arXiv subjects

Alain Pautrat

Publications and source records attributed to Alain Pautrat.

At least 19 recordsLinked to original sources

arXiv2026

Atomic structure and formation mechanism of a newly discovered charge density wave in the m=2 monophosphate tungsten bronze

The $m$=2 member of the monophosphate tungsten bronze family has been considered the only one in the family without an electronic instability at low temperature. In this paper, we report the discovery of a charge density wave phase in this compound, with a transition temperature of 290 K and an incommensurate modulation vector \textbf{q}=0.245\textbf{b*}+ $\upxi$\textbf{c*}. The presence of this new phase is confirmed by diffraction and resistivity measurements. Pre-transitional dynamics are investigated using diffuse and inelastic x-ray scattering, revealing a clear Kohn anomaly. We analyze both structural and electronic contributions to the phase transition, providing a comprehensive picture of the mechanism driving this newly identified instability.

cond-mat.str-el↗

arXiv2025

Hyperuniform patterns nucleated at low temperatures: Insight from vortex matter imaged in unprecedentedly large fields-of-view

Hyperuniform patterns present enhanced physical properties that make them the new generation of cutting-edge technological devices. Synthesizing devices with tens of thousands of components arranged in a hyperuniform fashion has thus become a breakthrough to achieve in order to implement these technologies. Here we provide evidence that extended two-dimensional hyperuniform patterns spanning tens of thousands of components can be nucleated using as a template the low-temperature vortex structure obtained in pristine Bi2Sr2CaCu2O8 samples after following a field-cooling protocol.

cond-mat.supr-con↗

arXiv2025

Tracing the horizon of tetragonal-to-monoclinic distortion in pressurized trilayer nickelate La4Ni3O10

The crux of understanding the superconducting mechanism in pressurized Ruddlesden-Popper nickelates hinges on elucidating their structural phases. Under ambient conditions, the trilayer nickelate La4Ni3O10 stabilizes in a twinned monoclinic structure with space group P21/c. Upon heating, it undergoes a structural transition to the tetragonal I4/mmm phase at Ts ~ 1030 K, while a second transition associated with the onset of density-weave (DW) ordering emerges upon cooling below TDW ~ 135 K. Here from pressure-temperature x-ray diffraction on high quality flux-grown single crystals we demonstrate a direct tetragonal-to-monoclinic transition without an intermediate orthorhombic Bmab phase. Ab initio density-functional theory calculations as a function of pressure corroborate the experimental observations. The tetragonal-to-monoclinic transition unfolds as the formation of a two-fold superstructure, as evidenced by the emergence of commensurate superlattice reflections and can be progressively suppressed from 1030 K down to 20 K under 14 GPa. Notably, from XRD we establish the observation of weak incommensurate satellite reflections associated with the DW ordering in flux-grown samples, as previous findings were confined only to crystals grown by the floating-zone technique. This is further reinforced by Raman spectroscopy that reveal the emergence of additional phonon modes below 130 K, concomitant with the onset of the incommensurate DW state.

cond-mat.str-el↗

arXiv2020

Magnetodielectric coupling and multi-blocking effect in the Ising-chain magnet Sr2Ca2CoMn2O9

We have demonstrated magnetodielectric (MD) coupling in an Ising-chain magnet Sr2Ca2CoMn2O9, via detailed investigation of ac susceptibility and dielectric constant as a function of temperature, magnetic field and frequency. Sr2Ca2CoMn2O9 consists of spin-chains, made of the regular stacking of one CoO6 trigonal prism with two MnO6 octahedra. The (Co2+ Mn4+ Mn4+) unit stabilizes a (up-down-up) spin-state along the chains which are distributed on a triangular lattice. This compound undergoes a partially disordered antiferromagnetic transition at TN ~ 28 K. The dielectric constant exhibits a clear peak at TN only in presence of an external magnetic field (above 5 kOe), evidencing the presence of MD coupling, which is further confirmed by field-dependent dielectric measurements. We argue that spatial inversion symmetry can be broken as a result of exchange-striction along each spin chain, inducing uncompensated local dipoles. At low temperatures, a dipolar relaxation phenomenon is observed, bearing strong similarities with the blocking effect typical of the spin dynamics in this compound. Such a spin-dipole relationship is referred to as a multi-blocking effect, in relation with the concept of magnetodielectric multiglass previously introduced for related materials.

cond-mat.str-el↗

arXiv2020

Weak localization competes with the quantum oscillations in a natural electronic superlattice: the case of Na$_{1.5}$(PO$_2$)$_4$(WO$_3$)$_{20}$

We report an investigation of the combined structural and electronic properties of the bronze Na$_{1.5}$(PO$_{2}$)$_{4}$(WO$_{3}$)$_{20}$. Its low dimensional structure and possible large reconstruction of the Fermi surface due to charge density wave instability makes this bulk material a natural superlattice with a reduced number of carriers and Fermi energy. Signatures of multilayered 2D electron weak localization are consequently reported, with an enhanced influence of quantum oscillations. A crossover between these two antagonistic entities previously observed only in genuine low dimensional materials and devices, is shown to occur in a bulk crystal due to its hidden 2D nature.

cond-mat.str-el↗

arXiv2019

Enhancement of magnetodielectric coupling in 6H-perovskites Ba3RRu2O9 for heavier rare earth cations (R=Ho,Tb)

The role of rare-earth (R) ions on the magnetodielectric (MD) coupling is always intriguing and markedly different for different systems. Although many reports are available concerning this aspect in frustrated 3d-transition metal oxides, no such reports exist on higher d (4d/5d)-orbital based systems due to the rare availability of highly insulating 4d/5d-systems. Here, we systematically investigated the magnetic, dielectric, ferroelectric and magnetodielectric behavior of the 6H-perovskites Ba3RRu2O9 for different R-ions, namely, R= Sm, Tb and Ho, which magnetically order at 12, 9.5 and 10.2 K respectively. For R=Tb and Ho, the temperature and magnetic-field dependent complex dielectric constant traces the magnetic features, which manifests MD coupling in this system. A weak magnetic-field (H) induced transition is observed for ~30 kOe, which is clearly captured in H-dependent dielectric measurements. No MD coupling is observed for Ba3SmRu2O9. The MD coupling is enhanced by a factor of 3 and 20 times for R=Tb and Ho, respectively, when compared to that of the Nd-counterpart. These results evidence the gradual enhancement of MD coupling with the introduction of heavier R-ions in this series, which is attributed to their larger moment values. A weak improper ferroelectricity is documented for Ho-member, which is absent for Nd. Our investigation establishes dominating 4d(Ru)-4f(R) magnetic correlation in this series for the heavier R-members.

cond-mat.str-el↗

arXiv2019

Designing of a magnetodielectric system in hybrid organic-inorganic framework, a perovskite layered phosphonate MnO3PC6H4-m-Br.H2O

The research on multiferrocity and magnetoelectric coupling in metal-organic system is rare. Very few hybrid organic-inorganic frameworks (HOIF) exhibit direct magnetoelectric coupling (coupling between spins and dipoles) and also restricted to particular COOH-based system. We show how one can design a hybrid system to obtain such coupling based on the rational design of the organic ligands. The layered phosphonate, MnO3PC6H5.H2O, consisting of perovskite layers stacked with organic phenyl layers, is used as a starting potential candidate. To introduce dipole moment, a closely related metal-phosphonate, MnO3PC6H4-m-Br.H2O is designed. For this purpose, this phosphonate is prepared from 3-bromophenylphosphonic acid that features one electronegative bromine atom directly attached on the aromatic ring in meta position, lowering the symmetry of precursor itself. Thus, bromobenzene moieties in MnO3PC6H4-m-Br.H2O induce a finite dipole moment. This new designed compound exhibits complex magnetism, as observed in layered alkyl chains MnO3PCnH2n+1.H2O materials, namely, 2D magnetic ordering around 20 K followed by weak ferromagnetic ordering below 12 K(T1) with a magnetic field (H)-induced transition around 25 kOe below T1. All these magnetic features are exactly captured in T and H-dependent dielectric constant, epsilon(T) and epsilon(H). This demonstrates direct magnetodielectric coupling in this designed hybrid and yields a new path to tune multiferroic ordering and magnetodielectric coupling.

cond-mat.mtrl-sci↗

arXiv2019

Unveiling the vortex glass phase in the surface and volume of a type-II superconductor

Order-disorder transitions between glassy phases are quite common in nature and yet a comprehensive survey of the microscopic structural changes remains elusive since the scale of the constituents is tiny and in most cases few of them take part in the transformation. Vortex matter in type-II superconductors is a model system where some of the experimental challenges inherent to this general question can be tackled by adequately choosing the host superconducting sample. For instance, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8 + \delta}$ is a type-II superconductor with weak point disorder that presents a transition between two glassy phases on increasing the constituents' (vortices) density. At low vortex densities, the impact of disorder produces the nucleation of a glassy yet quasi-crystalline phase, the Bragg glass. For high vortex densities the stable phase, coined as $\textit{vortex glass}$, was proposed to be disordered, but its structural properties have remained elusive up to now. Here we answer this question by combining surface and bulk vortex imaging techniques, and show that the vortex glass is neither a messy nor a hexatic phase: in the plane of vortices it presents large crystallites with positional correlations growing algebraically and short-ranged orientational order. However, no dramatic change in the correlation length along the direction of vortices is observed on traversing the order-disorder transformation.

cond-mat.supr-con↗

arXiv2019

Magnetism tailored by mechanical strain engineering in PrVO$_3$ thin films

Transition-metal oxides with an ABO$_3$ perovskite structure exhibit strongly entangled structural and electronic degrees of freedom and thus, one expects to unveil exotic phases and properties by acting on the lattice through various external stimuli. Using the Jahn-Teller active praseodymium vanadate Pr$^{3+}$V$^{3+}$O$_3$ compound as a model system, we show that PrVO$_3$ Néel temperature T$_N$ can be raised by 40 K with respect to the bulk when grown as thin films. Using advanced experimental techniques, this enhancement is unambiguously ascribed to a tetragonality resulting from the epitaxial compressive strain experienced by the films. First-principles simulations not only confirm experimental results, but they also reveal that the strain promotes an unprecedented orbital-ordering of the V$^{3+}$ d electrons, strongly favouring antiferromagnetic interactions. These results show that an accurate control of structural aspects is the key for unveiling unexpected phases in oxides.

cond-mat.mtrl-sci↗

arXiv2018

Room temperature biaxial magnetic anisotropy in La0.67Sr0.33MnO3 thin films on SrTiO3 buffered MgO (001) substrates for spintronic applications

Spintronics exploits the magnetoresistance effects to store or sense the magnetic information. Since the magnetoresistance strictly depends on the magnetic anisotropy of the system, it is fundamental to set a defined anisotropy to the system. Here, we investigate by means of vectorial Magneto-Optical Kerr Magnetometry (v-MOKE), half-metallic La0.67Sr0.33MnO3 (LSMO) thin films that exhibit at room temperature pure biaxial magnetic anisotropy if grown onto MgO (001) substrate with a thin SrTiO3 (STO) buffer. In this way, we can avoid unwanted uniaxial magnetic anisotropy contributions that may be detrimental for specific applications. The detailed study of the angular evolution of the magnetization reversal pathways, critical fields (coercivity and switching) allows for disclosing the origin of the magnetic anisotropy, which is magnetocrystalline in nature and shows four-fold symmetry at any temperature.

cond-mat.mtrl-sci↗

arXiv2018

Incipient spin-dipole coupling in a 1D helical-chain metal-organic hybrid

Low dimensional magnetic systems (such as spin-chain) are extensively studied due to their exotic magnetic properties. Here, we would like to address that such systems should also be interesting in the field of dielectric, ferroelectricity and magnetodielectric coupling. As a prototype example, we have investigated a one-dimensional (1D) helical-chain metal-organic hybrid system with a chiral structure which shows a broad hump in magnetic susceptibility around 55 K (Tmax). The complex dielectric constant exactly traces this feature, which suggests intrinsic magnetodielectric coupling in this chiral system. The dipolar ordering at Tmax occurs due to lattice-distortion which helps to minimize the magnetic energy accompanied by 1D-magnetic ordering or vice-versa. This experimental demonstration initiates a step to design and investigate hybrid organic-inorganic magnetic systems consisting of chiral structure towards ferroelectricity and magnetodielectric coupling.

cond-mat.mtrl-sci↗

arXiv2018

Magnetodielectric coupling in a Ru-based 6H-perovskite, Ba3NdRu2O9

A large spin-orbit coupling is a way to control strong magnetodielectric (MD) coupling in a higher d-orbital materials. However reports are rare on such compounds due to often leaky conductive behavior. Here, we demonstrate MD coupling in a Ru-based 6H-perovskite system, Ba3NdRu2O9. The rare-earth ion in a 6H-perovskite makes the system insulating enough to carry out MD investigation. The compound is ferromagnetically ordered below 24 K (TC), followed by another magnetic feature at T~ 17 K (T2). The dielectric constant clearly traces the magnetic ordering, manifesting a peak at the onset of TC, which is suppressed by the application of an external magnetic field (H). The results indicate the presence of MD coupling in this compound, which is further confirmed by the H-dependence of the dielectric constant. Interestingly, a cross-over of the sign of MD coupling is observed at T ~ T2. We conclude that two different mechanism controls the MD coupling which yields positive and negative coupling, respectively. Both mechanisms are competing as a function of temperature and magnetic field. This brings us a step closer to design and control the magnetodielectric effect in 6H-perovskites containing higher d-orbital elements.

cond-mat.mtrl-sci↗

arXiv2016

Giant Nernst effect in the incommensurate charge density wave state of P4W12O44

We report the study of Nernst effect in quasi-low dimensional tungsten bronze P4W12O44 showing a sequence of Peierls instabilities. We demonstrate that both condensation of the electronic carriers in the CDW state and the existence of high-mobility electrons and holes originating from the small pockets remaining in the incompletely nested Fermi surface give rise to a Nernst effect of a magnitude similar to that observed in heavy fermion compounds.

cond-mat.str-el↗

arXiv2016

Change of surface critical current in the surface superconductivity and mixed states of superconducting Niobium

A systematic study of irreversible magnetization was performed in bulk Niobium after different surface treatments. Starting with smooth surfaces and abrading them, a strong increase of the critical current is observed up an apparent limiting value. An impressive change of the critical current is also observed in the surface superconductivity (SSC) state, reaching values of the same order of magnitude as in the mixed state. We explain also the observation of strong SSC for magnetic field perpendicular to larges facets in terms of nucleation of SC along bumps of a corrugated surface.

cond-mat.supr-con↗

arXiv2016

Detailed investigation of the phase transition in K$_{x}$P$_4$W$_{8}$O$_{32}$ and experimental arguments for a charge density wave due to hidden nesting

Detailed structural and magnetotransport properties of the monophosphate tungsten bronze K$_{x}$P$_4$W$_{8}$O$_{32}$ single crystals are reported. Both galvanomagnetic and thermal properties are shown to be consistent with a charge density wave electronic transition due to hidden nesting of quasi - 1D portion of the Fermi surface. We also observe the enhancement of electronic anisotropy due to reconstruction of the Fermi surface at the Peierls transition. The resistivity presents a thermal hysteresis suggesting a first order nature characteristics of a strong coupling scenario. However, other measurements such as the change of carriers density demonstrate a second order Peierls scenario with weak coupling features. We suggest that the structural transition driven by the residual strain in the K - P - O environment is responsible for the resistivity hysteresis and modifes the Fermi surface which then helps the rise to the second order Peierls instability.

cond-mat.str-el↗

arXiv2015

Kondo effect goes anisotropic in vanadate oxide superlattices

We study the transport properties in SrVO3/LaVO3 (SVO/LVO) superlattices deposited on SrTiO3 (STO) substrates. We show that the electronic conduction occurs in the metallic LVO layers with a galvanomagnetism typical of a 2D Fermi surface. In addition, a Kondo-like component appears in both the thermal variation of resistivity and the magnetoresistance. Surprisingly, in this system where the STO interface does not contribute to the measured conduction, the Kondo correction is strongly anisotropic. We show that the growth temperature allows a direct control of this contribution. Finally, the key role of vanadium mixed valency stabilized by oxygen vacancies is enlightened.

cond-mat.mtrl-sci↗

arXiv2015

Two components for one resistivity in LaVO3/SrTiO3 heterostructures

A series of 100 nm LaVO3 thin films have been synthesized on (001)-oriented SrTiO3 substrates using the pulsed laser deposition technique, and the effects of growth temperature are analyzed. Transport properties reveal a large electronic mobility and a non-linear Hall effect at low temperature. In addition, a cross-over from a semiconducting state at high-temperature to a metallic state at low-temperature is observed, with a clear enhancement of the metallic character as the growth temperature increases. Optical absorption measurements combined with the two-bands analysis of the Hall effect show that the metallicity is induced by the diffusion of oxygen vacancies in the SrTiO3 substrate. These results allow to understand that the film/substrate heterostructure behaves as an original semiconducting-metallic parallel resistor, and electronic transport properties are consistently explained.

cond-mat.mtrl-sci↗

arXiv2014

Temperature dependence of clusters with attracting vortices in superconducting Niobium studied by neutron scattering

We have investigated the intermediate mixed state of a superconducting niobium sample by Very Small Angle Neutron Scattering.We show that this state is stabilized through a sequence where a regular vortex lattice appears, which then coexists with vortex clusters before vanishing at low temperature. Vortices in clusters have a constant periodicity regardless of the applied field, exhibit a temperature dependence close to the one of the penetration depth. The clusters disappear in the high temperature limit. All the results agree with an explanation in terms of vortex attraction due non local effects, and indicate a negligible role of pinning. Phase coexistence between Abrikosov vortex lattice and vortex clusters is reported showing the first order nature of the boundary line.

cond-mat.supr-con↗