Jonathan Casey

Laura Beskow

Jeremy Brown

Samuel Brown

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Pierre-François Laterre

Use of pragmatic and explanatory trial designs in acute care research: lessons from COVID-19

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Reza Jabbari

François Roubille

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Kumar Narayanan

Jean-Claude Deharo

Wearable cardioverter-defibrillator to reduce the transient risk of sudden cardiac death in coronary artery disease

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F Grollau

J.-M Mestdagh

B Soep

E Gloaguen

V Brenner

L Poisson

M Mons

M Briant

Spectroscopie Du

Spectroscopie du 4(5)-méthylimidazole et de ses hydrates, hébergés en gouttelette d'hélium

ouverte pluridisciplinaire HAL, est destinée au dépôt

L'histidine est l'un des deux acides aminés qui présente un équilibre tautomérique. Cette molécule est par ailleurs, à la fois accepteur et donneur de proton, ce qui lui donne un rôle essentiel dans les processus biologiques où le transfert d'un proton intervient (ex. régulation du pH sanguin). Des études antérieures ont montré que l'équilibre tautomérique de cet acide aminé est modifié par son environnement [1] , ce qui rend ses propriétés intrinsèques délicates à obtenir expérimentalement. Connaître ces propriétés est pourtant important pour comprendre le comportement de cet acide aminé lorsqu'il est environné. Les caractéristiques mentionnées ci-dessus sont uniquement portées par le cycle imidazole qui est la chaîne latérale de l'histidine. Ainsi, nous nous sommes intéressés à une molécule modèle de l'histidine, le 4(5)-méthylimidazole (MeIm) (Figure 1). Sur le plan expérimental, nous avons caractérisé les isomères du Melm et ses hydrates, par spectroscopie infrarouge (IR). Le dispositif Gouttelium [2] a permis d'enregistrer les spectres IR de ces espèces en utilisant la technique "Helium NanoDroplet Isolation" [3] . Les gouttelettes d'hélium sont des systèmes quantiques froids (0,37 K [4] ), superfluides [5] et très fragiles. Elles thermalisent les espèces qui y sont hébergées et interagissent peu avec ces dernières, ce qui permet d'obtenir des spectres d'absorption IR proche de ceux attendus sur des molécules totalement isolées. Néanmoins, cette observation porte essentiellement sur la structure vibrationnelle des spectres car la structure rotationnelle est affectée par la présence de la gouttelette.

Nous avons caractérisé sans ambiguïté les deux tautomères du Melm via l'élongation NH dont la fréquence est très sensible à la position du groupe méthyle. L'attribution des bandes IR observées à un conformère spécifique a été faite par comparaison avec des calculs de DFT-D. Les intensités relatives entre bandes nous ont permis d'estimer la constante de tautomérisation, K5 ↔ 4(T=43°C)= 5,3 ± 0.8, qui n'était pas connue avec précision. Les constantes de rotation du 4-méthylimidazole dans les gouttelettes ont été déterminées grâce à une simulation du spectre et montrent une interaction forte de la rotation avec la gouttelette d'hélium. Ce résultat est attendu car la molécule entraîne une couche d'hélium non superfluide dans sa rotation.

Enfin, les complexes 4(5)-MeIm-eau ont été formés par le dépôt des deux espèces dans les gouttelettes d'hélium. Une recherche du signal de l'élongation NH du MeIm et de l'élongation symétrique et antisymétrique OH de l'eau a été effectuée.

Les différents spectres et leur interprétation seront présentés, ainsi qu'une discussion. 1. J. A. Vila, et al., PNAS 108, 5602 (2011). http://dx.doi.org/10.1073/pnas.1102373108 2. M. Briant, et al., PCCP 18, 16414 (2016). http://dx.doi.org/10.1039/c6cp02989b. 3. F. Stienkemeier, et al., Phys. Rev. Lett. 74, 3592 (1995). http://dx.doi.org/10.1103/PhysRevLett.74.3592 4. M. Hartmann, et al., Phys. Rev. Lett. 75, 1566(1995). http://dx.doi.org/10.1103/PhysRevLett.75.1566 5. M. Hartmann, et al., Phys. Rev. Lett. 76, 4560 (1996). http://dx.doi.org/10.1103/PhysRevLett.76.4560

Figure 1 :

 1 Figure 1: L'histidine et sa molécule modèle, le méthylimidazole.

Spectroscopie du 4(5)-méthylimidazole et de ses hydrates, hébergés en gouttelette d'hélium

  . Grollau 1 , J.-M. Mestdagh 1 , B. Soep 2 , E. Gloaguen 1 , V. Brenner 1 , L. Poisson 2 , M. Mons 1 et M.

	Briant 3*
	1 Univ. Paris-Saclay, CEA, CNRS, LIDYL, 91191 Gif-sur-Yvette, France
	2 Univ. Paris-Saclay, CNRS, ISMO, 91405, Orsay, France
	3 Univ. Paris-Saclay, CEA, CNRS, NIMBE, 91191 Gif-sur-Yvette, France

F


Stéphane Fusil
email: stephane.fusil@cnrs-thales.fr

Jean-Yves Chauleau

Xiaoyan Li

Johanna Fischer

Pauline Dufour

Cyril Leveille

Cécile Carrétéro

Nicolas Jaouen

Michel Viret

Alexandre Gloter

Vincent Garcia
email: vincent.garcia@cnrs-thales.fr

J.-Y Chauleau

Polar chirality in BiFeO3 emerging from a peculiar domain wall sequence

Keywords: ferroelectric, chirality, domain wall, epitaxial strain

Topological states are currently gathering intensive investigation in condensed matter physics due to their potential as configurable electronic devices for the future era coined "topotronics".

Beyond numerous breakthroughs in magnetism over the last decade, a new paradigm is emerging with the proposal of topologically-protected objects in ferroelectric materials.

Recently, ferroelectric skyrmions and vortices were observed in PbTiO3/SrTiO3 superlattices, opening the path towards ultra-small topological objects with low-power electric-field control.

Here we report the observation of chiral polar windings in a single epitaxial thin film, triggered by its self-organized stripe domain pattern arrangement. Combining resonant elastic X-ray scattering and scanning transmission electron microscopy, we show signatures of polar chirality in epitaxial BiFeO3 thin films corroborated with a complex ferroelectric domain wall structure.

The net chirality suggests that domain walls induce a polar rotation through a small path alternating with an unexpected long path at every second domain wall. In addition, scanning probe microscopy reveals singularities associated to this peculiar domain wall structure. These results bring new insights into the unexpected complexity of standard striped-domain BiFeO3

thin films and open questions as for the driving force of this polar chirality.

Introduction

Chirality is a ubiquitous concept in chemistry with numerous chiral molecules such as glucids, amino acids and the most famous elemental brick of life: DNA. Chirality was first revealed in crystals in 1848, when Louis Pasteur sorted out levogyre and dextrogyre ammonium tartrate crystals with a pair of tweezers under the microscope. [1] Chirality is also a key property in physics and especially in magnetism where an intense research effort has been recently devoted to chiral textures such as skyrmions, merons, and hopfions among many others. [2] These chiral objects are envisioned as the most stable and smallest magnetic objects achievable thanks to their topological protection: topology being a branch of mathematics that focuses on the properties of geometrical objects that are preserved under continuous deformation. [3] Recently, the concept of chirality has been extended to ferroelectric materials. [START_REF] Seidel | Topological Structures in Ferroic Materials[END_REF][START_REF] Seidel | [END_REF][6][7] Chiral ferroelectric textures are indeed tremendously appealing because they can reach ultimate sizes, they harbor negative capacitance [8,9] and might be switchable with low-power electric fields. Nanometerscale polar vortices [10] and skyrmions [11] have recently been observed in oxide PbTiO3/SrTiO3 superlattices, triggered by a subtle interplay between electrical and elastic boundary conditions at interfaces. In such superlattices, polar chirality was revealed using resonant X-ray diffraction. [12] Topological defects were also reported in single ferroelectric layers [13][14][15] as well as in ferroelectric nanostructures. [16,17] Actually, ferroelectric chirality might be even more common than expected, emerging from simple ferroelectric domain walls when dipoles have to reorient between domains. [18] Ferroelectric domain walls are known to be tiny objects with only a few unit cells width, typically one order of magnitude smaller than their ferromagnetic counterpart. Nevertheless, their structure is nowadays considered very similar to the one observed in ferromagnets: numerous recent experimental observations suggest that they are not pure Ising-like but possess a Bloch or Néel component. [19][20][21][22] Here we report pristine chirality in a single epitaxial ferroelectric thin film, combining resonant elastic X-ray scattering and scanning transmission electron microscopy. Investigating "classical" striped-domain BiFeO3 thin films grown on (110) orthorhombic scandate substrates, [23][24][25][26][27][28] we show that a net polar chirality emerges from a peculiar arrangement of domain walls. While the polarization evolves from one domain to another through the expected short path 71° domain wall, the following domain wall is no longer 71° but consists of a pair of 109° domain wall plus a hidden 71° domain wall. Interestingly, through both short and long paths, the polarization rotates in the easy plane imposed by the monoclinic distortion of the substrate. This domain wall sequence allows a full (360°) polarization rotation, and the sequence repetition locks this rotation sense, therefore building polar chirality.

Results and discussion

Polar chirality revealed with resonant elastic X-ray scattering

BiFeO3 epitaxial thin films, with typical thicknesses in the 30-60 nm range, were grown by pulsed laser deposition on DyScO3 and GdScO3 (110) orthorhombic substrates using SrRuO3 buffer oxide electrodes, as described elsewhere. [29] The (001) pseudo-cubic oxide heterostructure is fully strained in both cases with high structural quality attested by X-ray diffraction. [29] While the oxide electrode induces a preferential downward orientation of the ferroelectric polarization, [30] the monoclinic symmetry of the (110) scandate substrates favors only two ferroelastic variants of the ferroelectric, which results in a self-organized stripedomain pattern with 71° domain walls (Figure 1). [24] The single direction of the stripes over the whole sample is imposed by the charge neutrality of the domain walls. The striped-domain ferroelectric period is typically of the order of 200  30 nm, as revealed by in-plane piezoresponse force microscopy (PFM) images (Figure 1a). Taking advantage of the system periodicity, we investigated the properties of the ferroelectric domain structures using resonant elastic X-ray scattering (REXS). The scattering patterns at the oxygen K-edge (530 eV) are displayed in Figure 2a and e for BiFeO3 thin films grown on DyScO3 [18] and GdScO3, respectively. For both cases, these scattering patterns feature two pairs of spots, labelled '1' and '2', symmetric with respect to the specular spot. Real-space imaging using atomic force microscopy (AFM) allows to disentangle the two sources of periodicity.

First, a purely surface-related periodicity is induced by the atomic steps mimicking those of the parent substrate and reproduced by the epitaxial layer-by-layer growth. In these DyScO3 and GdScO3 substrates, the direction of the miscut changes by 90° from one to the other (Figure 2c and g), corresponding to the spots labelled '1' in Figure 2a ande. The fast Fourier transform (FFT) patterns of the AFM images confirm the correspondence of the '1' spots in reciprocal space, with periodicities of 148  20 nm (Figure 2c) and 137  20 nm (Figure 2g), matching those obtained by REXS (164  20 nm and 142  20 nm in Figure 2a and 2e, respectively). We can then attribute the first pair of spots (labelled '1') to the periodic steps on the surface topography of the films. We now turn to the second pair of spots (labelled '2'). Here again, for both samples, the orientation as well as the spacing between the diffracted spots are matching when comparing REXS (218  20 nm and 175  20 nm in Figure 2a 6%-+3% and -40%-+40% for (b) and (f), respectively. The data in (a) and (b) are adapted from [18] . "1" and "2" spots stand for the topography and the ferroelectric periods, respectively. c) REXS experiments were performed for both helicities of circularly polarized X-rays. Figure 2b and 2f display the circular dichroism patterns, i.e. the difference between left and right circularly polarized X-rays normalized by their sum. For both BiFeO3 samples, while the periodic structural features ('1') show no circular dichroism, the spots corresponding to the periodic ferroelectric domains ('2') show a clear dichroic signal. This circular dichroism contrast inversion (blue and red spots for +q and -q) at the oxygen K edge reveals the presence of a chiral polar order in the ferroelectric thin films. [31] As we expect the ferroelectric domains to be uniform with collinear polarization, the periodic chiral polar textures are likely to emerge from the ferroelectric domain walls of BiFeO3. This implies that the domain walls are not Isinglike but possess a Bloch or Néel-like character with the polarization rotating always in the same sense. We note that this observation is valid for BiFeO3 thin films grown on DyScO3 and GdScO3 substrates, imposing compressive and tensile strain, respectively. This is contrary to the expected rotation of polarization along the back and forth 71° domain walls, where the shortest path (red arrows in Figure 1c) would imply an opposite sense of rotation between neighboring domain walls and hence no net chirality. As BiFeO3 is also antiferromagnetic with a periodic spin cycloid, one could wonder if a magnetic periodic texture could emerge from the ferroelectric domain arrangement, thus at the same period. We can rule out any magnetic contribution from the datasets acquired at the O K edge where the magnetic signal is negligible, if not absent. Nevertheless, the spots associated to the antiferromagnetic cycloid can be observed at the Fe L edge. [18,29]

Complex domain wall path using scanning transmission electron microscopy

In order to get more insights into the chiral domain walls present in striped-domain BiFeO3 thin films, we performed scanning transmission electron microscopy (STEM) experiments on crosssection specimens. Using medium angle annular dark field (MA-ADF), we were able to identify the expected 71° domain walls (Figure 3a). This 71° domain wall appears as a faint contrast in MA-ADF with a tilted plane with respect the growth axis. These walls are difficult to detect as the projection of polarization, perpendicularly to the [100] zone axis, is equivalent between both domains, and MA-ADF turned out to enhance their contrast as compared to more conventional high-angle annular dark field. Surprisingly, pairs of vertical domain walls (Figure 3b) were also identified, typically separated by about 10 nm. These can presumably be interpreted as 109° domain walls. Such observations imply that the ferroelectric landscape is much more complex than expected from PFM and X-ray diffraction characterizations. High-resolution high angle annular dark field (HA-ADF) images were also obtained in areas containing one or the other domain wall (Figure 4). Considering the different observations, we came up with the following scenario. The net polar chirality suggests that the periodic system alternates between a short path (domain 1 to domain 2, red boxed region in Figure 4), with a rotation of polarization in a standard 71° domain wall (Figure 4a), and a long path (domain 2 to domain 1, blue boxed region in Figure 4), within a 10-nm area delimited by the two 109° domain walls (Figure 4b). From the relative displacements of the Fe atoms with respect to the barycenter of the Bi lattice, we were able to map the distribution of polarization across the domain walls. [19] One can clearly see that the polarization rotates continuously at the domain walls and its orientation is locally upward in the 10-nm gap defined by the two vertical walls.

The polarization rotation, as projected on the plane perpendicular to the [100] zone axis, is fully compatible with a pair of 109° domain walls. To complete the long path of polar rotation from domain 2 to domain 1, we suggest that an additional 71° domain wall may be present in between the 109° domain walls, but again hidden due to the [100] zone axis. We point out that all the ferroelectric variants being at play in this scenario are lying in the "easy" (110) plane, in which the polarization should be favored thanks to the monoclinic distortion arising from the substrate symmetry.

Scanning probe microscopy insights into chiral windings

Given the small width (10 nm) of the long path area delimited by the pair of 109° domain walls, we are not able to resolve the local upward polarization orientation using PFM. By essence, PFM is not the most suitable approach to characterize domain walls as the PFM amplitude signal is decreased at their vicinity, and is even supposed to cancel at the border of two domains with out-of-phase responses. We are then unable to detect the upward polar nanoregions by PFM because of the resolution limitation in case of three domain walls stacked in a 10-nm width. This length scale even questions the standard definitions of domain walls in BiFeO3 (namely 71°, 109° and 180°) as each of these walls are a few unit cells wide so that we can consider the long path as a continuous domain wall with 289° (i.e., 109° + 71° + 109°) polarization rotation.

Consequently, we looked more into the details of these ferroelectric domain walls using scanning probe microscopy. As displayed in Figure 5a, the high-resolution in-plane PFM phase image shows the two domains alternating and the chiral polar rotation at the domain walls is sketched below the image, as inferred from REXS and STEM observations. The red and blue arrows indicate the short and long paths, respectively. A careful look at the high-resolution outof-plane phase image (Figure 5b) reveals drops of signal associated to the ferroelectric domain walls. Interestingly, the width of these walls alternates between narrow and wider lines, possibly due to the short and long path, respectively. This is corroborated with the in-plane PFM amplitude, which shows alternatively narrow and wider drops of signal, again possibly associated to the short and long path domain walls, respectively. These scanning probe microscopy investigations support the fact that the system alternates between a simple 71° domain walls (short path) and a complex set of 109°/71°/109° domain walls (or 289° long path). Furthermore, conducting-AFM was performed applying a bias voltage of 1.7 V between the conducting AFM tip and the SrRuO3 bottom electrode. Domain walls are clearly detected, defining the edges of the ferroelectric stripes seen by PFM. Their conduction remains weak and comparable to that of domains. Nevertheless, current maps indicate a different conduction between subsequent domain walls: one of them seems less conducting than the domains while the other appears as a double line with depressed and enhanced conductivities (Figure 5d). Notably, considering the domain wall sequences depicted in Figure 4, the "short path" 71° domain walls are neutral (Figure 4a), while the "long path" actually contains two charged 109° domain walls (Figure 4b), i.e. head to head and tail to tail for the left and right domain walls, respectively. Therefore, the enhanced or depressed conductions of the domain walls in Figure 5d could be related to their charged state. [17,33] However, these conducting-AFM currents maps do not allow us to conclude on the direct link between the complex domain wall sequences and their distinct conduction states, as these results are also reminiscent of those reported by Zhang et al. [32] on thick BiFeO3 films grown by molecular beam epitaxy on TbScO3 substrates, in which pure 71° domain walls showed enhanced conductions related to domain wall bending.

Conclusion

We show that the archetypical striped domain pattern in BiFeO3 thin films displays an unexpected polar chirality emerging from ferroelectric domain walls. Resorting to resonant elastic X-ray scattering, we show that polar chirality is developed in striped-domains of BiFeO3 thin films grown both under compressive or tensile strain. This is in contradiction with the simple picture linking two different polarization domains by the shortest path available, giving no net chirality. Using scanning transmission electron microscopy, we reveal that there are actually two types of domain walls which coexist in the films: the simple short path alternates with a long path nanoregion schematized by three stacked domain walls. Despite the complexity of the domain wall sequence, the complete 360° polar rotation always occurs in the plane favored by the monoclinic distortion of the scandate substrates. The synergy between advanced real-space and reciprocal-space techniques allows to decipher how such a chiral winding proceeds. Nevertheless, the driving force for the emergence of polar chirality remains to be understood and further theoretical investigations are highly desirable. [34][35][36] From an experimental point of view, chirality may be extended to other ferroelectrics as this selforganized stripe domain landscape is common for standard ferroelectric thin films (as for example BaTiO3 or PbTiO3 [15,[37][38][39][40] ).

Experimental Section

Sample preparation and characterization

The films are grown by pulsed laser deposition on DyScO3(110) and GdScO3(110) oxide single crystals using a KrF excimer laser (248 nm) with a laser fluence of 1 J/cm 2 . Prior to the growth, the scandate substrates were annealed under oxygen flow for 3 hours at 1000°C. The SrRuO3 bottom electrode was first grown at 660°C under 0.2 mbar of oxygen pressure and with a laser frequency of 5 Hz. The BiFeO3 thin films were subsequently grown at the same temperature under 0.36 mbar of oxygen pressure and a laser frequency of 2 Hz. After the growth, the samples were cooled down to room temperature under an oxygen pressure of 300 mbar. The thickness of the BiFeO3 thin films was determined from X-ray reflectivity and double-checked from the Laue fringes of the 2- diffraction patterns. The thicknesses of the BiFeO3 and SrRuO3 layers grown on GdScO3(110) (Figure 2e-h) are 56 nm and 5 nm, respectively. All the other experiments presented in the manuscript were performed on a BiFeO3 (32 nm) / SrRuO3 (10 nm) // DyScO3(110) sample.

Resonant elastic X-ray scattering

Resonant elastic X-ray scattering (REXS) measurements were performed in reflectivity geometry at the O K edge using the RESOXS diffractometer [41] at the SEXTANTS beamline [42] of SOLEIL synchrotron. The data were collected using nearly fully circular left and right X-ray polarizations delivered by the HU44 Apple2 undulator located at the I14-M straight section of the storage ring. REXS experiments were performed with the incoming X-rays at about 45° from the periodic stripes in Figure 2a,b while they were almost 90° from the periodic stripes in Figure 2e,f. These different azimuthal geometries explain the differences in the amplitude of the dichroic signals.

Scanning transmission electron microscopy

Scanning transmission electron microscopy (STEM) experiments were performed using a Cs corrected Nion microscope operated at 100 keV. High angle annular dark field (HA-ADF) and medium angle annular dark field (MA-ADF) images have been acquired with respectively 80-100 mrad and 40-80 mrad collection angle. The samples were prepared for STEM observation by gallium focus ion beam (FIB) where final surface polishing was done with a 2 keV ion beam.

Scanning probe microscopy

Piezoresponse force microscopy (PFM) experiments were performed in a Nanoscope V, multimode (Bruker) using an external ac source (DS360, Stanford Research) to excite the SrRuO3 bottom electrode at a frequency of 35 kHz (far off resonance) with typical ac voltage excitations of 2 V peak to peak, and external lock-in amplifiers (SR830, Standford Research).

In the high-resolution PFM experiments of Figure 5a-c, the pixel size is 2 nm. The conducting-AFM experiments were performed using a TUNA current amplifier. A voltage of 1.7 V was applied to the bottom electrode of SrRuO3 while the tip was grounded during the current measurements of Figure 5d. We used super sharp diamond probes from Adama  with a cantilever stiffness of 40 N/m for both high-resolution PFM and conducting-AFM experiments.

Table of contents

The polar chirality of striped-domain multiferroic thin films of BiFeO3 is associated to a complex set of ferroelectric domain walls. Interestingly, the complete polar winding proceeds as an alternation between short path and long path Bloch-like domain walls, in which the polarization remains in the easy plane imposed by the symmetry of the substrate.

Stéphane

Figure 1 .

 1 Figure 1. a) In-plane piezoresponse force microscopy phase image on a BiFeO3 thin film grown on DyScO3(110), illustrating the striped-domain pattern produced by two alternating ferroelastic domains. b) Sketch of the alternating ferroelectric domains with 71° domain walls. c) The monoclinic distortion of the substrate favors only two ferroelastic variants and the ferroelectric microstructure harbouring these two variants is sketched. The shortest path for polarization reorientation is represented by back and forth red arrows, giving rise to no net chirality.

  and 2e, respectively) and the FFT patterns from in-plane PFM phase images (225  20 nm and 180  20 nm in Figure2dand 2h, respectively). The second pair of spots (labelled '2') is then attributed to the periodic ferroelectric domain structure.

Figure 2 .

 2 Figure 2. Periodic features in BiFeO3 epitaxial thin films revealed by resonant elastic X-ray scattering and scanning probe microscopy. The BiFeO3 thin films are grown on DyScO3(110) (a-d) and GdScO3(110) (e-h) substrates. a) and e) Resonant elastic X-ray scattering patterns at the oxygen K-edge. b) and f) Corresponding circular dichroic patterns. The vertical scales are -

  and g) Unit-cell staircase surface of BiFeO3 epitaxial films observed by atomic force microscopy. The vertical scales are 1 nm and 2 nm for (c) and (g), respectively. d) and h) Inplane piezoresponse force microscopy phase images showing striped domain patterns with two ferroelastic variants. The vertical scales are 360° in (d) and (h). The fast Fourier transform (FFT) patterns are displayed on the right of each image in (c), (d), (g), (h).

[ 29 ]Figure 3 .

 293 Figure 3. Medium angle annular dark field (MA-ADF) scanning transmission electron microscopy images of a) a 71° domain wall tilted away from the [001] growth axis and b) a pair of 109° vertical domain walls with a typical gap of 10 nm. The red arrows are guides to the eyes showing the locations of the domain walls. The 71° and 109° domain walls are sketched with the BiFeO3 unit cell on the right of each panel.

Figure 4 .

 4 Figure 4. High-resolution high angle annular dark field (HA-ADF) images. a) A sketch of the striped domain structure of the BiFeO3 thin film together with the HA-ADF image of the "simple" 71° domain wall (red box). b) The complex domain wall sequence (blue box in a) containing the pair of 109° domain walls. The middle panel shows an HA-ADF image of the area of interest together with sketches of the polar configurations. The top panel shows selected high-resolution HA-ADF images while the lower panel displays maps of polar displacements (in yellow) across this 10-nm area.

Figure 5 .

 5 Figure 5. Scanning probe microscopy investigations of the BiFeO3 domain walls. a) In-plane PFM phase image showing alternating striped domains. Sketch of the domains and the polar chirality involving short paths (red) and long paths (blue). b) Out-of-plane PFM phase image showing a small decrease of signal at the domain walls. Note that the vertical scale has been decreased to 90° to visualize this small phase contrast. c) In-plane PFM amplitude image showing a decreased signal at the domain walls. From (b) and (c), the width of every second domain wall is wider. d) Conductive atomic force microscopy map showing an enhanced conductivity at every second domain wall. The images in (a), (b), (c) and (d) were taken in different areas of the sample.

  Fusil*, Jean-Yves Chauleau, Xiaoyan Li, Johanna Fischer, Pauline Dufour, Cyril Leveille, Cécile Carrétéro, Nicolas Jaouen, Michel Viret, Alexandre Gloter, and Vincent Garcia* Polar chirality in striped-domain BiFeO3 thin films

Acknowledgements

We thank support from the French Agence Nationale de la Recherche (ANR) through the PIAF and TATOO (ANR-21-CE09-0033-01) projects. This work was supported by a public grant overseen by the ANR as part of the 'Investissement d'Avenir' programme (LABEX NanoSaclay, ref. ANR-10-LABX-0035). This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 964931 (TSAR).

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Thomas S Hartmann

Valérie Hayot-Sasson

Stephan Heunis

Felix Hoffstaedter

Daniela M Hohmann

Corey Horien

Horea-Ioan Ioanas

Alexandru Iordan

Chao Jiang

Michael Joseph

Jason Kai

Agah Karakuzu

David N Kennedy

Anisha Keshavan

Ali R Khan

Gregory Kiar

P Christiaan Klink

Vincent Koppelmans

Serge Koudoro

Angela R Laird

Georg Langs

Marissa Laws

Roxane Licandro

Sook-Lei Liew

Tomislav Lipic

Krisanne Litinas

Daniel J Lurie

Désirée Lussier

Christopher R Madan

Lea-Theresa Mais

Sina Mansour

L 126

J P Manzano- Patron

Dimitra Maoutsa

Matheus Marcon

Daniel S Margulies

Giorgio Marinato

Daniele Marinazzo

Christopher J Markiewicz

Camille Maumet

Felipe Meneguzzi

David Meunier

Michael P Milham

Kathryn L Mills

Davide Momi

Clara A Moreau

Aysha Motala

Iska Moxon-Emre

Thomas E Nichols

Dylan M Nielson

Gustav Nilsonne

Lisa Novello

Caroline O'brien

Emily Olafson

Lindsay D Oliver

John A Onofrey

Edwina R Orchard

Kendra Oudyk

Patrick J Park

Mahboobeh Parsapoor

Lorenzo Pasquini

Scott Peltier

Cyril R Pernet

Rudolph Pienaar

Pedro Pinheiro-Chagas

Jean-Baptiste Poline

Anqi Qiu

Tiago Quendera

Laura C Rice

Joscelin Rocha-Hidalgo

Saige Rutherford

Mathias Scharinger

Dustin Scheinost

Deena Shariq

Thomas B Shaw

Viviana Siless

Molly Simmonite

Nikoloz Sirmpilatze

Hayli Spence

Julia Sprenger

Andrija Stajduhar

Martin Szinte

Sylvain Takerkart

Angela Tam

Michel Thiebaut De Schotten

Ina Thome

Laura Tomaz Da Silva

Nicolas Traut

Lucina Q Uddin

Antonino Vallesi

John W Vanmeter

Nandita Vijayakumar

Matteo Visconti

Oleggio Castello

Jakub Vohryzek

Jakša Vukojević

Kirstie Jane Whitaker

Lucy Whitmore

Steve Wideman

Suzanne T Witt

Hua Xie

Ting Xu

Chao-Gan Yan

Brainhack: developing a culture of open, inclusive, community-driven neuroscience

Keywords: best practices, Brainhack, collaboration, community building, hackathon, inclusivity, neuroscience, open science, reproducibility, training Discipline: Life-science, Meta-science, Neuroscience

Brainhack is an innovative meeting format that promotes scientific collaboration and education in an open inclusive environment. The authors describe the myriad benefits for participants and the research community, and how Brainhacks complement conventional formats to augment scientific progress.

Introduction

Social factors play a crucial role in the advancement of science. New findings are discussed and theories emerge through social interactions, which usually take place within local research groups, and at academic events such as conferences, seminars or workshops. This system tends to amplify the voices of a select subset of the community-especially more established researchers-thus limiting opportunities for the larger community to contribute and connect. Brainhack events (or Brainhacks for short) complement these formats in neuroscience with decentralized two-to-five day gatherings, in which participants from diverse backgrounds and career stages collaborate and learn from each other in an informal setting. The Brainhack format was introduced in a previous publication [START_REF] Craddock | Brainhack: a collaborative workshop for the open neuroscience community[END_REF]Fig. 1A&B). It is inspired by the hackathon model (see Glossary), which originated in software development and has gained traction in science as a way to bring people together for collaborative work and educational courses. Unlike many hackathons, Brainhacks welcome participants from all disciplines and with any level of experience-from those who have never written a line of code to software developers and expert neuroscientists. Brainhacks additionally replace the sometimes competitive context of traditional hackathons with a purely collaborative one, and also feature informal dissemination of ongoing research through unconferences.

In the following, we aim to address two key questions about the merits of a Brainhack. First, how do participants benefit from attending a Brainhack event? Second, what is the relevance and importance of Brainhacks for neuroscience more broadly? To answer these questions, we discuss the five defining Brainhack features: 1) a Project-Oriented Approach that fosters active participation and community-driven problem-solving; 2) Learning By Doing, which enables participants to gain more intensive training, particularly in computational methods; 3) training in Open Science & Collaborative Coding, which helps participants become more effective collaborators; 4) Focus on Reproducibility, which leads to more robust scientific research; and 5) accelerated Building and Bridging of Communities, which encourages inclusivity and seamless collaboration between researchers at different career stages. Altogether, Brainhacks and similar formats are increasingly recognized as a new way of providing academic training and conducting research that extends traditional settings. These events foster a new research culture that celebrates open science, collaboration and diversity, unlocking opportunities for scientific progress.

A Project-Oriented Approach Brainhacks are fundamentally centered around attendee-led projects. At the beginning of each Brainhack, participants pitch project ideas and form teams to realize some of these ideas during the "hacking" sessions (Fig. 1B). The teams are dynamic, and their composition can change throughout the course of a project. As participants group themselves based on their common interest in a question, method, goal or idea, interdisciplinary teams naturally emerge. Each participant can hone diverse skills by playing an active part in multiple projects. This format avoids the scientific silos that often arise when scientists connect over a specific methodological or conceptual approach. Brainhack projects promote the flow of information between specialized domains within the multidisciplinary field of neuroscience.

The project-oriented structure of Brainhack enables everyone to be an active participant at the event, with contributions taking a variety of forms. Importantly, the term "hacking" is not used to refer to coding in particular, but to describe an intensive form of work, eschewing strict conventions, and often targeted at prototyping an idea within a short period of time. Participants are therefore not required to have coding skills in order to make meaningful contributions. An example of an impactful project that did not focus on coding is Open Brain Consent (Open Brain Consent working group, 2021). This project developed consent form templates for the collection and sharing of human neuroimaging data, incorporating data protection standards such as the General Data Protection Regulation (GDPR) of the European Union. The consent forms can be used in ethics approval procedures to ensure that the collected data is shareable, while the participants' privacy is protected.

Unlike some traditional hackathons, there is no competitive element to Brainhacks. The focus is on collective and community-driven work, making the events more welcoming for inexperienced participants. Neither the level of completeness nor the publication potential determine the success of a project. Instead, Brainhacks emphasize the value of collaborating, exploring unconventional ideas, group thinking and building tools that benefit the community. Exemplary of these values is a project that originated at the first Brainhack in 2012: The Brain Catalogue provides magnetic resonance (MR) brain images of a range of different species and allows multiple users to view and segment them on the web simultaneously. Its successor, BrainBox, has evolved to enable real-time collaborative segmentation of any MR image accessible online [START_REF] Heuer | Open Neuroimaging Laboratory[END_REF]. BrainBox has been used in many subsequent Brainhack projects, research collaborations, and science outreach events. Similarly, braindr fosters citizen science while solving visual quality control for massive datasets [START_REF] Keshavan | Combining Citizen Science and Deep Learning to Amplify Expertise in Neuroimaging[END_REF]. This app enables anyone to contribute to scientific progress by swiping left or right on brain images to classify them as clean or corrupted. The project originated from a hackathon in 2017, was extended in a Brainhack project in 2018, and recently led to the development of the extensible SwipesforScience citizen science template.

Many Brainhack projects take on a life of their own and grow beyond a single event. The open science approach embraced by the Brainhack community makes it easier for anyone to contribute to, or take the lead in pushing projects forward beyond their initial creation. For example, the Autism Gradients project, exploring the cortical hierarchy in individuals with autism, was conceptualised at Brainhack Global 2016. It was subsequently picked up by another group who expanded the original idea and invited the initial Brainhack team to collaborate. This resulted in a peer-reviewed publication [START_REF] Hong | Atypical functional connectome hierarchy in autism[END_REF], multiple follow-up projects, two exchange grants, and international workshops (Autism workshop at INSAR and gradient workshop at OHBM) on the same topic. Another example is the development of Nighres, a Python package for processing high-resolution neuroimaging data. The initial project spanned two Brainhacks in 2016, and resulted in a toolbox that made algorithms for layer-specific analysis of the cortex easier to install and use. This sparked the development of the full Nighres package, with a broader range of functions and various contributors across several Brainhacks. Nighres has been presented in a peer-reviewed publication [START_REF] Huntenburg | Nighres: processing tools for high-resolution neuroimaging[END_REF], is actively maintained, and has been used and cited in multiple studies. Some projects transcend the domain of brain sciences; for example, DueCredit is a project promoting citable code that emerged at Brainhack OHBM 2015 and is now used in molecular dynamics, geophysics, and other sciences. Many more projects have been part of Brainhacks over the years, attracting users and developers and evolving together with the community. An expanding list lives in the accompanying Jupyter Book.

All these examples highlight how the project-oriented approach of Brainhacks encourages active participation and interdisciplinary collaboration that can reach beyond a single event. The projects directly benefit participants, who can explore new ideas in a stimulating setting, leverage their projects for career advancement, and gain new skills by collaborating with experienced researchers and developers. Brainhack projects also contribute to the progress of the wider neuroscience community by fostering exchange across scientific silos, resulting in multidisciplinary tools, community-driven guidelines and reference data, as well as traditional publication output.

Learning by Doing

Alongside projects, educational activities lie at the heart of Brainhacks. Such activities include informal teaching between project teammates, theoretical discussions in self-organized groups, unconference presentations, and structured workshops on a particular tool or topic. A recent format for major Brainhack events is the TrainTrack, entirely education-focused sessions that run in parallel with project work (Fig. 1A-B). This format lowers the entry barrier for new participants, enabling them to build relevant skills and familiarize themselves with the structure and environment of a Brainhack before diving into their first project. The variety of educational approaches supports different ways of learning. Furthermore, the informal nature of these activities empowers participants to be proactive about learning and asking for help.

Brainhack instructors strive to share their materials with the scientific community, including recorded presentations, slide decks, or interactive tutorials. For example, all the materials developed for the TrainTrack of OHBM Brainhack 2020 have been made publicly available under a permissive license so as to encourage reuse, redistribution, and reproduction of the content. Educational content developed for Brainhacks covers a range of topics including analytical and statistical methods (e.g., machine learning, data preprocessing), reproducible workflows (e.g., automated pipelines, automated data standardization, version control, software containers), and other relevant concepts (e.g., preregistration, p-hacking). Brainhacks represent an ideal place to showcase neuroscientific tools in the form of presentations or training sessions. These sessions are designed to be hands-on and interactive as they typically feature small groups having active discussions. Participants are explicitly encouraged to adopt what they learned at a Brainhack event to their own context and to improve the teaching material with their own ideas. Skills learned at Brainhacks are not constrained to those of a technical nature; the event format provides a unique opportunity for early career researchers to develop transferable skills such as teamwork and leadership. Project teams are often interdisciplinary, allowing participants to practice communicating beyond their own field. Everyone is encouraged to propose and lead their own projects, and the informal structure of the events often empowers more junior participants to also take on a leadership role. The growing pool of training materials provides a ready route to extend teaching opportunities to any member of the community, including trainees. Such experiences are rare for early career researchers, but crucial for their advancement given that they can potentially mold future interests, boost the quality of their research, and widen their scientific horizons. Altogether, the broad range of scientific and professional training opportunities equips participants with a skill set that can be applicable across many domains and career stages and may therefore open up a greater range of career opportunities.

Open Science & Collaborative Coding

Despite the increasingly central role of programming in neuroscience research, formal training in coding is not common in the neuroscience curriculum. In addition, code is seldom shared across more than a few labs, and too often read and executed by only a single individual. As a result, many scripts and workflows are hard to reuse and share and may contain undiscovered errors [START_REF] Merali | Computational science[END_REF].

By putting cross-disciplinary collaboration at its heart, Brainhacks have brought awareness to the need for usability, reusability, and long-term maintenance of tools. This comes with a shift of efforts, from individuals creating tools for their own needs to a community actively contributing to an existing resource, solving the aforementioned issues. Practices such as writing good code and documentation, improving code readability, performing basic version control, working collaboratively on a codebase on GitHub, GitLab, or BitBucket, and using appropriate open licenses have become essential within the community. These open practices and tools facilitate community-driven development and ensure that tools are available to all researchers, fostering global inclusivity. Brainhacks have highlighted the utility of producing a variety of research deliverables other than scientific papers (such as software, tutorials, workflows, and datasets), a concept that is increasingly endorsed by publishing venues such as F1000, RIO, eLife, Aperture, and others over the years.

Mastering collaborative programming skills enables Brainhack participants to contribute to open research objects which impact the wider scientific community. It can also make them more efficient at conducting their own research; for example, skills such as version control can be transferred to their own research group and foster more seamless collaboration amongst lab members. The wider neuroscience community benefits from the creation of transparent, reproducible tools, and from researchers equipped with the skills to maintain and extend them.

A Focus on Reproducibility

In line with their open, transparent, and collaborative nature, Brainhacks promote increased awareness of the importance of reproducible practices that integrate easily into research workflows. In addition to the coding practices mentioned above, an important aspect of reproducibility is data sharing. Public datasets are featured extensively in Brainhack projects and training sessions, since they are ideal for testing out new ideas or learning how to use a new tool.

First-time users thus experiment with these datasets and related tools under the guidance of expert users, which lowers the barrier to working with public data in the future. This approach establishes open data sharing as a standard practice and teaches participants how to curate their own data and metadata to make them accessible and reusable by others.

Resources that help researchers handle their data in a reproducible fashion are integral to Brainhacks; many of these have been introduced to the Brainhack community through structured efforts from the Center for Reproducible Neuroimaging Computation [START_REF] Kennedy | Everything Matters: The ReproNim Perspective on Reproducible Neuroimaging[END_REF]. For example, many projects and trainings use DataLad, a tool that not only lets participants versioncontrol their own data, but also helps them find, access, share, and work with increasingly large publicly available datasets. Similarly, a growing number of projects build on the Brain Imaging Data Structure (BIDS, [START_REF] Gorgolewski | The brain imaging data structure, a format for organizing and describing outputs of neuroimaging experiments[END_REF], a community standard for the organization of brain imaging data and metadata founded with the International Neuroinformatics Coordinating Facility (INCF). Introducing participants to data standards, such as BIDS, in the environment of a Brainhack allows them to experience the benefits of a unified data organization and provides them with the skillset to use these formats in their own research. Additionally, past Brainhacks have highlighted best practices in neuroimaging data analysis as defined by the Committee on Best Practice in Data Analysis and Sharing (COBIDAS guidelines for MRI as well as for EEG and MEG). By creating a scientific culture around open and standardized data, metadata, and methods, as well as detailed documentation and reporting, Brainhacks promote fundamental building blocks of a more efficient and reliable scientific research process.

Building and Bridging Communities

All aspects of Brainhacks discussed above build upon an active commitment to a diverse, inclusive, and non-hierarchically organized community. This commitment has been formalized in a Code of Conduct that aims to ensure a safe and welcoming environment for participants from all backgrounds. The Code of Conduct is discussed at the beginning of a Brainhack and adherence is monitored throughout the event. There have also been dedicated efforts to raise awareness about equity, diversity and inclusivity, such as a recent panel discussion at Brainhack Ontario 2020. While far from perfect or bias-free, we feel that the Brainhack community itself is continuously growing more diverse in terms of race, ethnicity, gender identity and expression, sexual orientation, career stage, and other aspects of personal background and identity. The enthusiastically inclusive culture helps members hold each other to a standard of mutual respect that empowers individuals from typically underrepresented groups to claim their space and take on central roles in the community.

Brainhacks are designed to promote intensive networking. The project-oriented and decentralized setting puts participants on an equal footing regardless of backgrounds and career stage. Unconferences provide a unique opportunity for people interested in the same topic to meet and discuss, sometimes sparking new collaborations. Working in small groups during projects, workshops, and unconferences over the course of several days encourages frequent interactions that often go deeper than relatively short encounters at traditional conferences. These interactions contribute to building lasting collaborations that bridge across disciplines, research contexts, career stages, and geographical borders. Sometimes they lead to job opportunities, grant proposals, new ideas, and new projects. Often they turn into friendships. We firmly believe that growing this diverse community and insisting on a culture of collaboration and inclusivity has untold benefits for the retention and well-being of all scientists doing brain research.

In addition to year-round locally organized Brainhacks, Brainhack Global has emerged as a major yearly initiative that has sparked numerous simultaneous events around the world (Fig. 1C-D).

The focus on open collaboration through virtual spaces throughout the years meant that the community had the infrastructure, knowledge and motivation to go fully virtual in 2020, accommodating restrictions due to the COVID-19 pandemic, budget constraints, and increased awareness of the climate cost of travel. The general format of project-oriented, communitybuilding events has gained traction in the field of neuroscience and beyond. Large initiatives such as the Human Brain Project, Neurodata Without Borders, and the Society for the Improvement of Psychological Science have also chosen hackathons as a primary work format. Summer schools like the ABCD-ReproNim course, NeuroHackademy, the ABCD Workshop and the Brainhack School are based on the same principles as the original Brainhack events. Brainhacks have been organized with other communities such as Network Neuroscience, thereby forming a bridge with those communities. Many brainhack community members also play active roles in like-minded initiatives such as the Open Science Special Interest Group of OHBM or Neuromatch Academy, amongst others. Thus, a community of individuals and practices has emerged that transports the benefits and values of the brainhack format far beyond any individual event or organization. Brainhacks are spreading to an increasingly wide community, as their nonhierarchical, self-organising structure enables individuals to organize a Brainhack anywhere in the world (Fig. 1C), while events are kept financially accessible. An online community with over 4000 members and 500 channels uses the Mattermost messaging platform to provide continuity across time and space. Posts range from questions about a specific resource, to job openings and discussions about research ethics; and anyone can join regardless of having attended a Brainhack. The community evolves with every new member and their ideas, and many members become local advocates for the principles of open and collaborative science in their home institutions.

Conclusion & Future Directions

Brainhacks complement traditional academic settings and offer additional opportunities for participants to achieve their scientific and professional goals. The focus on building a community that promotes open science and inclusivity has naturally led to better coding practices, more reproducible methods, accelerated knowledge dissemination and ample opportunities for collaboration. Brainhacks differ from many scientific meetings, as they are more project-oriented, less formal, and have broadened the notion of what constitutes successful outputs in science. Within neuroscience, Brainhacks have the potential to evolve beyond their initial focus on neuroimaging data and include more projects on theory, hardware, and different types of neural data. With a growing global community, and an iteratively improving format (Fig. 1D), Brainhacks provide a successful template that can be extended to other scientific fields. Nearly a decade of successful Brainhacks have already brought about positive change for individual researchers and the field as a whole, in the form of improved skills, reusable resources, new collaborations, and a diverse and inclusive community.

Hackathon

The term hackathon is a portmanteau of ''hacking'' and ''marathon.'' Traditionally, it is an event in which people and teams gather to collaboratively work on projects over the course of multiple days. These events often historically feature competitions between teams. Brainhacks instead emphasize collaboration over competition.

Hacking

In this context, hacking does not indicate trying to break into computer systems by breaching security. Instead, it refers to tinkering with a system to better understand its working and subsequently laying a foundation for its advancement.

Unconference

This refers to a short session in which participants either present their research or prompt a discourse on any topic of interest in an informal setting.

The content of an unconference may be decided impromptu and is often inspired by ongoing team discussions during the course of the Brainhack.

TrainTrack

A series of educational workshops that run in parallel with the projects enabling attendees to acquire specific skills during the course of their projects. The content of TrainTrack ranges from tutorials teaching skills useful to successfully navigate diverse projects in the Brainhack (such as code version control using Git) to more generalized education relevant to neuroscientific research (like MATLAB, BIDS, etc.).

Terms highlighted here either are defined in a unique way for this community or are important for appreciating the text. For detailed definitions, see Cameron [START_REF] Craddock | Brainhack: a collaborative workshop for the open neuroscience community[END_REF] and the Brainhack Jupyter Book Glossary (http://brainhack.org/brainhack_jupyter_book/glossary.html).

Figure 1 .

 1 Figure 1. Brainhack in time and space. (A) Anatomy of a Brainhack shows how the components of Brainhack events relate to overarching topics that lead to scientific and professional opportunities in neuroscience. (B) Timeline of a Single Event describes the typical daily schedule throughout a brainhack. Project work and educational activities occur simultaneously interspersed with unconferences. (C) Brainhack Cartography displays cities across the world that have hosted a Brainhack. Marker color indicates the year in which each city hosted its first Brainhack, and marker size indicates the number of events hosted in each city. (D) The Brainhack Timeline displays the number of events per month since the inaugural Brainhack in September 2012, along with notable happenings throughout the years. Months are only denoted in the first year with a single letter; this ordering is repeated for all subsequent years. See the latest version of this figure here.

Figure 1 .

 1 Figure 1. Brainhack in time and space (A) Anatomy of a Brainhack shows how the components of Brainhack events relate to overarching topics that lead to scientific and professional opportunities in neuroscience. (B) Timeline of a single event describes the typical daily schedule throughout a Brainhack. Project work and educational activities occur simultaneously interspersed with unconferences. (C) Brainhack cartography displays cities across the world that have hosted a Brainhack. Marker color indicates the year in which each city hosted its first Brainhack, and marker size indicates the number of events hosted in each city. (D) The Brainhack timeline displays the number of events per month since the inaugural Brainhack in September 2012, along with notable happenings throughout the years. Months are only denoted in the first year with a single letter; this ordering is repeated for all subsequent years. See the latest version of this figure at http://brainhack.org/brainhack_jupyter_book/neuroview_figure.html.

Table 1 .

 1 Glossary of selected terminology

	Term	Definition
	Open Science	Movement and practices within science aimed at
		increasing the transparency, accessibility,
		diversity, and inclusivity of scientific practices and
		output. This is often reflected in open science
		practices, such as publishing open access
		manuscripts; making research data findable,
		accessible, interoperable, and reusable (FAIR);
		and open sourcing code and software, etc.

Acknowledgments

The present manuscript is part of a growing community effort to collate BrainHack related insights and expertise into a Jupyter Book that will serve as a centralized set of resources for the community; we acknowledge all the individuals who contributed and will make ongoing contributions to these resources. A pre-print version of the present manuscript is available as part of the Jupyter Book here. Moreover, we would like to acknowledge all Brainhack organizers, supporters, presenters, and participants for their contribution to growing and maintaining this community. The benefits described in this manuscript would not be possible without them. We also thank all institutions, labs, and organizations who have helped this community grow, meet in stimulating environments and add an excellent educational resource pool and agenda. With an expanding community, Brainhack's support network keeps growing and we thank all labs and individual researchers for their dedication and expertise offered to this community (see Supplementary Materials for a full list of individual acknowledgments; an updated list will be maintained in the Jupyter Book here).

Declaration of Interests

Anisha Keshavan is an employee of Octave Bioscience


Clara Locher
email: clara.locher@chu-rennes.fr

David Moher

Ioana A Cristea

Florian Naudet

Didier Raoult

During the COVID-19 pandemic, the rush to scientific and political judgments on the merits of hydroxychloroquine was fuelled by dubious papers which may have been published because the authors were not independent from the practices of the journals in which they appeared. This example leads us to consider a new type of illegitimate publishing entity, "self-promotion journals" which could be deployed to serve the instrumentalisation of productivity-based metrics, with a ripple effect on decisions about promotion, tenure, and grant funding, but also on the quality of manuscripts that are disseminated to the medical community and form the foundation of evidence-based medicine.

was delegated to an Associate Editor [8]. The journal has not implemented an Open Peer Review; we asked the authors to share the peer reviews, but to no avail. The team published four other papers (see Supplementary table 1), which were below general research standards (e.g. ICH guidelines, relevant reporting guidelines), in journals where members of the team were part of the editorial board or indeed editors-in-chief. Among these, a so-called meta-analysis on the therapeutic efficacy of hydroxycholoroquine [START_REF] Million | Clinical Efficacy of Chloroquine derivatives in COVID-19 Infection: Comparative meta-analysis between the Big data and the real world[END_REF] was published in New Microbes and New Infections (NMNI), and was at odds with all best practices in the field of meta-analyses (for instance it included a withdrawn preprint and it pooled different outcomes extracted from the same studies). It also received 7 critical comments on Pubpeer. As the reporting did not respect the PRISMA statement, the methods and results were not reproducible. NMNI's editor-in-chief also works for Raoult. A further six associate editors of the journal also work for Raoult. The scope of the journal is to serve the field "as a peerreviewed, open access journal for rapid dissemination of the latest research, with a particular focus on new genomes, new microbes, and new technology applied to the diagnosis of infectious/tropical diseases" [10], an unusual definition for publishing a meta-analysis on a therapeutic issue.

A highway to publication

In its 2017 report on Didier Raoult's unit, the French "Haut Conseil de l'évaluation de la recherche et de l'enseignement supérieur" (High Council for the Evaluation of Research and Higher Education), an independent authority that inspects French research units, noted that the "creation of this journal, which serves to publish papers rejected by other journals, is a somewhat desperate initiative". A careful inspection of the NMNI publication output (see Supplementary table 2) showed that the journal, created in 2013, had published 728 papers up to June the 25th 2020. Of these, 231 (32%) were published by at least one author on the current editorial board, 226 (31%) by one editor from Marseille, and 235 (32%) by Didier Raoult, who is not part of the editorial board.

Computing the proportion of contributions published in a journal by any single author can provide a very rough index to spot problematic journals. We explored scientific journals specialized in infectious diseases selected from the National Library of Medicine (NLM) Catalog using NMNI MeSH terms (see Supplementary Methods). Among these 789 journals, 239 published at least 50 papers between 2015 and 2019.

Figure 1 (panel A) shows this indicator for the most prolific author for each journal in relation

to the volume of the journal's published output. NMNI is a strikingly clear outlier, with both a large proportion of published papers by the same author (36.6%) and a large publication volume over the last 5 years (N=598 articles). A sensitivity analysis was computed with 'journal articles' only (using the NCBI publication type) in order to exclude contributions such as editorials, news items, or comments. The results of this sensitivity analysis were consistent with those for all articles. However, the case of Nature Review Microbiology raises the risk of misclassification bias as this author only contributes to "News & Comment" or "In Brief" section articles and should not have been flagged in the sensitivity analyses. We therefore think that a large proportion of papers published by one author could be used as a red flag -to identify journals that are suspected of dubious editorial practice -but deserves a subsequent qualitative investigation of the journal.

We therefore explored the 12 journals with an Index value > 10.2% corresponding to the 95 th percentile threshold. The key features of these journals are shown in Table 1. Figure 1 (panel B), shows the distribution of the index for each author, among the 5 journals ranking respectively at the minimum, 1st quartile, median, 3rd quartile and maximum, over the 5 years, by year. Details for the whole sample of journals are presented in Supplementary Figure 1. NMNI appears consistently as an outlier over the past five years.

Self-promotion journals: a new type of illegitimate publishing entity?

Of course, to avoid publication bias it is expected that all researchers transparently submit all outputs of their research. However, publications are presumed successful if the research is sound enough and a rigorous, unbiased peer-review actually took place. Elsevier's general policies explicitly state that "the editor must not be involved in decisions about papers which s/he has written him/herself, or which have been written by […] colleagues." In application of this policy, more than 40% of published papers should not have been handled by Michel Drancourt, the Editor-in-Chief. This is a very large proportion for the editor, supposed to be responsible for the whole journal content. It is also expected that an editor of a journal should publish editorials delineating the agenda of the journal. However, a high proportion of this type of article raises questions about the plurality of viewpoints and the independence of the journal. A similar case was described in 2008 with Elsevier's theoretical physics journal Chaos, Solitons and Fractals, whose Editor in Chief M.S. El Nashie published 332 papers in the journal as an author [START_REF]Publish Your Work The Easy Way[END_REF].

In contrast with the El Nashie case, NMNI appears to prioritize the productivity of a larger network of editors/authors. We suggest that there is (i) a consistently large proportion of papers published by a group of authors, (ii) particularly in the presence of relationships between the editors and these authors, and (iii) publication of low-quality research all key characteristics of a new type of illegitimate publishing entity, i.e. "self-promotion journals".

The first two criteria have the advantage of being simple and easy to obtain for a given journal, but they are likely to detect only the more problematic journals deserving further investigation. Coincidentally, Dorothy Bishop in a blog post reported a similar analysis for the addiction subfield of psychology [START_REF] Deevybee | Percent by most prolific' author score: a red flag for possible editorial bias[END_REF] and found that 3 of 99 journals had more than 8% by the most prolific author. These 2 preliminary studies -in the field of infectious diseases and psychology-are being extended in a comprehensive survey of biomedical journals to refine the description of such dubious editorial practice [START_REF] Scanff | Nepotistic journals': a survey of biomedical journals[END_REF]. This survey showed that prolific authors were often associated with shorter lags between submission and publication which reinforces the idea of "Self-promotion journals". "Self-promotion journals" could be deployed to game productivity-based metrics, with a ripple effect on decisions about promotion, tenure, and grant funding. COVID-19 has clearly shown the detrimental effects of such practices: authorizations issued in March 2020 for chloroquine and hydroxychloroquine for emergency use, were suspended by the Food and Drug Administration three months later [START_REF]Coronavirus (COVID-19) Update: FDA Revokes Emergency Use Authorization for Chloroquine and Hydroxychloroquine[END_REF]. Didier Raoult implicitly acknowledged using his publication capacity as clout in his own research ecosystem, even threatening to go on strike over the signing of his own publications [START_REF] Vaudoit | IHU méditérranée infection -Le défi de la recherche et de la médecine intégrées[END_REF]. Indeed, in France, hospitals are rewarded according to the volume of publications. Various initiatives, including the Declaration on Research Assessment (DORA) [START_REF] Moher | Assessing scientists for hiring, promotion, and tenure[END_REF], warn against the use of incentives based on scientific productivity, which can easily be gamed and could be related to a kind of natural selection of bad science [START_REF] Smaldino | The natural selection of bad science[END_REF]. In the case of NMNI, it is not possible to ascertain the integrity or quality of the peer review process because the journal does not have an open peer-review policy.

Possible consequences of dubious editorial practice on evidence-based medicine

A cornerstone of evidence-based medicine is the use of the best available evidence that have to be obtained from trustworthy findings. By carefully managing the peer-review process, editors are responsible for the quality of manuscripts that are accepted and consequently disseminated to the medical community [18]. The International Committee of Medical Journal Editors (ICMJE) explicitly states that "in return for the altruism and trust that make clinical research possible, the research enterprise has an obligation to conduct research ethically and to report it honestly" [START_REF] Deangelis | Clinical Trial RegistrationA Statement From the International Committee of Medical Journal Editors[END_REF]. In our opinion, such relationships between authors and editorial board members could (i) facilitate selective publication of clinical results driven by cronyism rather than the peer-review process, and (ii) facilitate publication of studies with high risk of bias, studies underpowered or misreporting/selective reporting research. Once published, these low-quality positive studies disrupt evidence-based medicine at several levels (Table 2).

Rewarding integrity instead of productivity

Authorship is an important component of scientific integrity, it entails responsibilities [20] and any doubts on actual authorship call the trustworthiness of the science into question.

Publishers such as Elsevier can easily screen their catalogue using the indicators we propose to detect outliers such as NMNI and to audit the specific processes in these ACCEPTED MANUSCRIPT -CLEAN COPY journals. Independent researchers can explore and refine the index we propose on the basis of an exhaustive study across a broad range of scientific journals to explore its validity and possible variations according to the field. It is indeed time to reward scientific integrity instead of productivity, institutions, journals, or publishers. Following the DORA initiative [START_REF] Moher | Assessing scientists for hiring, promotion, and tenure[END_REF] or the Hong Kong principles for assessing researchers [21] affords a good opportunity to determine which values matter: productivity-based metrics, research quality, or the societal consequences of research.

Figure 1. Description of the contributions of prolific authors across 239 infectious disease journals that published at least 50 papers between 2015 and 2019

Panel A: Percentage of contributions of the most prolific authors and numbers of published outputs for all journals (2015-2019). Grey lines correspond to the numbers of articles signed by the most prolific authors. The 5 journals presented in panel B are identified by colours.

Panel B: Distribution of the contributions of each author, across the 5 journals ranking respectively at the minimum, 1st quartile, median, 3rd quartile and maximum (2015-2019) All queries for PubMed extraction were performed using the easyPubMed library in R. The code to reproduce this analysis is available here: https://osf.io/dqvea/.

Table 1. Description of journals with a 5-year index > 95 percentile. Percentages of contributions by the most prolific authors and numbers of published outputs were calculated for all articles published during the 2015-2019 period and for articles labeled 'journal article'. This sensitive analysis allows the exclusion of outputs such as news items, comments, editorials and letters.

Journal

Publisher / country Low-quality studies (e.g. studies with high risk of bias, or underpowered studies) lead to biased intervention effect estimates and increased between-trial heterogeneity in meta-analyses.

Decreased confidence in trusted sources

The overabundance of low-quality publications with conflicting data may be particularly confusing for patients, which may decrease confidence in trusted sources of health information. Misdirected research Once published, low-quality positive studies may distort the rationales of future clinical trials, leading to waste of effort, time, and resources, and are hence unethical to patients who participated in these clinical studies. Inflate the influence of Scientists that do not respect clinical best practices A larger total number of scientific publications is perceived as giving a legitimacy to being recognized as an expert, regardless of the quality of publications. By promoting some authors by accepting large numbers of publications, editors risk advancing some pseudo-experts. Influence patient care decisions Patients are often not trained to detect bias and low-quality studies and could be influenced by low-quality studies.

Influence medical practice

Low-quality positive studies may be used to support prescriptions of drugs with limited benefit/possible harms. Moher

Table 2 :

 2 Possible consequences of dubious editorial practice on evidence-based medicine.

	Increase publication
	bias and 'garbage in,
	garbage out'

table 2 .

 2 D, Bouter L, Kleinert S, et al. The Hong Kong Principles for assessing researchers: Fostering research integrity. PLOS Biology 2020;18:e3000737. doi:10.1371/journal.pbio.3000737 Contribution of the editorial board and Didier Raoult in the New Microbes and New Infections (June 25, 2020)

	A	95th percentil	B	60%	min Case Rep Infect Dis	60%	Q1 AIDS Patient Care STDS
					40%		40%
					20%		20%
					0%		0%
		Posit Aware				2015 2016 2017 2018 2019		2015 2016 2017 2018 2019
	Number of articles 2015-2019	AIDS Rev Curr Med Mycol Drug Resist. Updat_ Eur J Microbiol Immunol (Bp) Int J MCH AIDS J Arthropod Borne Dis Nat. Rev. Microbiol_ Trop Parasitol	New Microbes New Infect	0% 20% 40% 60%	median 2015 2016 2017 2018 2019 Health Secur	0% 20% 40% 60%	Q3 2015 2016 2017 2018 2019 Sex Transm Infect
				40 articles
		Commun Dis Intell	20 articles		New Microbes New Infect
					60%	max
				10 articles	40%
		Klin. Mikrobiol. Infekc. Lek_	5 articles	20%
	0%	10%	0% ACCEPTED MANUSCRIPT -CLEAN COPY 20% 30% 40% 2015 2016 2017 2018 2019
		Percentage of contribution of the most prolific author [2015-2019]

('New microbes and new infections')[Journal]) AND (Drancourt[au] OR Fournier[au] OR De Lamballerie[au] OR Eremeeva[au] OR Mediannikov[au] OR Opota[au] OR Polkinghorne[au] OR Rolain[au] OR Yang[au] OR Raoult[au])

Moher D. Along with the privilege of authorship come important responsibilities. BMC Med 2014;12:214. doi:10.1186/s12916-014-0214-2

using the RISmed library in R. The code is available here: https://osf