R El Hage

D Sánchez-Manzano

V Humbert

S Carreira

V Rouco

A Sander

F Cuellar

K Seurre

A Lagarrigue

J Briatico

J Trastoy

J Santamaría

Javier E Villegas
email: javier.villegas@cnrs-thales.fr

Disentangling photodoping, photoconductivity, and photosuperconductivity in the cuprates

The normal-state conductivity and superconducting critical temperature of oxygen-deficient YBa2Cu3O7- can be persistently enhanced by illumination. Strongly debated for years, the origin of those effects -termed persistent photoconductivity (PPC) and photosuperconductivity (PPS)-has remained an unsolved critical problem, whose comprehension may provide key insights to harness the origin of high-temperature superconductivity itself. Here we make essential steps toward understanding PPS. While the models proposed so far assume that it is caused by a carrier-density increase (photodoping) observed concomitantly, our experiments contradict such conventional belief: we demonstrate that it is instead linked to a photo-induced decrease of the electronic scattering rate. Furthermore, we find that the latter effect and photodoping are completely disconnected and originate from different microscopic mechanisms since they present different wavelength and oxygen-content dependences as well as strikingly different relaxation dynamics. Besides helping disentangle photodoping, PPC, and PPS, our results provide new evidence for the intimate relation between critical temperature and scattering rate, a key ingredient in modern theories on high-temperature superconductivity.

The transport properties of underdoped YBa2Cu3Ox (YBCO) (6.3 < 𝑥 < 6.9) can be altered via illumination with visible-UV light [1][2][3][4][START_REF] Kudinov | Persistent Photoconductivity in YBa2Cu3O6+x Films as a Method of Photodoping toward Metallic and Superconducting Phases[END_REF][START_REF] Kudinov | Mechanisms for the Persistent Photoconductivity of Oxygen Deficient YBa2Cu3O6+x[END_REF][START_REF] Osquiguil | Photoexcitation and Oxygen Ordering in YBCO Films[END_REF][START_REF] Tanabe | Effects of Photoinduced Hole Doping on Normal-State and Superconducting Transport in Oxygen-Deficient YBa2Cu3Oy[END_REF][START_REF] Lederman | Photoinduced Superconductivity and Structural Changes in High Temperature Superconducting Films[END_REF][START_REF] Gob | Erasable Photoinduced Change of Carrier Density and Coherence Lengths in Oxygen Deficient YBCO[END_REF][START_REF] Chew | Infrared Quenching of Photoinduced Persistent Conductivity in YBa2Cu3O6+x[END_REF][START_REF] Endo | Enhancement of Persistent Photoconductivity by Uv Excitation in Gd<span Class[END_REF][START_REF] Markowitsch | Dynamics of Photodoping and Photoconductivity Relaxation in Oxygen Deficient YBCO[END_REF][START_REF] Stockinger | Mechanisms of Photodoping in Oxygen-Deficient Films Studied by in Situ Transport Measurements[END_REF]. Following photoexcitation, and once in the dark, a decrease of the resistivity and an increase of the superconducting critical temperature TC are observed. These effects, termed persistent photoconductivity (PPC) and photosuperconductivity (PPS), are accompanied by electronic changes (a decrease in the Hall coefficient and an increase in Hall mobility) [3,[START_REF] Tanabe | Effects of Photoinduced Hole Doping on Normal-State and Superconducting Transport in Oxygen-Deficient YBa2Cu3Oy[END_REF][START_REF] Stockinger | Mechanisms of Photodoping in Oxygen-Deficient Films Studied by in Situ Transport Measurements[END_REF], an enhancement of the superconducting coherence lengths [START_REF] Gob | Erasable Photoinduced Change of Carrier Density and Coherence Lengths in Oxygen Deficient YBCO[END_REF], as well as by a contraction of the c axis parameter [START_REF] Lederman | Photoinduced Superconductivity and Structural Changes in High Temperature Superconducting Films[END_REF]. These photoexcited states relax only at a high temperature (typically above 250K) [4,[START_REF] Kudinov | Persistent Photoconductivity in YBa2Cu3O6+x Films as a Method of Photodoping toward Metallic and Superconducting Phases[END_REF][START_REF] Markowitsch | Dynamics of Photodoping and Photoconductivity Relaxation in Oxygen Deficient YBCO[END_REF]. Because PPC and PPS become stronger with decreasing x and vanish in samples with optimum oxygen stoichiometry, it is widely accepted that they are linked to the presence of oxygen vacancies, particularly in the basal CuO chains [START_REF] Lederman | Photoinduced Superconductivity and Structural Changes in High Temperature Superconducting Films[END_REF][START_REF] Hasen | Enhancement of Persistent Photoconductivity in Insulating High-T c Thin Films[END_REF]. PPC and PPS attracted a great deal of attention since their discovery, in the early times of high-TC superconductivity. Their understanding is crucial for various reasons. First, explaining PPS may contribute to understanding the crux of the matter -the origin of high-TC superconductivity itself. Second, an analogous photoinduced switching of ground-states has been recently observed in other strongly correlated oxides, for example, manganites [START_REF] Takubo | Photoinduced Metalto-Insulator Transition in a Manganite Thin Film[END_REF][START_REF] Lin | Unexpected Intermediate State Photoinduced in the Metal-Insulator Transition of Submicrometer Phase-Separated Manganites[END_REF],

iridates [START_REF] Li | Recovery of Photoexcited Magnetic Ordering in Sr2IrO4[END_REF], nickelates [START_REF] Esposito | Dynamics of the Photoinduced Insulator-to-Metal Transition in a Nickelate Film[END_REF][START_REF] Liao | Metal-Insulator-Transition Engineering by Modulation Tilt-Control in Perovskite Nickelates for Room Temperature Optical Switching[END_REF] and LaAlO3/SrTIO3 heterostructures [START_REF] Lin | Control of the Metal-Insulator Transition at Complex Oxide Heterointerfaces through Visible Light[END_REF], which are also far from being fully understood. Since these systems share many electronic properties and structural features with cuprates, it is likely they also share some of the unknown mechanisms underlying their photosensitivity. (with 𝜌 𝑥𝑦 the Hall resistivity, 𝐵 the applied field, and 𝑒 the electron charge), existing models generally consider they both primarily originate from an enhancement of the carrier concentration (photodoping) whose microscopic origin was largely debated (see [1] for a review). Kudinov et al. [START_REF] Kudinov | Persistent Photoconductivity in YBa2Cu3O6+x Films as a Method of Photodoping toward Metallic and Superconducting Phases[END_REF] first proposed that photoexcitation produces electronhole pairs which, followed by electron trapping and the promotion of holes to the conduction band, leads to the photodoping that ultimately enhances the conductivity and TC. Electrons would be trapped by charged defects, such as oxygen vacancies or by CuO chain fragments [START_REF] Kudinov | Mechanisms for the Persistent Photoconductivity of Oxygen Deficient YBa2Cu3O6+x[END_REF].

In this scenario, the photoinduced effects relax as T increases as electrons are released from their traps, and recombine. An alternative scenario was proposed by Osquiguil et al. [START_REF] Osquiguil | Photoexcitation and Oxygen Ordering in YBCO Films[END_REF] based on the idea that photoexcitation promotes the reordering of oxygen vacancies within the CuO chains, lengthening the average chain fragment, which expectedly produces an increase of carrier density in CuO2 planes [START_REF] Fontaine | Low-Temperature Long-Range Oxygen Order in YBa2Cu3Oz[END_REF] and thus the enhancement of the YBCO superconducting properties.

While hybrid scenarios have also been proposed [START_REF] Bubb | Cellular Automata Model for Persistent Photoconductivity in YBCO[END_REF][START_REF] Markowitsch | Persistent Photoconductivity in Oxygen-Deficient and Ion-Irradiated YBa 2 Cu 3 O x[END_REF], in all cases, PPS is primarily attributed to photodoping. Contrarily, it is generally assumed that the concomitant changes in the Hall mobility 𝜇 𝐵 [START_REF] Stockinger | Mechanisms of Photodoping in Oxygen-Deficient Films Studied by in Situ Transport Measurements[END_REF] have no causal effect on the enhancement of superconducting

properties. Yet, changes in 𝜇 𝐵 reflect changes in the electron scattering [START_REF] Anderson | Hall Effect in the Two-Dimensional Luttinger Liquid[END_REF][START_REF] Chien | Effect of Zn Impurities on the Normal-State Hall Angle in Single-Crystal YBa2Cu3-XZnxO7[END_REF], which has strong ties with the superconducting pairing in the cuprates in various ways. For example, it is well known that scattering by impurities [START_REF] Chien | Effect of Zn Impurities on the Normal-State Hall Angle in Single-Crystal YBa2Cu3-XZnxO7[END_REF] or disorder [START_REF] Lesueur | Depairing-like Variation of Tc in YBa2Cu3O7-δ[END_REF][START_REF] Tolpygo | Effect of Oxygen Defects on Transport Properties and: Displacement Energy for Plane and Chain Oxygen and Implications for Irradiation-Induced Resistivity and Suppression[END_REF] depresses TC via pair breaking.

Another example is the observation of quantum criticality -a scaling between TC and the Tlinear scattering rate [START_REF] Cooper | Anomalous Criticality in the Electrical Resistivity of La 2-XSrxCuO4[END_REF], which furthermore exhibits the same anisotropy as the d-wave gap [START_REF] Abdel-Jawad | Anisotropic Scattering and Anomalous Normal-State Transport in a High-Temperature Superconductor[END_REF][START_REF] Abdel-Jawad | Correlation between $T_c$ and Anisotropic Scattering in Tl$_2$Ba$[END_REF]-pointing to a common origin for anisotropic scattering and pairing [START_REF] Taillefer | Scattering and Pairing in Cuprate Superconductors[END_REF]. Because illumination modifies both 𝜇 𝐵 and TC, the question is whether a causal relationship exists and whether the mechanisms behind PPS go beyond the photodoping scenario.

The present experiments show that PPS is indeed strongly correlated with the photoinduced enhancement of 𝜇 𝐵 , but not with the variations of 𝑛 𝐻 . This finding challenges the idea that PPS is caused by photodoping and suggests instead that it is related to a change in the scattering rate. Furthermore, not only are the variations of the 𝜇 𝐵 and 𝑛 𝐻 decorrelated, but they also relax over very different time and temperature scales, implying that they originate from different microscopic mechanisms. Our observations allow for reconciling the main scenarios for the microscopic mechanisms by assigning separate roles to each of them. We propose that photodoping can be explained within the photocarrier excitation/electron trapping model, which would not play a major role in PPS, while oxygen ordering explains the photoinduced decrease of the scattering rate, and would more substantially contribute to PPS.

In our experiments, we compare oxygen-deficient c-axis YBa2Cu3Ox (YBCO) films with optimally doped ones in which disorder has been induced by ion irradiation. All the films were grown on (001) SrTiO3. Oxygen-deficient films (~30 nm thick) were grown by sputtering in a pure oxygen (3.4 mbar) atmosphere at T= 900º C. Samples with a nominal oxygen content 6.3<x <6.9 were prepared by adjusting the pressure during sample cooldown following different lines of the pressure-temperature phase diagram [START_REF] Sefrioui | Crossover from a Three-Dimensional to Purely Two-Dimensional Vortex-Glass Transition in Deoxygenated YBa2Cu3O7-δ Thin Films[END_REF][START_REF] Osquiguil | Controlled Preparation of Oxygen Deficient YBa2Cu 3Ox Films[END_REF]. Two samples were prepared for irradiation, the one (~ 30 nm thick) by sputtering as described above and the second (~50 nm thick) by pulsed laser deposition (=248 nm, energy ~1 J.cm -2 , repetition rate 5Hz) with oxygen pressure 0.36 mbar and T= 700°C. In the PLD, the oxygen pressure was raised to 800 mbar right after deposition and kept constant during cool-down to obtain nearly optimally doped films (𝑇 𝐶 ~85K). Each of these samples was diced in four pieces and subsequently irradiated with O + (E=110 keV) at different doses 10 13 < 𝑑 < 10 14 ions cm -2 . As discussed elsewhere [START_REF] Bergeal | Using Ion Irradiation to Make High-Tc Josephson Junctions[END_REF][START_REF] Swiecicki | Strong Field-Matching Effects in Superconducting YBa 2Cu 3O 7-δ Films with Vortex Energy Landscapes Engineered via Masked Ion Irradiation[END_REF][START_REF] Trastoy | Freezing and Thawing of Artificial Ice by Thermal Switching of Geometric Frustration in Magnetic Flux Lattices[END_REF], in these conditions the O + track length across YBCO is ~150 nm. Thus, ions finish their course in the substrate and are not implanted within YBCO, where they create point defects (vacancies/interstitials) due to the displacement of atoms from the structure (mainly oxygen), at a ratio (Displacements Per Atom in the structure) 𝐷𝑃𝐴 = 𝑑 • 3.82 • 10 -16 cm -2 , as calculated using SRIM simulations [START_REF] Thomé | Radiation Effects in Nuclear Ceramics[END_REF][START_REF] Biersack | The Stopping and Range of Ions in Solids[END_REF]. X-ray diffraction confirms the quality of the films, shows the expected dependence on x and d [START_REF] Ye | Quantitative Structure Analyses of YBa2Cu3O7-δ Thin Films: Determination of Oxygen Content from x-Ray-Diffraction Patterns[END_REF][START_REF] Navacerrada | Critical Temperature Depression and Persistent Photoconductivity in Ion Irradiated YBa2Cu3O7-x Films and YBa2Cu3O7-x/PrBa2Cu 3O7 Superlattices[END_REF], and demonstrates that the structural coherence is not affected by oxygen depletion but diminishes as d increases (see Supplemental Material).

Photolithography was used to define a Hall bar for longitudinal 𝜌 𝑥𝑥 and Hall 𝜌 𝑥𝑦 resistivity measurements, carried out by injecting current and sensing voltages with a nanovoltmeter.

These experiments were performed in a closed-cycle refrigerator equipped with an electromagnet to apply a magnetic field 𝐵, and an optical window for illumination. Three characteristic [START_REF] Osquiguil | Controlled Preparation of Oxygen Deficient YBa2Cu 3Ox Films[END_REF] plateau for 6.4 < x < 6.6 and a fast drop for x < 6.4. Supplemental Fig. S5 further supports that our samples present the characteristics of high-quality oxygen-deficient YBCO [START_REF] Arpaia | Probing the Phase Diagram of Cuprates with YBa2Cu3 O7-δ Thin Films and Nanowires[END_REF][START_REF] Barisic | Universal Sheet Resistance and Revised Phase Diagram of the Cuprate High-Temperature Superconductors[END_REF]. Irradiated samples show a distinct behavior, with a rapid 𝑇 𝐶 decrease as 𝑑 is increased until an insulating state is obtained for 𝑑 = 2 10 14 ions cm -2 . Here superconductivity depression is not due to changes in the oxygen content, but to irradiation-induced structural disorder that enhances electronic scattering and pair-breaking [START_REF] Lesueur | Depairing-like Variation of Tc in YBa2Cu3O7-δ[END_REF]. We stress that no relationship exists between x in the oxygen-deficient series and 𝑑 in the irradiated one: data are displayed in the same graphs for compacity, and to illustrate that 𝜌 𝑥𝑥 and 𝑇 𝐶 span over the same range for both sets of samples, which calls for a comparison of their response to illumination. Notice also that the relative mobility variation ∆𝜇 𝐻 /µ 𝐻 (𝑝𝑟𝑖𝑠𝑡𝑖𝑛𝑒) ~ 30%-50 % is large (particularly in the low oxygen-content samples that show the largest PPC and PPS) despite a relatively modest ∆𝑛 𝐻 /𝑛 𝐻 (𝑝𝑟𝑖𝑠𝑡𝑖𝑛𝑒) < 5 %. Furthermore, and most strikingly, the very different relaxation of ∆𝑛 𝐻 and ∆𝜇 𝐻 univocally demonstrates that they stem from different microscopic mechanisms. This is consistent with the earlier observation that, during illumination, 𝜇 𝐻 and 𝑛 𝐻 evolve differently [START_REF] Stockinger | Mechanisms of Photodoping in Oxygen-Deficient Films Studied by in Situ Transport Measurements[END_REF][START_REF] Markowitsch | Persistent Photoconductivity in Oxygen-Deficient and Ion-Irradiated YBa 2 Cu 3 O x[END_REF]. It is therefore clear that the ∆𝑛 𝐻 and ∆𝜇 𝐻 are concomitant but essentially unrelated illumination effects.

Second, and crucially, our experiments show that the photoinduced ∆𝑇 𝐶 is correlated with ∆𝜇 𝐻 , but not with ∆𝑛 𝐻 Clear-cut evidence is provided by i) the oxygen-deficient samples, for which ∆𝑇 𝐶 and ∆𝜇 𝐻 are proportional and show the same dependence on the oxygen content and the illumination wavelength; and ii) the ion-irradiated samples in which, despite ∆𝑛 𝐻 being similar that in oxygen-deficient ones, no ∆𝜇 𝐻 and consequently no ∆𝑇 𝐶 are observed.

The above conclusions call for an interpretation in terms of two different microscopic mechanisms, the one leading to ∆𝑛 𝐻 and the other to ∆𝜇 𝐻 and ∆𝑇 𝐶 .

Regarding ∆𝜇 𝐻 , it is helpful to recall Anderson's theory [START_REF] Anderson | Hall Effect in the Two-Dimensional Luttinger Liquid[END_REF] that explains Hall effects in the cuprates [START_REF] Chien | Effect of Zn Impurities on the Normal-State Hall Angle in Single-Crystal YBa2Cu3-XZnxO7[END_REF] based on the relaxation rate of the quasiparticle-like excitations (spinons) in the interacting-electron (Luttinger) liquid. In this framework the Hall angle tan 𝜃 𝐻 ≡ 𝜌 𝑥𝑦 𝜌 𝑥𝑥 ⁄ =𝜇 𝐻 𝐵, reflects i) spinon-spinon scattering and ii) impurity (or defect) scattering [START_REF] Anderson | Hall Effect in the Two-Dimensional Luttinger Liquid[END_REF].

While (i) depends on the doping level, (ii) does not [START_REF] Anderson | Hall Effect in the Two-Dimensional Luttinger Liquid[END_REF][START_REF] Chien | Effect of Zn Impurities on the Normal-State Hall Angle in Single-Crystal YBa2Cu3-XZnxO7[END_REF]. If we also consider the fact that photodoping and mobility (Hall angle) variations are decorrelated, we conclude that the 𝜇 𝐻 enhancement reflects a decrease in the impurity scattering rate.

One needs to explain now at a microscopic level why illumination reduces the impurity scattering rate. If we recall the two scenarios debated, namely i) photoexcitation plus electron trapping and ii) photoinduced oxygen ordering within the CuO chains, we find that the latter naturally explains the decrease in the impurity scattering if we consider that oxygen ordering reduces the impurity potentials introduced by oxygen vacancies. Furthermore, this also allows understanding of the associated TC increase, as it is well-known that in d-wave superconductors lower impurity scattering implies weaker pair-breaking [START_REF] Tolpygo | Effect of Oxygen Defects on Transport Properties and: Displacement Energy for Plane and Chain Oxygen and Implications for Irradiation-Induced Resistivity and Suppression[END_REF] and thus higher TC. Within this scenario, the presence of PPS in oxygen-depleted samples and its absence in ion-irradiated ones can be understood as follows. In oxygen-depleted YBCO, vacancies (and the related disorder) are essentially found within the CuO chains [1]. Here, the barrier for ion displacement and more specifically for oxygen jump between chain fragments is relatively low (~1 eV) [2],

allowing for optical activation. However, in irradiated samples, vacancy-interstitial pairs are created for all atomic species. Furthermore, these defects, particularly oxygen ones (44% of the total according to SRIM simulations [START_REF] Biersack | The Stopping and Range of Ions in Solids[END_REF]), are not solely located in the CuO chains, but also in the CuO2 planes, where the activation energy for ion displacement is up to ten times higher [START_REF] Tolpygo | Effect of Oxygen Defects on Transport Properties and: Displacement Energy for Plane and Chain Oxygen and Implications for Irradiation-Induced Resistivity and Suppression[END_REF] than in the chains, thus precluding re-ordering by optical activation. Yet, defects in the CuO2 planes play the dominant role in the large depression of carrier mobility and Tc produced upon ion irradiation [START_REF] Lesueur | Depairing-like Variation of Tc in YBa2Cu3O7-δ[END_REF][START_REF] Tolpygo | Effect of Oxygen Defects on Transport Properties and: Displacement Energy for Plane and Chain Oxygen and Implications for Irradiation-Induced Resistivity and Suppression[END_REF]. In conclusion, the absence of mobility enhancement and PPS in ionirradiated samples results from the fact that illumination cannot "heal" the broader disorder that causes the superconductivity depression in them.

If oxygen ordering can explain the 𝜇 𝐻 and TC enhancements, it cannot account for the concomitant photodoping because it is uncorrelated to those effects, as demonstrated by all our experimental observations. However, photodoping can be understood independently, in terms of photoexcitation of electrons that are captured either by charged defects (such as oxygen vacancies) [START_REF] Stockinger | Mechanisms of Photodoping in Oxygen-Deficient Films Studied by in Situ Transport Measurements[END_REF][START_REF] Hasen | Enhancement of Persistent Photoconductivity in Insulating High-T c Thin Films[END_REF] or by unoccupied p-levels of the oxygen atoms at the CuO chains [START_REF] Kudinov | Persistent Photoconductivity in YBa2Cu3O6+x Films as a Method of Photodoping toward Metallic and Superconducting Phases[END_REF][START_REF] Kudinov | Mechanisms for the Persistent Photoconductivity of Oxygen Deficient YBa2Cu3O6+x[END_REF],

leaving excess holes in the CuO2 planes. The fact that the observed carrier density variation is similar in all samples indicates that the density of "electron traps" is always large enough so that, within the chosen illumination time, there is no saturation. Indeed, this experimental condition has allowed us to show that photodoping is not the main ingredient for PPS.

In summary, we have studied oxygen-deficient and ion-irradiated YBCO films and evidenced that, contrary to conventional belief, photodoping and PPS have no causal relationship. We find instead that PPS is observed only in YBCO films in which illumination produces a significant carrier mobility enhancement, and particularly that the size of PPS scales with the photoinduced mobility enhancement. This unearths an interesting facet of the link between scattering and pairing in the superconducting cuprates [START_REF] Taillefer | Scattering and Pairing in Cuprate Superconductors[END_REF], i.e. that it is controllable by light. These results call for examining the effect of VIS-UV light on other phenomena that emerge in the underdoped region of the cuprate's phase diagram. For example, charge density wave (CDW) order [START_REF] Ghiringhelli | Long-Range Incommensurate Charge Fluctuations in (Y,Nd)Ba2Cu3O(6+x)[END_REF][START_REF] Comin | Charge Order Driven by Fermi-Arc Instability in Bi2Sr 2-XlaxCuO6+δ[END_REF][START_REF] Da | Ubiquitous Interplay between Charge Ordering and High-Temperature Superconductivity in Cuprates[END_REF][START_REF] Tabis | Charge Order and Its Connection with Fermi-Liquid Charge Transport in a Pristine High-Tc Cuprate[END_REF] whose competition with superconductivity can be tipped by disorder and scattering [START_REF] Chang | Direct Observation of Competition between Superconductivity and Charge Density Wave Order in YBa2Cu3O6.67[END_REF][START_REF] Leroux | Disorder Raises the Critical Temperature of a Cuprate Superconductor[END_REF][START_REF] Choi | Spatially Inhomogeneous Competition between Superconductivity and the Charge Density Wave in YBa2Cu3O6.67[END_REF] as it has been shown in the case of external perturbations such as large magnetic fields [START_REF] Gerber | Three-Dimensional Charge Density Wave Order in YBa2Cu3O6.67 at High Magnetic Fields[END_REF] or strain [START_REF] Bluschke | Stabilization of Three-Dimensional Charge Order in YBa2Cu3O6+x via Epitaxial Growth[END_REF][START_REF] Kim | Uniaxial Pressure Control of Competing Orders in a High-Temperature Superconductor[END_REF]. These effects could be studied using techniques such as resonant inelastic X-ray Scattering [START_REF] Arpaia | Dynamical Charge Density Fluctuations Pervading the Phase Diagram of a Cu-Based High-Tc Superconductor[END_REF][START_REF] Wahlberg | Restored Strange Metal Phase through Suppression of Charge Density Waves in Underdoped YBa2Cu3O7-d[END_REF] after illumination. Another worthwhile avenue of research stemming from the present study is looking into the possible universality of the microscopic mechanisms underlying light effects, in systems showing similar quantum critical transport phenomena [START_REF] Takubo | Photoinduced Metalto-Insulator Transition in a Manganite Thin Film[END_REF][START_REF] Lin | Unexpected Intermediate State Photoinduced in the Metal-Insulator Transition of Submicrometer Phase-Separated Manganites[END_REF][START_REF] Li | Recovery of Photoexcited Magnetic Ordering in Sr2IrO4[END_REF][START_REF] Esposito | Dynamics of the Photoinduced Insulator-to-Metal Transition in a Nickelate Film[END_REF][START_REF] Liao | Metal-Insulator-Transition Engineering by Modulation Tilt-Control in Perovskite Nickelates for Room Temperature Optical Switching[END_REF][START_REF] Lin | Control of the Metal-Insulator Transition at Complex Oxide Heterointerfaces through Visible Light[END_REF]. ). The YBCO (005) peak centered around 2𝜃 ≈ 38 ° is shown in the inset to highlight the effect of both oxygen deficiency and ion irradiation on the YBCO structural properties. The spectra have been vertically shifted for clarity. (c) c-axis parameter as a function of the nominal oxygen content (red) and ion irradiation dose (blue). Calculated from the XRD spectra using Bragg law 2𝑑 ℎ𝑘𝑙 𝑠𝑖𝑛𝜃 = 𝑛𝜆 and assuming orthorhombic structure. As expected, the 𝑐 axis parameter increases as the oxygen content 𝑥 decreases [1] and also as the irradiation dose increases [2]. The straight line is the best fit for the oxygen-depleted series, which yields 𝑐 = 𝐴 + 𝐵𝑥, with 𝐴=13.1±0.4 Å and B=-0.20±0.1 Å. These parameters are comparable to those found in the literature 𝐴 = 12.85 ± 0.4 Å and B = -0.171, see e.g. [3]. From the data scatering around the linear regression, we can estimate the uncertainty of the nominal oxygen content 𝑥~0.1 for the samples where photoinduced effects are studied 6.4 ≤ 𝑥 ≤ 6.6. (d) Structural correlation length along the c-axis as obtained from the Scherrer formula 𝜉 ≈ 0.9𝜆 Γ 𝑐𝑜𝑠 𝜃 ⁄ , with 𝜃 the diffraction angle, 𝜆~1.54 Å the X-ray wavelength, and Γ the 005 peak full-width at half-maximum. In the case of oxygen-depleted samples (red), the structural coherence is independent of the oxygen content and is limited by the sample thickness ~ 30 nm. On the contrary, for ion-irradiated samples (blue), the structural coherence is shortened below the sample thickness (~50 nm) as the dose is increased. , with 𝑇 𝐶 𝑚𝑎𝑥 the critical temperature for optimum doping. The hollow data points correspond to samples from this work, and the solid data points are taken from the literature [4] . Lines are a guide to the eye. We can see that, despite somewhat lower T**, the studied YBCO films (hollow symbols) present a phase diagram similar that found in the literature (solid symbols).

(

  Despite much work, a clear picture of the microscopic mechanisms of PPS has not emerged. Because PPC and PPS are accompanied by an increase in the Hall number 𝑛 𝐻 ≡

  Fig. 1 summarizes the native properties of the films. (a) and (b) respectively show

Fig. 2

 2 Fig. 2 summarizes the illumination experiments, carried out always with the same

Figure 3

 3 Figure3shows the typical temperature-dependent relaxation of the photo-induced

Figure 1 :

 1 Figure 1: Temperature-dependent resistivity for (a) oxygen-deficient (6.3 < 𝑥 < 6.9) and (b) ion-irradiated (3 10 13 < 𝑑 < 2 10 14 ion cm -2 ) samples. (c) 𝑇 𝐶 and (d) 𝜌 𝑥𝑥 (95 𝐾) vs. oxygen content x (red data) and irradiation dose (blue data).

Figure 2 :

 2 Figure 2: (a) Temperature-dependent resistivity before and after illumination, for an oxygendeficient film (𝑥 = 6.4). The inset shows the corresponding Hall resistance at 𝑇 = 95𝐾. (b) Photoinduced changes ∆𝑇 𝑐 (c) Hall number ∆𝑛 𝐻 and (d) Hall mobility ∆𝜇 𝐻 as a function of the nominal oxygen content (lower axis, red) and irradiation dose (upper x-axis), for different wavelengths (see legend). The irradiated set includes sputtering and PLD-grown samples (see legend), both types of samples behaving similarly. The insets in (c) and (d) show the ∆𝑇 𝑐 vs.∆𝑛 𝐻 and ∆𝑇 𝑐 vs. ∆𝜇 𝐻 respectively. Notice that the latter shows a strong correlation. All the lines are guides to the eye.

Figure 3 :

 3 Figure 3: Time-dependent Hall number (a) and Hall mobility (b) measured in the dark at different temperatures (see legend), following illumination at T = 95K with 𝜆 = 565𝑛𝑚 and 𝑃 = 100 𝑚𝑊. 𝑐𝑚 -2 (for a sample 𝑥=6.4). The reference levels on the right were measured before illumination. The double-sided arrows highlight the remnant enhancement at t=200 min. The inset in (a) shows the time-dependent relative variation of the carrier density ∆𝑛/∆𝑛 0 where ∆𝑛 0 = 𝑛 𝑡=0 -𝑛 𝑝𝑟𝑖𝑠𝑡𝑖𝑛𝑒 .

Fig. S1

 S1 Fig. S1 TC (up) and Hall mobility (down) vs-Hall number for the two sets of samples. Grey critical temperature labels indicate upper limits of the measurements. Nonsuperconducting samples are not discussed in the main text.

Fig. S3

 S3 Fig. S3 Light-induced variations of the critical temperature ∆𝑇 𝑐 as a function of the photoinduced variations of the Hall number (a) and the Hall mobility (b) for the two sets of samples and under all types of illumination. The photoinduced changes in Tc do not correlate with the changes in the Hall number, but scale with the changes in mobility for all the experimental configurations considered. The red line is a guide to the eye.

Fig. S4 :

 S4 Fig. S4: Time-dependent Hall number (a) and Hall mobility (b) measured in the dark at different temperatures (see legend), following illumination at T = 95K with 𝜆 = 565𝑛𝑚 and 𝑃 = 100 𝑚𝑊. 𝑐𝑚 -2 (for a sample with 𝑥=6.4). The reference levels on the right were measured before illumination. The double-sided arrows highlight the remnant enhancement at t=200 min. These measurements demonstrate the reproducibility of the results reported in Fig.3(main text), which were obtained with another sample of the same nominal oxygen content.

Figure S5 :

 S5 Figure S5: (a) R(T) of an oxygen-deficient sample (𝑥=6.6). The TC is taken as the temperature across the transition in which the derivative peaks (lower inset). The R(T) shows the characteristic crossover from the high-temperature linear dependence (strange metal phase) to the quadratic dependence of the pseudogap phase. The departure from linear behaviour occurs at T* (upper inset). (b) Resistance vs. T 2 for the same sample. The crossover into the pseudogap phase leads to a range where the temperature dependence is quadratic, between an upper bond T** and a lower one T' which is usually attributed to the onset of superconducting fluctuations [4] . T** and T' are determined from the derivative dR/dT 2 as shown in the inset. (c) Phase diagram showing the characteristic temperatures as a function of the hole doping p per Cu. The hole doping 𝑝 is estimated from the usual [4] phenomenological relationship 1 -𝑇 𝐶 𝑇 𝐶 𝑚𝑎𝑥 = 82.6(𝑝 -0.16) 2 ⁄, with 𝑇 𝐶 𝑚𝑎𝑥 the critical temperature for optimum doping. The

  1 , D. Sánchez-Manzano 1 , V. Humbert 1 , S. Carreira 1 , V. Rouco 1 , A Sander 1 , F. Cuellar 2 , K. Seurre 1 , A. Lagarrigue 1 , J. Briatico 1 , J. Trastoy 1 , J. Santamaría 2 and Javier E.

	Villegas 1,*
	1 Unité Mixte de Physique, CNRS, Thales, Université Paris Saclay, 91767 Palaiseau, France
	2 GFMC, Dpto. Física de Materiales. Universidad de Ciencias Físicas, Universidad
	Complutense de Madrid, 28040 Madrid, Spain

Acknowledgments

Work supported by ERC grant N° 647100 "SUSPINTRONICS", French ANR grant ANR-17-CE30-0018-04 "OPTOFLUXONICS", COST action "Nanocohybri", Spanish AEI grants PID2020-118078RB-I00 and PCI2020-112093, and Madrid Regional Government grant Y2020/NMT-6661. JS thanks a Scholarship program Alembert funded by the IDEX Paris-Saclay, ANR-11-IDEX-0003-02. We (JS, J-E V) acknowledge funding from Flag ERA ERA-NET To2Dox project. We thank Y. Legall for ion irradiation at ICube (Strasbourg).


Aurélien Lagarrigue

Carolina De Dios

Salvatore Mesoraca

Santiago Carreira

Vincent Humbert

Javier Briatico

Juan Trastoy

Javier E Villegas
email: javier.villegas@cnrs-thales.fr

Memristive effects in YBa2Cu3O7-x devices with transistor-like structure

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Introduction

Memristors, named and defined in 1971 by Leon Chua as resistors whose properties depend on their electrical history [START_REF] Chua | Memristor-The Missing Circuit Element[END_REF], are multi-state memories [START_REF] Ho | Nonvolatile memristor memory: Device characteristics and design implications[END_REF][START_REF] Parajuli | Nonvolatile Memory Cell Based on Memristor Univers[END_REF] that play a prominent role in the development of novel neuromorphic computing [START_REF] Zhang | Organismic materials for beyond von Neumann machines[END_REF], logic operations [START_REF] Gao | Memristor-Based Logic Gate Circuit[END_REF] as well as signal processing circuits [START_REF] Zhong | A memristor-based analogue reservoir computing system for real-time and power-efficient signal processing[END_REF][START_REF] Mouttet | Proposal for memristors in signal processing[END_REF]. The realization of memristors based on superconducting materials is interesting for various reasons. On the one hand, the development of non-volatile memory in superconducting circuits attracts continued interest [8] as it constitutes a missing, gamechanging ingredient in the field of conventional superconducting electronics [START_REF] Anders | European roadmap on superconductive electronics status and perspectives[END_REF][START_REF] Koelle | High-transition-temperature superconducting quantum interference devices[END_REF][START_REF] Cybart | Nano Josephson superconducting tunnel junctions in YBa 2 Cu 3 O 7 -δ directly patterned with a focused helium ion beam[END_REF][START_REF] Bauch | Quantum dynamics of a d-wave Josephson junction[END_REF][START_REF] Ortlepp | Flip-flopping fractional flux quanta[END_REF][START_REF] Ouanani | High-Tc superconducting quantum interference filters (SQIFs) made by ion irradiation[END_REF]. On the other hand, the implementation of memristive effects in superconducting devices is arguably pivotal for the nascent field of superconducting neuromorphics [START_REF] Shainline | Superconducting Optoelectronic Circuits for Neuromorphic Computing[END_REF][START_REF] Cheng | Superconducting neuromorphic computing using quantum phase-slip junctions[END_REF][START_REF] Zhang | Quantum nickelate platform for future multidisciplinary research[END_REF][START_REF] Schneider | SuperMind: A survey of the potential of superconducting electronics for neuromorphic computing[END_REF], and will expectedly open new avenues in other areas such as quantum computing [START_REF] Salmilehto | Quantum memristors with superconducting circuits[END_REF].

Among the superconducting materials in which memory effects could be sought, highcritical temperature (𝑇 𝐶 ) cuprates [START_REF] Bednorz | Possible high Tc superconductivity in the Ba-La-Cu-O system Zeitschrift für[END_REF][START_REF] Proust | The remarkable underlying ground states of cuprate superconductors[END_REF] are especially appealing. This is, first and most obviously, because devices based on them can operate at temperatures in the tens of K range, dramatically reducing the burden of cryogenics as compared to low-𝑇 𝐶 superconductors. This has indeed fostered a variety of electronic applications and technologies [START_REF] Anders | European roadmap on superconductive electronics status and perspectives[END_REF][START_REF] Koelle | High-transition-temperature superconducting quantum interference devices[END_REF][START_REF] Cybart | Nano Josephson superconducting tunnel junctions in YBa 2 Cu 3 O 7 -δ directly patterned with a focused helium ion beam[END_REF][START_REF] Bauch | Quantum dynamics of a d-wave Josephson junction[END_REF][START_REF] Ortlepp | Flip-flopping fractional flux quanta[END_REF][START_REF] Ouanani | High-Tc superconducting quantum interference filters (SQIFs) made by ion irradiation[END_REF][START_REF] Bai | A compact high temperature superconducting bandpass filter for integration with a Josephson mixer[END_REF][START_REF] Faley | High-Tc DC SQUIDs for magnetoencephalography[END_REF]. Second, and importantly, cuprates belong to the vast family of strongly-correlated functional oxides [START_REF] Bibes | Ultrathin oxide films and interfaces for electronics and spintronics[END_REF] in which the presence of oxygen vacancies and disorder dramatically changes the ground state.

Beyond its fundamental interest, this characteristic offers possibilities for engineering interesting functionalities, as we illustrate with the current experiments.

While the microscopic mechanism for high-𝑇 𝐶 superconductivity in the cuprates remains a long-standing problem, it is well established that the CuO2 planes characteristic of their crystal structure play a significant role. Indeed, the superconducting properties are strongly sensitive to the presence of disorder as well as the oxygen stoichiometry in such CuO2 planes [START_REF] Matsunaka | Effects of oxygen-vacancies in hightemperature cuprate superconductors[END_REF][START_REF] Lee | Doping a Mott insulator: Physics of hightemperature superconductivity[END_REF][START_REF] Chu | Hole-doped cuprate high temperature superconductors[END_REF]. For example, the oxygen stoichiometry controls the doping level, leading to a complex temperature-doping phase diagram which includes the superconducting and insulator phases, as well as other exotic ones, such as the pseudo-gap [START_REF] Timusk | The pseudogap in high-temperature superconductors: An experimental survey Reports[END_REF] or the strange metal phase [START_REF] Varma | Colloquium: Linear in temperature resistivity and associated mysteries including high temperature superconductivity[END_REF]. An interesting consequence of the intricate phase diagram and its connection to oxygen content and disorder is the possibility of tuning the ground state through the controlled migration of ions. Such control can be achieved electrically, via the application of currents or voltages. One of the possible oxygen-control approaches consists of circulating large current densities, as reported in thin films and nanoconstrictions [START_REF] Vitta | Electromigration failure in YBa2Cu3O7-x thin films[END_REF][START_REF] Moeckly | Electromigration study of oxygen disorder and grain-boundary effects in YBa2Cu3O7-thin films[END_REF][START_REF] Moeckly | Micro-Raman spectroscopy of electromigration-induced oxygen vacancy aggregation in YBa2Cu3O7-δ[END_REF][START_REF] Baumans | Electromigration in the dissipative state of hightemperature superconducting bridges[END_REF][START_REF] Marinković | Direct Visualization of Current-Stimulated Oxygen Migration in YBa2Cu3O7-δThin Films[END_REF][START_REF] Trabaldo | Mapping the Phase Diagram of a YBa2Cu3 O7-δ Nanowire Through Electromigration[END_REF]. It was shown in these experiments that a sufficiently large current (typically above 7 𝑀𝐴 • 𝑐𝑚 -2 ) [START_REF] Baumans | Electromigration in the dissipative state of hightemperature superconducting bridges[END_REF][START_REF] Marinković | Direct Visualization of Current-Stimulated Oxygen Migration in YBa2Cu3O7-δThin Films[END_REF][START_REF] Trabaldo | Mapping the Phase Diagram of a YBa2Cu3 O7-δ Nanowire Through Electromigration[END_REF][START_REF] Trabaldo | Electromigration tuning of the voltage modulation depth in YBa2Cu3O7-δnanowire-based SQUIDs[END_REF] can produce direct electromigration and, under certain conditions, the local heating produced by lower current densities can lead to the thermomigration of oxygen ions [START_REF] Marinković | Direct Visualization of Current-Stimulated Oxygen Migration in YBa2Cu3O7-δThin Films[END_REF][START_REF] Marinković | Oxygen ordering in untwinned YBa2Cu3 O7-δ films driven by electrothermal stress[END_REF]. Both current-driven mechanisms can change the cuprate's doping level and physical properties at a local scale [START_REF] Strauven | Oxygen evolution from YBa2Cu3O6.85 high Tc superconductors[END_REF][START_REF] Bruynseraede | Oxygen Disorder Effects in High Tc Superconductors[END_REF]. A different approach for electrically controlling a cuprate's ground state consists of placing a low-reduction-potential material in contact with YBCO to exploit the redox reaction that occurs at the interface between both materials. The oxygen exchange with the second material can be tuned by applying voltage pulses of different polarity and amplitude, which drive oxygen in and out of the cuprate at will, thus allowing for a reversible switching of the transport and superconducting properties [START_REF] Leng | Electrostatic control of the evolution from a superconducting phase to an insulating phase in ultrathin YBa2Cu3O7-x films[END_REF][START_REF] Palau | Electrochemical Tuning of Metal Insulator Transition and Nonvolatile Resistive Switching in Superconducting Films[END_REF][START_REF] Murray | Interfacial-Redox-Induced Tuning of Superconductivity in YBa2Cu3O7-Î&acute[END_REF][START_REF] Rouco | Quasiparticle tunnel electroresistance in superconducting junctions[END_REF].

In this work, we study devices with a structure analogous to that of a transistor, in which the channel is made of the archetypal high-critical temperature superconductor YBa2Cu3O7- (YBCO) and the gate is made of Al (see Fig. 1). Interestingly, we show that both redox and current-induced mechanisms play a role in these devices. On the one hand, the redox mechanism allows for reversibly switching the gate conductance via changes in the oxidation state at the Al/YBCO interface. In particular, the gate conductance shows a memristive behaviour: it can be switched across a continuum of non-volatile levels depending on the history of applied gate pulses. Those levels are intermediate between a high-conductance (hereafter ON) state and a low-conductance (OFF) state, the ratio between the ON/OFF conductance being as high as 10000 %. This memristive behaviour is observed within a range of low to moderate gate voltages (depending on the device, this range can be as high as ~15 𝑉). On the other hand, the gate voltage also produces a non-reversible tuning of the YBCO channel's ground state, namely a superconductor-to-insulator transition, which is caused by ion migration within the bulk of the YBCO film that changes the source-drain resistance. This effect becomes stronger at higher gate voltages, for which the electrical power dissipated in the channel is significantly higher and introduces bulk disorder via thermal effects (structural damage caused by local heating, likely ion thermomigration). The obtained results and understanding pave the way for devices in which the dominance and/or interplay of redox and dissipation effects can be controlled to optimize specific applications -for example superconducting memristive or current-limiting elements in superconducting circuits-and contribute to enlarging the technological prospects of cuprate superconductors.

Method

The structure of the studied devices is shown schematically in Fig. 1(c), and presents 3 terminals, named by analogy with a conventional transistor: gate (G), drain (D) and source (S).

The device fabrication is based on commercial (Ceraco ceramic coating GmbH) 150 𝑛𝑚-thick c-axis YBCO films grown on r-cut sapphire substrates, with a buffer layer of cerium oxide (CeO2) and capped in situ with 100 𝑛𝑚 of gold. Those films were patterned using various steps of optical lithography and Ar ion-beam etching. The first step is to define the outline of the devices, in which the YBCO/Au bilayer is etched down to the CeO2 layer [Fig. 1(a)]. Then, the thickness of the superconducting channel 𝑡 𝑌𝐵𝐶𝑂 was defined by local etching [Fig. 1(b)]. This process was monitored using a secondary ion mass spectrometer (SIMS). The sample topography was subsequently studied with atomic force microscopy (AFM), which allowed us to precisely determine the YBCO thicknesses 𝑡 𝑌𝐵𝐶𝑂 . We fabricated different samples with 35 nm < 𝑡 𝑌𝐵𝐶𝑂 < 140 nm. Reducing 𝑡 𝑌𝐵𝐶𝑂 from 150 𝑛𝑚 to 140 𝑛𝑚 yields 𝑇 𝐶 ≈ 86 𝐾. Further thinning to 𝑡 𝑌𝐵𝐶𝑂 = 35 𝑛𝑚 lowers the 𝑇 𝐶 to ~72 𝐾, which is likely due to moderate oxygen loss caused by heating during the ion etching. Subsequent optical lithography, plasma cleaning, sputtering deposition and lift-off steps allow defining the gate (denoted G), made of aluminium (𝑡 𝐴𝑙 = 2 𝑛𝑚 or 𝑡 𝐴𝑙 = 10 𝑛𝑚) covered by 100 𝑛𝑚 of gold [Fig. 1(c)]. The gate width 𝑊 𝐴𝑙 was either 60 m or 5 m, depending on the device, and thus the interface's area was either 60 × 100 µ𝑚 2 or 5 × 100 µ𝑚 2 . In addition to the main devices, in which Al is in direct contact with YBCO under the gate, we fabricated two control samples whose gates are made solely of Au.

To electrically characterize the devices, we measured the gate-drain conductance as well as probed the source-drain channel resistance after applying gate voltages, always in a twoprobe configuration. For the former type of measurement, the gate-drain was voltage-biased and the current was probed using a Keithley 2450 source-meter [upper sketch in Fig. 1(d)]. For the latter, a Keysight B2901B precision current source and a Keithley 2182A nanovoltmeter were used for the current biasing and voltage measurements, respectively [lower sketch in Fig. 1(d)].

Results

Gate-drain measurements

A first set of measurements demonstrating the switching behaviour of the gate is shown in Fig. 2. In these measurements, the gate voltage 𝑉 𝐺 is ramped from zero to a final value 𝑉 𝑤𝑟𝑖𝑡𝑒 (which can be positive or negative) and then back to zero in a fixed time (typically 20 seconds) using a large number of steps (typically 100), as illustrated by the inset of Fig. 2(a). The gatedrain current is measured during the 𝑉 𝐺 sweep to obtain 𝐼 𝐺 (𝑉 𝐺 ) characteristics as those displayed in the main panel of Fig. 2(a). Typically, a data set contains 𝐼 𝐺 (𝑉 𝐺 ) curves for forty or eighty different 𝑉 𝑤𝑟𝑖𝑡𝑒 , although the figure only displays a selected subset for clarity. Notice that 𝑉 𝑤𝑟𝑖𝑡𝑒 is varied in a particular chronological sequence, which is indicated in the colour legend (top to bottom). Direct inspection of Fig. 2(a) reveals a jump between a low-conducting (OFF) state to a high-conducting (ON) one as 𝑉 𝑤𝑟𝑖𝑡𝑒 is increased above a positive threshold (~4.5 -5 V), and the opposite (yet more gradual) switching (ON to OFF) as 𝑉 𝑤𝑟𝑖𝑡𝑒 is decreased below ~ -1.5 V. To better describe that behaviour, we have selected the 𝐼 𝐺 (𝑉 𝐺 ) curve set in the six different panels of Fig. 2 First, the conductance level is overall three to five orders of magnitude higher for Au than for Al. Second, for Au, the conductance switching is extremely small -only ~10 % as opposed to ~10000 %. This suggests that a material with a tendency to oxidize (such as Al) is required to produce large conductance-switching effects. Second, with the Au gate the 𝐺(𝑉 𝑤𝑟𝑖𝑡𝑒 ) cycle is fully symmetric: |𝑉 𝑃𝑂𝐿 (+)| = |𝑉 𝑃𝑂𝐿 (-)|. Indeed, as it will be discussed later, the asymmetry of the conductance cycles characteristic of the devices with Al gate is a fingerprint of the redox reaction underlying the large resistance switching [START_REF] Rouco | Quasiparticle tunnel electroresistance in superconducting junctions[END_REF].

Source-drain measurements

We measured the source-drain conductance after applying 𝑉 𝑤𝑟𝑖𝑡𝑒 to determine its impact on the YBCO channel. An important key to understanding the irreversible channel's resistance variation across the sequence of 𝑉 𝑤𝑟𝑖𝑡𝑒 is given in Fig. 5(c). This figure displays (red curve) the maximum power dissipated in the device 𝑃 𝑚𝑎𝑥 = 𝑉 𝑤𝑟𝑖𝑡𝑒 • 𝐼(𝑉 𝑤𝑟𝑖𝑡𝑒 ) as the gate voltage is cycled to 𝑉 𝑤𝑟𝑖𝑡𝑒 . Direct comparison between the red curve in Fig. 5(c) and the black one in Fig. 5(b) demonstrates that the channel's resistance 𝑅 𝑆𝐷 is a step function of the dissipated power. Interestingly, the threshold power 𝑃 𝑡ℎ at which the channel's resistance increase is triggered is not constant, but rapidly increases during the first 𝑉 𝑤𝑟𝑖𝑡𝑒 cycles and then tends to saturate, as shown by the blue curve in Fig. 5(c). This means that the process that leads to the suppression of the channel's conducting (and superconducting) properties is self-limiting: once a given power threshold triggers the increase of the channel's resistance, this will not further change unless a higher power is applied. Interestingly, this allows reaching a regime, after a few 𝑉 𝑤𝑟𝑖𝑡𝑒 cycles (training), in which the memristive behaviour of the gate [green curve in Fig. 5(b)] can be exploited without inducing further changes in the channel. Conversely, if desired, the channel can be driven to a lower conductance state by increasing 𝑉 𝑤𝑟𝑖𝑡𝑒 , while the memristive behaviour of the gate is preserved. As demonstrated in Fig. 4(c) and further discussed below, the onset of the different regimes can be engineered by changing the thickness of the YBCO channel.

Discussion

The reversible switching of the conductance between ON, OFF and intermediate non-

volatile states [Fig. 3] can be explained by an interfacial redox reaction between aluminium and YBCO, similarly as for the memristive effects observed in YBCO/MoSi [START_REF] Rouco | Quasiparticle tunnel electroresistance in superconducting junctions[END_REF] and NdNiO3/MoSi [START_REF] Humbert | An Oxygen Vacancy Memristor Ruled by Electron Correlations[END_REF] micro junctions. The redox reaction naturally reduces the material with a higher reduction potential (here YBCO, given the high reduction potential of copper 𝐸 °(𝐶𝑢) = 2.4 𝑒𝑉) and oxidizes the material with a lower reduction potential (𝐸 °(𝐴𝑙) = -1.676 𝑒𝑉) [START_REF]CRC Handbook of Chemistry and[END_REF]. As a result, a 2-3 𝑛𝑚 thick oxygen-depleted YBCO layer is expected at one side of the interface [START_REF] Rouco | Quasiparticle tunnel electroresistance in superconducting junctions[END_REF], and conversely, an AlOx layer will form at the other side. Because both oxygen-depleted YBCO and AlOx are insulating [START_REF] Khoobiar | The electronic properties of aluminum oxide and the chemisorption of water, hydrogen, and oxygen[END_REF][START_REF] Abyzov | Aluminum Oxide and Alumina Ceramics (review). Part 1. Properties of Al2O3 and Commercial Production of Dispersed Al2O3[END_REF][START_REF] Lee | Doping a Mott insulator: Physics of hightemperature superconductivity[END_REF], a tunnel barrier forms at the interface, which strongly diminishes its electrical conductance. This explains the low-conductance OFF state. That redox reaction can be reversed by the application of a 𝑉 𝑤𝑟𝑖𝑡𝑒 that exceeds the difference between the reduction potentials, very much as a battery can be charged, leading to oxygen transfer from AlOx into YBCO, and thus to a thinning of the oxygen-depleted YBCO and AlOx tunnelling barrier. This explains the eventual switching into the highest-conductance ON state upon application of a sufficiently high positive 𝑉 𝑤𝑟𝑖𝑡𝑒 . This redox scenario is supported by various key observations. As shown by the examples displayed in Fig. 3(a), the conductance shows a non-ohmic behaviour similar to that observed earlier in YBCO/MoSi junctions [START_REF] Rouco | Quasiparticle tunnel electroresistance in superconducting junctions[END_REF], which is consistent with electron tunnelling across the Al/YBCO interface. The experimental values of 𝑉 𝑃𝑂𝐿 are consistent with the expectations [START_REF] Rouco | Quasiparticle tunnel electroresistance in superconducting junctions[END_REF][START_REF] Humbert | An Oxygen Vacancy Memristor Ruled by Electron Correlations[END_REF]. In particular, we find that the voltage 𝑉 𝑃𝑂𝐿 (+)

required for switching into the ON state is around 4.7 𝑉, which is above the minimum ∆𝐸 = 𝐸 °(𝐶𝑢) -𝐸 °(𝐴𝑙) = 2.4 -(-1.676) = 4.076 𝑒𝑉 [START_REF]CRC Handbook of Chemistry and[END_REF] expected from electrochemical arguments. The excess voltage of nearly ~0.7 𝑉 is required to overcome the barrier for oxygen ion diffusion, that is, to accelerate the dynamics of the electrochemical reaction. This is consistent with 𝑉 𝑃𝑂𝐿 (-) ≈ -0.7𝑉 required to switch back into the OFF state, which is spontaneous from the electrochemical point of view and thus only requires activation over the barrier for oxygen diffusion. Therefore, the redox scenario naturally explains the asymmetry in the switching loop of Fig. 3(b). This scenario is also supported by the behaviour of the control samples whose gate is made of Au instead of Al. These show three to four orders-of-magnitude larger conductance, and hundredfold weaker conductance-switching effects. That is as expected from the redox scenario, which cannot occur at the interface between YBCO and a noble metal.

The small conductance switching observed in Fig. 3(c) for Au can be explained by changes in the distribution of the fewer, native oxygen vacancies in YBCO, which are attracted towards/repelled from the interface under the influence of the local electric field induced by 𝑉 𝑤𝑟𝑖𝑡𝑒 [START_REF] Rouco | Quasiparticle tunnel electroresistance in superconducting junctions[END_REF]. This is consistent with the fact that, in the case of the Au gate, the switching loop [Fig. 3(c)] is fully hysteretic with |𝑉 𝑃𝑂𝐿 (+)| = |𝑉 𝑃𝑂𝐿 (-)| ≈ 0.7 𝑉. As discussed above, this is identical to the voltage required to switch from the ON to OFF state in the samples with the Al gate, which further substantiates that 0.7 V is the voltage necessary to overcome the barrier for oxygen ion diffusion.

Because the changes in the YBCO channel resistance are irreversible, similar in devices with Al and Au gates [Fig. 4 and Fig. 5], and ultimately triggered by the dissipated power [Fig.

5(c)],

we conclude that they are not caused by the interfacial redox reaction. Having ruled out this mechanism, we turn to current-induced effects. The normal-state resistivity increase and 𝑇 𝐶 depression indicates either that the oxygen content of the channel is reduced (underdoping) [START_REF] Trabaldo | Mapping the Phase Diagram of a YBa2Cu3 O7-δ Nanowire Through Electromigration[END_REF][START_REF] Xi | Effects of field-induced hole-density modulation on normal-state and superconducting transport in YBa2Cu3O7-x[END_REF], that disorder is being introduced [START_REF] Swiecicki | Strong field-matching effects in superconducting YBa 2Cu 3O 7-δ films with vortex energy landscapes engineered via masked ion irradiation[END_REF], or both. The current densities circulating across the device upon application of 𝑉 𝑤𝑟𝑖𝑡𝑒 range between 1 𝑘𝐴 • 𝑐𝑚 -2 and 10 𝑘𝐴 • 𝑐𝑚 -2 , based on the measured currents and considering the section of the YBCO channel. However, these current densities are well below the value typically required to produce electromigration in YBCO (~7 𝑀𝐴 • 𝑐𝑚 -2 ) [START_REF] Baumans | Electromigration in the dissipative state of hightemperature superconducting bridges[END_REF][START_REF] Marinković | Direct Visualization of Current-Stimulated Oxygen Migration in YBa2Cu3O7-δThin Films[END_REF][START_REF] Trabaldo | Mapping the Phase Diagram of a YBa2Cu3 O7-δ Nanowire Through Electromigration[END_REF][START_REF] Trabaldo | Electromigration tuning of the voltage modulation depth in YBa2Cu3O7-δnanowire-based SQUIDs[END_REF]. Consequently, it is unlikely that oxygen displacement due to electron collisions may explain the resistance increase and 𝑇 𝐶 depression observed in Fig. 4.

Furthermore, electromigration is expectedly reversible [START_REF] Trabaldo | Mapping the Phase Diagram of a YBa2Cu3 O7-δ Nanowire Through Electromigration[END_REF], meaning that reversing the polarity of the current should allow moving oxygen back and recovering both 𝑇 𝐶 and resistivity. This is not observed. Altogether, the experimental observations rule out that electrochemistry and electromigration may contribute significantly to the changes in the channel's conducting and superconducting properties. Instead, the fact that the resistance enhancement is triggered by power dissipation [Fig. 5(c)] suggests that the observed effects are linked to Joule heating. A related mechanism is thermomigration [START_REF] Oriani | Thermomigration in solid metals[END_REF]: the heating produced by local dissipation creates a temperature gradient across the device, which leads to migration of the most mobile ions. This mechanism has been discussed earlier in experiments with YBCO nanowires [START_REF] Marinković | Direct Visualization of Current-Stimulated Oxygen Migration in YBa2Cu3O7-δThin Films[END_REF][START_REF] Marinković | Oxygen ordering in untwinned YBa2Cu3 O7-δ films driven by electrothermal stress[END_REF]. In these, thermomigration of the highly mobile oxygen O -2 in the CuO2 planes leads to underdoping. In our experiments, this power dissipation seems to be producing an inhomogeneous effect.

Should thermomigration lead to homogeneous underdoping of the YBCO channel in our devices, we would observe a depression of 𝑇 𝐶 with no significant widening of the transition, as well as a characteristic departure from the linear temperature dependence of the normal-state resistivity [START_REF] Trabaldo | Mapping the Phase Diagram of a YBa2Cu3 O7-δ Nanowire Through Electromigration[END_REF][START_REF] Arpaia | Probing the phase diagram of cuprates with YBa2Cu3 O7-δ thin films and nanowires[END_REF]. In contrast, Fig. 4(a) and (b) show a widening of the superconducting transition with an onset temperature that remains essentially unchanged (86 𝐾 -88 𝐾) for increasing 𝑉 𝑤𝑟𝑖𝑡𝑒 . Furthermore, despite the strong overall enhancement of the normal-state resistivity, its linear temperature dependence is preserved for all 𝑉 𝑤𝑟𝑖𝑡𝑒 , except the highest for which the channel becomes insulating (figure not shown). All of that indicates that in our experiments power dissipation is not producing a homogeneous underdoping of the YBCO channel. Instead, the experimental observations consistently point to inhomogeneous changes within the channel, and to a dominant role of heat-induced local structural disorder which increses resistivity and depresses 𝑇 𝐶 due to pair-breaking by enhanced eletronic scattering, to which d-wave superconductors are extremely sensitive [START_REF] Swiecicki | Strong field-matching effects in superconducting YBa 2Cu 3O 7-δ films with vortex energy landscapes engineered via masked ion irradiation[END_REF][START_REF] Lesueur | Depairing-like variation of Tc in YBa2Cu3O7-δ[END_REF]. In particular, a consistent picture sketched in Fig. 1(d) is that bulk structural disorder is created locally under the gate, where YBCO is thinner, over a volume that extends deeper after each writing cycle. This explains the observation of an almost constant superconduting transition temperature onset (that corresponds to the YBCO volume further from the gate) despite the large resistance increase (created by the the growing volume of disordered YBCO) as well as to the large widening of the superonducting transition.

Conclusions

We have realized three-terminal superconducting devices with a transistor-like structure based on YBCO and Al. The Al/YBCO gate interface shows memristive behaviour: a large, reversible switching of the conductance across a continuum of non-volatile states driven by the application of gate voltages, which is produced by a reversible redox reaction. On the other hand, the channel's resistance and superconducting 𝑇 𝐶 are modified by the power dissipated in the device, which depresses the superconducting properties due to induced disorder.

Interestingly, the latter process is self-limiting and, after a few training cycles, allows exploiting the memristive function without further modification of the channel's properties. Conversely, by increasing the dissipated power, one can tune the properties of the channel without affecting the memristive functions. The latter could be exploited, for example, for current-limiting functions intended at protecting superconducting circuitry, or for detection of (cumulative) power dissipation. Those functions could be applied in areas like superconducting signal processing [START_REF] Zhong | A memristor-based analogue reservoir computing system for real-time and power-efficient signal processing[END_REF][START_REF] Mouttet | Proposal for memristors in signal processing[END_REF], where memory and switching functions are required but hard to implement [8], or in the nascent area of neuromorphic computing with superconductors [START_REF] Schneider | SuperMind: A survey of the potential of superconducting electronics for neuromorphic computing[END_REF]. The technological prospects in these areas are enhanced by the fact that the present demonstration is based on high-temperature superconductors. Furthermore, we believe that the identification of the different involved mechanisms we provide will be beneficial for the design of other cuprate-based (or more generally, complex-oxide-based) electronic devices subject to moderate to high voltages and power dissipation. Finally, it is worth mentioning that the observation of large redox-induced tunnel-conductance switching was somewhat unexpected because the gate areas here are orders of magnitude bigger than those of the micro junction where they have been demonstrated earlier [START_REF] Rouco | Quasiparticle tunnel electroresistance in superconducting junctions[END_REF]. Indeed, one would have anticipated that, in such large junctions, the likely presence of defects would have prevented the observation of the very large conductance switching effects. Their present realization shows that the redox mechanism is very robust against defects and can be easily implemented at a larger scale, which can be relevant for specific applications.

  (b). Each panel corresponds to a different regime. Starting from the OFF state, the first regime [Fig. 2(b), 1] corresponds to positive voltage excursions with 0 < 𝑉 𝑤𝑟𝑖𝑡𝑒 < 4 𝑉. Here the I-V curves are reversible and overlap, that is, the gate's conducting state is unchanged by the voltage excursions. The second regime appears as 𝑉 𝑤𝑟𝑖𝑡𝑒 exceeds ~4.5 V [Fig. 2(b), 2]: here the 𝐼 𝐺 (𝑉 𝐺 ) becomes more and more hysteretic as 𝑉 𝑤𝑟𝑖𝑡𝑒 is increased, the conductance being the highest in the voltage-decreasing branch. Near 𝑉 𝑤𝑟𝑖𝑡𝑒 ~ 5 V the hysteresis becomes the largest, indicating the full switching from the OFF into the highest conductance ON state. This state remains stable both if 𝑉 𝑤𝑟𝑖𝑡𝑒 is further increased, and also in regimes 3 [Fig. 2(b), 3; 5 > 𝑉 𝑤𝑟𝑖𝑡𝑒 > 0 𝑉] and 4 [Fig. 2(b), 4; 0 > 𝑉 𝑤𝑟𝑖𝑡𝑒 > -1.5 𝑉]. Regime 5 [Fig. 2(b), 5] corresponds to the gradual switching from the ON into the OFF state, for voltage excursions with 𝑉 𝑤𝑟𝑖𝑡𝑒 below -1.5 V. Once 𝑉 𝑤𝑟𝑖𝑡𝑒 ~ -5 𝑉 is reached, the OFF state is fully set and we enter the 6 th regime [Fig. 2(b), 6], which corresponds to a stable OFF state as 𝑉 𝑤𝑟𝑖𝑡𝑒 is increased from -5 𝑉 to 0 𝑉. Additionally, the above memristive behaviour and the possibility to stabilize intermediate states between ON and OFF, can be easily sensed via low-bias conductance measurements. That is demonstrated in Fig. 3(a) for which we applied 𝑉 𝑤𝑟𝑖𝑡𝑒 voltages following the sequence shown in the legend (bottom to top), for a different device compared to Fig. 2 (see the captions). After each 𝑉 𝑤𝑟𝑖𝑡𝑒 , we measured a low-bias 𝐼 𝐺 (𝑉 𝐺 ) of the gate with |𝑉 𝐺 | < 0.5 𝑉, which is low enough to probe the remnant state of the gate without modifying it. This allow us to numerically calculate the differential conductance 𝐺 = 𝑑𝐼 𝐺 𝑑𝑉 ⁄ 𝐺 as a function of the bias. One can also observe the gradual switching between the OFF and ON states across intermediate ones, as 𝑉 𝑤𝑟𝑖𝑡𝑒 is increased from 4.7 𝑉 to 7 𝑉. Fig. 3(b) shows the differential conductance for a fixed 𝑉 𝐺 = 100 𝑚𝑉 as a function of 𝑉 𝑤𝑟𝑖𝑡𝑒 , which is cycled from positive to negative to drive the ON/OFF conductance switching. The behaviour is typical of all the samples with Al gating: a hysteretic, reversible ON/OFF switching, with the conductance change being as large as 10000 % (in the example, from 10 -6 𝑆 to 10 -8 𝑆). Notice also the strong asymmetry in the voltage required for initiating the switching from OFF to ON |𝑉 𝑃𝑂𝐿 (+)| = 4.7 𝑉 and vice versa |𝑉 𝑃𝑂𝐿 (-)| = 0.7 V. Fig. 3(c) shows the typical behaviour of the control samples, which have a noble-metal gate (Au instead of Al). Their behaviour shows three main differences as compared to the Al devices.

Fig. 4 ( 3 ,

 43 a) shows the channel's resistance 𝑅 𝑆𝐷 as a function of the temperature, measured after applying increasing 𝑉 𝑤𝑟𝑖𝑡𝑒 (see legend). The measurements shown in Fig.4(a) are representative of all the devices with an Al gate. One can distinguish two main features. First, increasing 𝑉 𝑤𝑟𝑖𝑡𝑒 leads to a gradual increase of the channel's normal-state resistance (for 𝑇 > ~90 𝐾) until, ultimately, for 𝑉 𝑤𝑟𝑖𝑡𝑒 = 15 𝑉 a highly resistive state is observed at all temperatures. Second, the normal-to-superconducting resistive transition, apparent for 𝑉 𝑤𝑟𝑖𝑡𝑒 ≤ 12 𝑉, is gradually enlarged as 𝑉 𝑤𝑟𝑖𝑡𝑒 increases. Notice that the superconducting transition onset occurs at a similar temperature for all 𝑉 𝑤𝑟𝑖𝑡𝑒 ≤ 12 𝑉, as it only decreases from 𝑇 𝐶 ~88 K for the pristine YBCO channel to 𝑇 𝐶 ~86 K for 𝑉 𝑤𝑟𝑖𝑡𝑒 = 12 𝑉. Fig.4(b)shows similar measurements carried out on a control sample whose gate is made of gold instead of Al. These measurements globally show the same behaviour as the one described for the Al-gated device: as 𝑉 𝑤𝑟𝑖𝑡𝑒 is increased, the normal-state resistance increases and the superconducting transition widens. To provide a more quantitative analysis of the latter observation, we define in our measurements the effective critical temperature 𝑇 𝐶 * as the temperature for which the channel resistance drops below 1 Ω (this roughly corresponds to the residual resistance due to the 2-probe configuration). This criterion is shown by the black dashdotted horizontal line displayed in Fig.4(a) and (b). Fig. 4(c) shows 𝑇 𝐶 * as a function of 𝑉 𝑤𝑟𝑖𝑡𝑒 for different devices with a channel thickness varying between 50 𝑛𝑚 < 𝑡 𝑌𝐵𝐶𝑂 < 125 𝑛𝑚. In all cases, we observe a monotonic decrease of 𝑇 𝐶 * with increasing 𝑉 𝑤𝑟𝑖𝑡𝑒 , which allows driving the channel from superconducting into a highly resistive state. The voltage required to suppress superconductivity scales with the thickness of the YBCO channel: the thicker the channel, the higher the required 𝑉 𝑤𝑟𝑖𝑡𝑒 . Interestingly, a similar suppression of superconductivity is observed for the control samples whose gate is made of Au [see Fig. 4(b) and the corresponding purple curve in Fig. 4(c)]. That is, contrary to the conductance switching effects discussed in Fig. here similar effects are observed regardless of whether the gate material can reduce YBCO or not. Another key observation is that, at variance to the gate conductance-switching effects, the modulation of superconductivity in the YBCO channel is irreversible: the application of negative 𝑉 𝑤𝑟𝑖𝑡𝑒 voltage pulses cannot enhance 𝑇 𝐶 * nor drive a decrease in the channel's resistance. This is well illustrated in Fig. 5, which monitors the concomitant gate and channel resistance changes induced by the application of a series of 𝑉 𝑤𝑟𝑖𝑡𝑒 cycles. Fig. 5(a) shows the sequence of 𝑉 𝑤𝑟𝑖𝑡𝑒 , which goes back and forth from positive to negative with increasing amplitudes. Fig. 5(b) shows the remnant channel's resistance 𝑅 𝑆𝐷 = 𝑉 𝑆𝐷 𝐼 𝑆𝐷 ⁄ for 𝐼 𝑆𝐷 = 1 µ𝐴, (black curve) and the gate's resistance 𝑅 𝐺 = 𝑉 𝐺 𝐼 𝐺 ⁄ for 𝐼 𝐺 = 1 µ𝐴 (green curve), measured following each writing cycle. One can see that the gate's resistance 𝑅 𝐺 is reversibly switching between two ON and OFF levels, which are roughly stable after the 1 st cycle, regardless of the gradual increase of 𝑉 𝑤𝑟𝑖𝑡𝑒 across the sequence. Contrarily, the channel displays irreversible resistance-increasing steps, one switching cycle after the other. The unavoidable conclusion is that the gate and channel resistance variations arise from different mechanisms.

FIG. 1 .

 1 FIG. 1. Fabrication process and electrical measuring setup. (a) Etching of the general layout of the device down to the substrate. (b) Etching of the superconducting channel with a thickness 35 𝑛𝑚 < 𝑡 𝑌𝐵𝐶𝑂 < 140 𝑛𝑚 while defining the source (S) and drain (D) contacts. The darker areas on the microscope picture correspond to the upper part of the YBCO channel. (c) Deposition step of the Al and Au gate (G) contacts with 𝑡 𝐴𝑙 = 2 𝑛𝑚 𝑜𝑟 10 𝑛𝑚 and 𝑊 𝐴𝑙 = 5 𝜇𝑚 𝑜𝑟 60 𝜇𝑚. (d) Schematic representation of the measuring setup used for the transport experiments. The electrical characterization is divided into two distinct sets: (top) the gate measurements are carried by applying a voltage bias 𝑉 𝐺 and measuring the current 𝐼 𝐺 between G and S and (bottom) the source-drain measurements are carried by injecting a current 𝐼 𝑆𝐷 and measuring the voltage 𝑉 𝑆𝐷 between D and S.

FIG. 2 .

 2 FIG. 2. (a) Semilog 𝐼 𝐺 -𝑉 𝐺 characteristics of the gate between -5 𝑉 and +5 𝑉 for a device with 𝑡 𝑌𝐵𝐶𝑂 = 110 𝑛𝑚 and 𝑡 𝐴𝑙 = 10 𝑛𝑚 at 40 𝐾. The inset shows the shape of the 𝑉 𝐺 sweep going from 0 𝑉 to 𝑉 𝑤𝑟𝑖𝑡𝑒 (positive or negative) and back to 0 𝑉 in a time 𝑡 𝐺 = 20 𝑠 and number of steps 𝑛 = 100. (b) Splitting of the switching process into 6 consecutive regimes. For clarity, the contour of the set of I-V characteristics is plotted as a guide to the eye along with selected curves. Graph 2 demonstrates the switching from the low conducting OFF state to the high conducting ON state for gate voltages between 4.4 𝑉 and 5 𝑉, whereas graph 5 shows the opposite switching as negative voltages are applied to the gate.

FIG. 3 .

 3 FIG. 3. (a) Differential conductance G measured between G and S in semilog scale as a function of a low reading voltage 𝑉 𝐺 between -0.5 𝑉 and 0.5 𝑉 showing a clear switching from the OFF state (blue curve) to the ON state (red curve) as well as intermediate stable states for a device with 𝑡 𝑌𝐵𝐶𝑂 = 50 𝑛𝑚 and 𝑡 𝐴𝑙 = 10 𝑛𝑚 at 35 𝐾. 𝑉 𝑤𝑟𝑖𝑡𝑒 is applied sequentially as indicated by the legend, from bottom to top. The vertical black line represents the chosen value of 𝑉 𝐺 = 100 𝑚𝑉 along which Fig. 3(b) is calculated. (b) G as a function of 𝑉 𝑤𝑟𝑖𝑡𝑒 for a fixed reading voltage of 100 𝑚𝑉 demonstrating the reversible switching (with an amplitude of 10000 %) behaviour of G between two non-volatile ON and OFF states. The polarization voltages 𝑉 𝑃𝑂𝐿 (+) and 𝑉 𝑃𝑂𝐿 (-) represent the minimum value of the voltage required to induce a switching from the OFF to the ON state and inversely, respectively. (c) G as a function of 𝑉 𝑤𝑟𝑖𝑡𝑒 for a control sample with 𝑡 𝑌𝐵𝐶𝑂 = 72 𝑛𝑚 at 90 𝐾.

FIG. 4 .FIG. 5 .

 45 FIG. 4. (a) Temperature dependence of the resistance 𝑅 𝑆𝐷 in semilog scale measured after applying 𝑉 𝑤𝑟𝑖𝑡𝑒 (see legend) for a device with 𝑡 𝑌𝐵𝐶𝑂 = 110 𝑛𝑚 and 𝑡 𝐴𝑙 = 10 𝑛𝑚. Notice that the resistance drops to a finite value of ~0.9 Ω instead of zero because the two-probe configuration measures the wiring and contact resistances. (b) Temperature dependence of 𝑅 𝑆𝐷 in semilog scale for a control device with no aluminium gating and 𝑡 𝑌𝐵𝐶𝑂 = 72 𝑛𝑚, measured after applying 𝑉 𝑤𝑟𝑖𝑡𝑒 (see legend). (c) Effective critical temperature 𝑇 𝐶 * of the superconducting channel as a function of 𝑉 𝑤𝑟𝑖𝑡𝑒 for different devices with a channel thickness varying between 50 𝑛𝑚 < 𝑡 𝑌𝐵𝐶𝑂 < 125 𝑛𝑚.

Acknowledgements

Work supported by ERC PoC 966735 "SUPERMEM", French ANR-17-CE30-0018-04 "OPTOFLUXONICS", Flag-ERA "To2DoX" and Cost Action 21144 "SUPERQUMAP". A.L. acknowledges the support from the French ANRT in the form of a CIFRE scholarship.


Alexis Pecheux

Morgan Almanza

Daniele Fournier

Laura Thevenard

Catherine Gourdon

Jon Ander

Vojtěch Uhlíř

Martino Lobue

Study of the Laser-Assisted Thermal Hysteresis in FeRh Films

des établissements d'enseignement et de recherche

Study of the Laser-Assisted Thermal Hysteresis in FeRh Films

❑ References Alexis Pecheux *(a) , Morgan Almanza (a) , Daniele Fournier (b) , Laura Thevenard (b) , Catherine Gourdon (b) , Jon Ander Arregi (c) , Vojtěch Uhlíř (c,d) , Martino LoBue (a) (a) Université Paris-Saclay, ENS Paris-Saclay, CNRS, SATIE, 91190 Gif-sur-Yvette, France, (b) Sorbonne Université, CNRS, Institut des NanoSciences de Paris, UMR7588, F-75252 Paris, France (c) CEITEC BUT, Brno University of Technology, Purkyňova 123, 612 00 Brno, Czech Republic (d) Institute of Physical Engineering, Brno University of Technology, Technická 2, 616 69 Brno, Czech Republic *Corresponding author: alexis.pecheux@ens-paris-saclay.fr Near equiatomic FeRh alloys, with their close to room temperature antiferromagnetic (AF) to ferromagnetic (FM) magnetostructural transition, have been studied in bulk, and in thin film form. The complex interplay between optical, electronic, magnetic, and structural properties underlying the phase transition makes FeRh a promising multifunctional material with possible applications in fields as different as spintronics, nanomagnetism, and magnetic cooling [1]. Whatever the aimed application, understanding FeRh phase change mechanism, and tackling the associated thermal hysteresis, is one of the key challenge for harnessing its functional properties.

Here we study the AF-FM transition using a modulated reflectance experiment [2] with a laser driven phase change taking place at a rather long timescale (μs range) where only thermal effects are at play. An optical microscopy setup where a pump and a probe beam (532 nm laser with square modulation at 100 kHz, and 488 nm laser respectively), are focused on a 2 μm radius spot, is used to measure the material's reflectance. The DC component (f=0 ), and the AC component (f=100Hz) of the reflectance are measured while the temperature of the sample is slowly varied across the transition. The average transition temperature shifts by 2.2°C per mW of the laser pump and, while the DC component under cooling is unaffected by the pump power, the one under heating merge with the cooling curve when pump power gets above 4 mW. On the other hand, the AC signal, rather weak at low pump power, increases (two orders of magnitude) when pump power gets above 2 mW.

❑ Abstract

❑ Introduction

❑ Perspectives

✓ Study the phase transition on the entire surface and in the film volume to probe the hysteresis properties at multiples scale to have a better understanding of the coupling involves inside the phase transition ✓ Further investigation without the lock-in ✓ Introduce a magnetic field in order to probe the impact of magnetic field on the phase transition kinetics

  [1] : L. Lewis, et al., J.Phys. D: App. Phys. 49, 323002 (2016) [2] : D. Fournier, M. Marangolo, and C. Fretigny, J. Appl. Phys., 128, 241101 (2020) [3] : Uhlíř, V., Arregi, J. A., and Fullerton, E. E. (2016) Nat.Com, 7(1), 13113

➢➢

  Photodiode