these films decreases from a few kΩ. cm (Ts < 620°C) to a few mΩ.cm and their electrical characteristics versus temperature evolve from semiconducting (at 690°C) to superconducting behavior (at 790°C), as shown in fig. 2. These results are in agreement with the structural properties of the films and prove the important role of substrate temperature on the film structural properties.

Deposition oxygen pressure

The previous results show that substrate temperature during deposition should not the only parameter to be studied in order to improve high-Tc superconducting film quality. According to the previous experiments and our aim (c-oriented films), the selected substrate temperature is 790°C. The oxygen pressure will vary in the range of 6x10 -3 -0.3 mbar. The other deposition parameters are identical as previously, that is to say: the laser fluence is 3 J/cm 2 and repetition rate 10 Hz. After deposition, samples are cooled under oxygen at 30 mbar. Fig. 3 shows the change of the resistivity versus temperature curves with the deposition pressure. All the films are superconducting. The critical temperature (Tonset) increases (from 80 to 86 K) and the transition width decreases (from 32 to 6 K) when the deposition pressure increases. These electrical properties are in agreement with the structural ones. Indeed, the mixture of (h00) and (00l) orientations which is observed at low deposition pressure disappears on behalf of the single orientation (001), at the highest pressures (fig. 4). The improvement of the critical temperature and transition width can be attributed to a more effective oxygen incorporation in the material at high pressures. This hypothesis is confirmed by the values of the lattice parameters c. In the present case, the c values, are respectively equal to 1.183 and 1.171 nm for 0.1 (sample D6) and 0.3 mbar (sample D8). Indeed, the lowering of the c-lattice parameter corresponds to an increase of the number of oxygen atoms in YBa2Cu3Ox [9]. In fact, these two parameters c and x are linked by the relation x = (1.2736 -c( nm)) /0.01501 established for bulk samples by Ohkubo et al. [10].

Choice of gas and pressure during cooling step

The former studies emphasize the influence of the substrate temperature and the deposition oxygen pressure on the crystalline orientation. When the temperature is 790°C and the pressure 0.3 mbar, c-oriented superconducting thin films are obtained, but their electrical properties are not optimal (transition width 6 K). That is why the influence of the nature (vacuum, argon and oxygen atmosphere) and the pressure of the gas present during cooling is investigated. The deposition conditions are reported in table 2. The films are found highly-c oriented independently on the nature and the pressure of the gas present during cooling. This observation demonstrates that the deposition conditions themselves (substrate temperature, oxygen pressure, laser fluence, target-substrate distance, etc.) determine the crystalline orientation. Fig. 5 shows the temperature dependence of resistivity for the films of this experiment. We notice that the film cooled down in vacuum is semiconductive, while those cooled down in argon or oxygen atmospheres are superconducting. This is attributed to the difference in c lattice parameter and, therefore, in the oxygen content x (in YBa2Cu3Ox). Indeed, the sample D12, cooled in vacuum condition, is oxygen deficient (x = 5.9). This result, in agreement to Ohkubo's observations [10], leads to the two following hypotheses: -the oxygen incorporation during the deposition process is insufficient, -the oxygen contained in the film at the end of the deposition process has outdiffused from the film during the cooling process in vacuum.

In order to try to conclude, we can examine the sample D13, which is cooled down in argon pressure of 330 mbar. Its oxygen content, greater than in the case of cooling in vacuum, yields to a superconducting behavior, as shown in fig. 5. The transition width, of about 20 K, indicates that the incorporation of oxygen into the film during deposition is certainly not sufficient, according to the first hypothesis. But, it is not possible now to confirm or cancel the second hypothesis. Indeed, oxygen can outdiffuse, like in the case of cooling in vacuum, from the film to the gaseous phase (argon).

However, this experiment shows that the superconducting YBaCuO phase grows epitaxially on MgO during the deposition. This result, evident from our experiment, disagree with the conclusions of Ohkubo et al. [10] and Ying et al. [11]. These results show that cooling in oxygen is primordial to obtain high quality superconducting thin films. So, we have grown several films samples which were cooled down in different oxygen pressures (table 2). All these films are superconducting (fig. 5) and have a good metallic behavior above the transition (ratio of the resistivities at 300 and 100 K of about 3). The transition width varies from 18 to 1 K and presents an optimum for the oxygen pressures during cooling in the range of 160-330 mbar.

Conclusion

In situ growth of high quality superconducting thin films depends not only on the deposition parameters but on the cooling process. The determinant role of the substrate temperature and the oxygen pressure during deposition on the crystalline orientation, has been shown. The optimization of the electrical properties needs adequate oxygen pressure during the cooling process.

Table 1

Deposition temperatures of the YBaCuO thin films on single crystal MgO(00l) samples (oxygen pressure 6x10 -3 mbar. laser fluence 3 J/cm 2 , natural cooling with 30 mbar of oxygen)

Table 2

Deposition parameters, structural and electrical properties of YBaCuO thin films deposited on heated single crystal MgO (001) substrates at 790°C (oxygen pressure 0.3 mbar and cooled down in different environments)

Fig. 1 .

 1 Fig. 1. X-ray diffraction patterns of YBaCuO films on MgO as functions of the substrate temperature.

Fig. 2 .

 2 Fig. 2. Resistivity versus temperature curves for samples D3 (Ts = 690°C) and D5 (Ts = 790°C).

Fig. 3 .

 3 Fig. 3. Resistivity versus temperature curves for different deposition oxygen pressures.

Fig. 4 .

 4 Fig. 4. X-ray diffraction patterns of YBaCuO films deposited on MgO at Ts =790°C as functions of the deposition pressure.

Fig. 5 .

 5 Fig. 5. Resistivity as a function of temperature for films cooled at different residual ambient gas.

Acknowledgement

This work is partly supported by DRET under contract No. 88-084.


Ricardo P S M Lobo

François Gervais

Corinne Champeaux

Pascal Marchet

Francois Gervais

Alain Catherinot

MATERIALS SCIENCE & ENGINEERING g Unexpected behaviour of IR reflectivity of a YBa2Cu307 6 oriented film

Keywords: YBCO thin films, Optical conductivity, Superconducting gap

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Since the discovery of high temperature superconductivity in layered copper-based oxides by Bednorz and Muller [1], an enormous effort to understand the physics of these compounds has been made. An even greater interest in this subject has come with the synthesis by YBa2Cu3O7 (YBCO), having a critical temperature above liquid nitrogen temperature [2]. One of the main features of layered oxide superconductors is the highly anisotropic, even bidimensional, conducting behaviour: the charge carriers are mainly confined within the basal a-b plane [START_REF]Physical Properties of High Temperature Superconductors[END_REF]. Thus, the obtention of good quality YBCO a-b oriented films, independent of their interest for future applications, is also of great interest for the understanding of their conducting properties.

A very powerful tool to study these systems is the IR (and more generally optical) reflectivity. IR-visible spectroscopy enables us to obtain information about, in addition to phonons, charge carriers (plasmons), superconducting gap and higher energy (mid-IR) absorptions. Although the phonon behaviour is nowadays well understood in YBCO [START_REF] Burns | [END_REF]5], the other features observable in the IR and visible spectrum of this material are far from giving rise to any consensus yet. (i) The plasmon in YBCO cannot be fitted with a simple Drude expression [6 12]. (ii) The explanations of mid-IR excitations are unclear [START_REF]Physical Properties of High Temperature Superconductors[END_REF]7,8]. (iii) The existence of an observable superconducting gap in the Bardeen-Cooper-Schrieffer (BCS) sense is controversial [6,8,19].

In this work, we present optical reflection over a YBCO film grown by laser ablation, between 4 K and room temperature. Spectra have been recorded with a single spectrometer covering the spectral range from 50 to 22 000 cm -1 , thus including all the excitations discussed above.

Experiment

The spectra have been recorded with a Fourier transform IFS 307 Bruker spectrometer composed of two interferometers with complementary spectral ranges, which cover the region from 8 cm -1 to 40000 cm -1 . Reference measurements have been done over an aluminium mirror and the reflectance error in our spectra does not exceed 2%. Temperature control accuracy, with the aid of a CLTS probe, was of the order of 0.5 K. The YBCO film has been deposited by pulsed laser ablation. The laser source is an excimer laser operating at 248 nm (KrF). After spatial filtering, the laser beam impinges on a bulk superconducting target. The vaporized material ejected from the target surface is deposited on a (100)-oriented single crystal of MgO substrate located opposite to the target. The film has a surface area of about 1 cm 2 and is 325 nm thick. Its X-ray diffraction pattern reproduced in Fig. 1 shows that it is highly a-b plane oriented (better than 0.7 °) and allows the determination of c lattice parameter: 11.695 ± 0.005 Å. The electrical resistivity has been measured by a standard DC four-probe technique. Thermal evolution of the DC resistivity is shown in Fig. 2. The superconducting transition has a critical temperature Tc = 90.5 K and a transition width ∆T = 0.6 K. A linear increase in resistivity is observed from Tc up to room temperature. The critical current density reaches a value as high as 5 × 10 6 A.cm -2 . All these data indicate a "δ" of about 0.05 and a relatively good quality film.

IR data fitting

Previous studies [6][7][8][9][10] have shown that a Drude term alone cannot explain the conducting behaviour of the a-b plane conductivity in YBCO. The best compromise with just a Drude term for the dielectric susceptibility χplasmon obtained in terms of the plasma frequency Ωp and its damping γp, i.e.

χ = -ε (1)
is shown in Fig. 3 and clearly is unable to fit the experimental result. Here Ωp is defined in such a manner that, in the absence of damping, one obtains unity reflection up to ω = Ωp. Note that in some reports ε∝ is included in Ωp².

In the best compromise obtained with this model, the input parameters were Ωp = 9100 cm -1 , γp =2500 cm -1 and ε∝ =3.60. Many models have attempted to reproduce the spectra with various phenomenological or microscopic bases. (i) The only model that keeps the Drude term requires several additional strong oscillators in the phonon and also mid-IR regions [6,8,11]. However, the dielectric strength of certain of these oscillators is unrealistically high for phonons (below 700 cm -1 ), compared with phonons in O6 compounds, for instance, and their origin is quite unclear in the mid-IR. In addition, the Drude contribution is not dominant. This seems physically artificial because, in a highly conducting medium, all oscillators are expected to be screened by charge carriers. (ii) Another way to fit the response in YBCO is to consider a frequency dependence of at least one term, either the scattering rate or the effective mass [5,7,20]. (iii) It is also possible to explain the plasmon-like response in YBCO as multiple contribution of different Drude terms but here again the microscopic origins are unclear [9,10].

Here we use an extension of the Drude model, which introduces a very simple frequency dependence in the plasmon scattering rate. This model has already been successfully used to fit IR spectra of other conducting oxides such as reduced TiO2 [START_REF] Gervais | Infrared and Millimeter Waves[END_REF][START_REF] Baumard | [END_REF], NbO2 [23] and strontium titanate [24]. According to the Maxwell equations, transverse optical (TO) and longitudinal optical (LO) modes are the poles and zeroes respectively of the dielectric response. Hence the dielectric function model consists in factorizing over TO and LO modes [START_REF] Gervais | Infrared and Millimeter Waves[END_REF]. The basic point added by such an extension is the possibility of having different damping terms of TO and LO modes. This possibility of different scattering rates has been shown to be very useful in cases where we have wide reflection bands (a large LO -TO splitting) [START_REF] Gervais | Infrared and Millimeter Waves[END_REF][START_REF] Baumard | [END_REF][23][24][25]. If we regard the charge carrier contribution to the dielectric function as an additional "oscillator" with it's TO frequency equal to zero (no restoring force constant for mobile charge carriers) and the plasma frequency as an LO mode, we obtain an extension of the Drude contribution for the dielectric susceptibility χplasmon in the form

χ = -ε (1)
In Eq. ( 2), Ωp is the plasma frequency as defined in Eq. ( 1) and γp and γ0 are the dampings evaluated at Ωp and in the static limit respectively. Setting γp = γ0 transforms Eq. ( 2) into the commonly used Drude expression Eq. (1). Eq. ( 2) implicitly introduces a monotonic variation in the plasmon damping function from γ0 at zero frequency to γp at the LO plasma frequency, whereas the usual Drude term Eq. ( 1) assumes a constant damping. More details about this model were recently given in Ref. [24].

The room temperature normal state

Fig. 3 shows spectra obtained at room temperature and 4 K. Above ca. 700 cm -1 . The difference between spectra is negligible within experimental error. This is unexpected because Fig. 2 shows a (common) temperature dependence of the DC conductivity and, therefore, the absence of significant dependence in the IR raises a problem. The overall behaviour of the room temperature spectrum can be obtained from the simple modified Drude term Eq. ( 2) with no additional feature. Phonons are completely screened by the plasmon, which is expected in view of the Tc of our sample, and so there is no need to introduce any phonon oscillator. (In fact, their introduction, with parameters known for the O6 compound, does not significantly modify the response). The modified-Drude fit shown in Fig. 3 has Ωp =9100 cm -1 , γp =7350 cm -1 , γ0 =1750 cm -1 and ε∝ = 3.60, values comparable with those found by Thomas et al. However, in Ref. [6], Thomas et al. used an additional mid-IR oscillator. The high damping values (when compared with Ωp), which are observed in virtually all conducting oxides, may be regarded as a strong interaction between change carriers and crystalline lattice. We are, then, allowed to think that conduction mechanisms in these systems are polaronic (trapped rather than free charge carriers).

We are tempted, here, to make a parallel between the present model and other models dealing with the same problem. The marginal Fermi liquid (MFL) [26], the nested Fermi liquid (NFL) [27] and the antiferromagnetic Fermi liquid (AFL) [28] models are the most important examples. The MFL model is based on the hypothesis that there are excitations, which contribute to both spin and charge polarizability. The form of this excitation is stated to behave as ω/T when |ℏω| < kBT and to be constant when |ℏω| > kBT. The IR data are then fitted by means of a Drude-like expression having frequency-dependent damping and frequency-dependent charge carrier effective mass. The NFL model, as its name states, requires the nesting of the Fermi surface and is also based on a Drude-like expression for optical conductivity. Both models predict a linear frequency dependence of damping. Both models fit quite well the frequency-dependent conductivity and are compatible with the linear thermal dependence of DC resistivity. The AFL Model is based on antiferromagnetic spin fluctuation interactions. If the model is less successful in describing IR-visible conductivity, it is still compatible with the thermal dependence of DC conductivity.

The model used in this paper also describes in a quite accurate manner the frequency dependence of optical conductivity. It is not possible to compare its formalism directly with the other models. The only situation in which we can do so is when γp -γ0 << Ωp. In this approximation one obtains γ(ω) ≈ γ0 + (γp -γ0) ω²/ωp for the present model. The quadratic term is then very weak and may be considered as a slight perturbation in the Drude classical form. However, when γp -γ0 ≈ Ωp (as is the case in the present work) the functional form of IR conductivity can no longer be compared with the Drude Model. The present model has already proved itself able to fit data from various conducting systems [START_REF] Gervais | Infrared and Millimeter Waves[END_REF][START_REF] Baumard | [END_REF][23][24][25] and we must emphasize that in YBCO, contrary to some analyses of other experimental data [6,8,29], the overall response is here obtained via this single modified Drude-like term Eq. ( 2). This YBCO film needs no additional contribution in the phonon or mid-IR regions. This means that, if such mid-IR absorptions appear at lower electronic concentration (lower oxygen content), they are masked here by the plasmon as well as phonons are screened. Moreover, no assumption about particle interactions (magnetism, low frequency dependence) or system configuration (nesting of Fermi surface) is made, making the present model quite general. If the other models describe optical data in YBCO with the same accuracy, applying them to other systems (especially non-cuprate systems) can lead to some physical inconsistency. The present model, on the contrary, can be used in any conducting system.

The superconducting state

At low frequency, the liquid helium temperature spectrum exhibits an additional reflection band which is absent at room temperature. The thermal evolution of the low frequency spectrum of our film is displayed in Fig. 4. Fig. 4 also shows a comparison of the reflection at all temperatures with the 300 K reflectivity at three different frequencies within the 200 400 cm -1 range. Below 200 cm -1 , experimental uncertainties limit the accuracy of the data. Above 400 cm -1 , the change in spectral data is too small with respect to the experimental error. Looking higher in frequency, we see that the spectrum at 4 K follows the normal state spectrum down to about 600 cm -1 and then has an enhanced reflectivity for the lower frequencies. The problem, thus, is to determine whether this departure starts at the superconducting transition temperature or not. In other words, we want to see whether this behaviour can be related to the condensation of Cooper pairs or whether it is just continuous evolution of the spectra. The plot of ∆R/R300 in Fig. 4 seems to be clear about the behaviour, at least at intermediate frequencies.

Between 300 K and 102 K, no enhancement of the reflectivity is observed. The reflectivity for temperatures between 4 K and 90 K also is clearly higher than the normal state reflectance. Within experimental uncertainty and on the basis of this plot in Fig. 4, it appears that this change to regime starts at the transition temperature. We can conclude, then, that this reflection band is related to the superconducting transition and thus to the formation of Cooper pairs. In BCS theory, the formation of a superconducting gap is expected. The IR response of such a gap is a flat unity level in reflectivity below the gap energy and a thermal evolution of this energy gap (the gap being the order parameter, we should expect a second-order transition behaviour). This is not what we observe in our sample. No flat unity level has been attained and no frequency shift of our onset frequency has been observed. Anyway, one may consider that a standard BCS gap is unlikely to exist in YBCO as, at least, phonon states lie within this "gap" region. We will not enter in a discussion of s wave or d wave superconductivity but, if we insist (for comparison with other data) on taking a frequency (about 500 cm -1 ) just below this onset in terms of a BCSlike gap, we find 2∆ / kBTc = 8, as already reported [30,31]. This value is also consistent with indications deduced from some other methods (nuclear magnetic resonance, Raman photoemission) [32][33][34][35][36]. Present experiment data are very similar to those reported by Kamarfis et al. [8] below 100 K, but contrary to their report, we do not observe any significant evolution of the reflectivity between 100 K and room temperature. They did not relate this low frequency increasing in reflection to a superconducting gap as they considered that a residue of this profile was still present at room temperature. This is not the case in our film. The extra band is clearly related to the superconducting transition

Optical conductivity

Fig. 5 shows the real part of the optical conductivity of our sample. The theoretical curve for room temperature spectrum has been obtained from Eq. ( 2) and the curves at 4 K and 300 K are Kramers Kronig transformations of reflection data in the superconducting and normal states respectively. Adopting different extrapolations to zero frequency leads to slightly different low frequency behaviours in conductivity. To avoid misinterpretation of data we present Kramers Kronig transformations above the 380 cm -1 region which, in our case, was independent of the low frequency extrapolation. Once again, the agreement between the model and the experiment is quite satisfactory. Moreover, the extrapolation of the fitted conductivity to zero frequency falls into the same range of DC conductivity measured for similar films [37]. In the experimental spectra, one sees the effect of the superconducting transition in the frequency region below 600 cm -1 while in the normal state the conductivity raises continuously on decreasing frequency, in the superconducting phase it drops for frequencies below 600 cm - 1 , an effect possibly linked to the superconducting transition. This possible relation is even more evident in Fig. 6 where we present conductivity obtained from Kramers Kronig transforms. No significant change has been observed between 300 K and 102 K. On the contrary, a sudden depart from this "room temperature behaviour" can be inferred below Tc for low frequencies.

Conclusion

In this work, we have presented the characterization of a YBa2Cu3O7 film obtained by laser ablation. The aim of high critical current densities has been achieved in this film, which presents an almost single-crystal structure configuration with a high a-b plane orientation. We have characterized the temperature dependence of the reflectivity and focused the low frequency extra absorptions that develop below Tc. Present results and their analysis follow a long-standing controversy about both of the main discussed points (superconducting gap and charge carrier response). We show the onset of an additional band that appears very probably at Tc and increase below. From both reflectivity and conductivity curves, we can associate this change in reflectivity with a superconducting condensate. If the cut-off frequency of this feature is assigned to a superconducting gap, then one confirms results previously obtained by others and commonly interpreted in terms of 2∆ / kBTc = 8. We have also been to fit the normal state spectrum with a single modified Drude-like term (no additional phonon or mid-IR contributions) which assumes a frequency-dependent damping or, equivalently, a frequency-dependent carrier effective mass.

Fig. 1 .

 1 Fig. 1. X-ray diffraction pattern of YBCO film showing its (001) orientation.

Fig. 2 .

 2 Fig. 2. DC resistivity of YBCO film. The 0.6 K width of the superconducting transition is also shown.

Fig. 3 .

 3 Fig. 3. IR and visible reflection of YBCO film at 4 K and 300 K and best fits obtained from a simple and a modified Drude behaviour. Fit parameters are given in the text.

Fig. 4 .

 4 Fig. 4. Low frequency behaviour of IR reflection in YBCO film. A comparison is also shown of the reflectivity at temperature T with the room temperature value via [R(T )-R(300 K)] / R(300 K) for three frequencies in the low energy part of the spectra. The uptake in reflectance below 600 cm -1 can be associated with the superconducting transition.

Fig. 5 .

 5 Fig. 5. Real part of IR and visible conductivity. Data at 4 K and 300 K have been obtained from Karmers Kronig transforms of reflection data and theoretical curve is obtained from our modified Drude expression with the same parameters used in Fig. 2. Double arrow shows DC conductivity measured by Flik et al. [37] for a set of similar thickness range films.

Fig. 6 .

 6 Fig. 6. Low frequency dependence of real part of conductivity between 4 K and 300 K. Departure from a "'room temperature behaviour" occurs at Tc and can be related to the superconducting transition.

Acknowledgements

The authors are grateful to Dr. P. Echegut for helpful discussions. One of us (R.P.S.M.L.) acknowledges fellowship support from the Brazilian agency CNPq.