arXiv subjects

L. Cario

Publications and source records attributed to L. Cario.

17 recordsLinked to original sources

arXiv2026

Anisotropy of Ultrafast Strain in $V_2O_3$ Thin Films: Out-of-Equilibrium Phase Transitions under Interfacial Clamping

Ultrafast photoinduced insulator-to-metal transitions in correlated materials are often mediated by lattice distortions, yet the role of interfacial lattice constraints in shaping nonequilibrium pathways remains largely unexplored. We use azimuth-resolved time-resolved X-ray diffraction to track orientation-dependent strain dynamics in granular V$_2$O$_3$ thin films on c-cut sapphire, where thermal-expansion mismatch imposes anisotropic interfacial strain. Across the thermal transition, the azimuthal profile of the (110)$_H$ strain inverts curvature, providing direct evidence of partial clamping of the hexagonal basal-plane lattice (a$_H$,b$_H$). After photoexcitation of the antiferromagnetic insulating phase, the structural response remains clamp-limited: weakly constrained grain families reach the full basal-plane contraction characteristic of the metallic-like state, whereas strongly constrained families exhibit a strongly reduced distortion. Fluence-dependent measurements further disentangle transformed fraction from clamping-limited lattice distortion. Our results show that interfacial clamping acts as a static selector for ultrafast phase switching and provide a general route to quantify anisotropic strain dynamics in heterostructures.

cond-mat.mtrl-sci↗

arXiv2021

Extreme in-plane upper critical magnetic fields of heavily doped quasi-two-dimensional transition metal dichalcogenides

Extreme in-plane upper critical magnetic fields $B_{c2//ab}$ strongly violating the Pauli paramagnetic limit have been observed in the misfit layer $(LaSe)_{1.14}(NbSe_2)$ and $(LaSe)_{1.14}(NbSe_2)_2$ single crystals with $T_c$ = 1.23 K and 5.7 K, respectively. The crystals show a two-dimensional to three-dimensional transition at temperatures slightly below $T_c$ with an upturn in the temperature dependence of $B_{c2//ab}$, a temperature dependent huge superconducting anisotropy and a cusplike behavior of the angular dependence of $B_{c2}$. Both misfits are characterized by a strong charge transfer from LaSe to $NbSe_2$. As shown in our previous work, $(LaSe)_{1.14}(NbSe_2)$ is electronically equivalent to the highly doped $NbSe_2$ monolayers. Then, the strong upper critical field can be attributed to the Ising coupling recently discovered in atomically thin transition metal dichalcogenides with strong spin-orbit coupling and a lack of inversion symmetry. A very similar behavior is found in $(LaSe)_{1.14}(NbSe_2)_2$, where the charge transfer is nominally twice as big, which could eventually lead to complete filling of the $NbSe_2$ conduction band and opening superconductivity in LaSe. Whatever the particular superconducting mechanism would be, a common denominator in both misfits is that they behave as a stack of almost decoupled superconducting atomic layers, proving that Ising superconductivity can also exist in bulk materials.

cond-mat.supr-con↗

arXiv2021

Lattice contraction induced by resistive switching in chromium-doped V2O3: a hallmark of Mott physics

Since the beginnings of the electronic age, a quest for ever faster and smaller switches has been initiated, since this element is ubiquitous and foundational in any electronic circuit to regulate the flow of current. Mott insulators are promising candidates to meet this need as they undergo extremely fast resistive switching under electric field. However the mechanism of this transition is still under debate. Our spatially-resolved {\mu}-XRD imaging experiments carried out on the prototypal Mott insulator (V0.95Cr0.05)2O3 show that the resistive switching is associated with the creation of a conducting filamentary path consisting in an isostructural compressed phase without any chemical nor symmetry change. This clearly evidences that the resistive switching mechanism is inherited from the bandwidth-controlled Mott transition. This discovery might hence ease the development of a new branch of electronics dubbed Mottronics.

cond-mat.str-el↗

arXiv2020

Strain Wave Pathway to Semiconductor-to-Metal Transition revealed by time resolved X-ray powder diffraction

Thanks to the remarkable developments of ultrafast science, one of today's challenges is to modify material state by controlling with a light pulse the coherent motions that connect two different phases. Here we show how strain waves, launched by electronic and structural precursor phenomena, determine a macroscopic transformation pathway for the semiconducting-to-metal transition with large volume change in bistable Ti$_3$O$_5$ nanocrystals. Femtosecond powder X-ray diffraction allowed us to quantify the structural deformations associated with the photoinduced phase transition on relevant time scales. We monitored the early intra-cell distortions around absorbing metal dimers, but also long range crystalline deformations dynamically governed by acoustic waves launched at the laser-exposed Ti$_3$O$_5$ surface. We rationalize these observations with a simplified elastic model, demonstrating that a macroscopic transformation occurs concomitantly with the propagating acoustic wavefront on the picosecond timescale, several decades earlier than the subsequent thermal processes governed by heat diffusion.

cond-mat.mtrl-sci↗

arXiv2017

Domain size effects on the dynamics of a charge density wave in 1T-TaS2

Recent experiments have shown that the high temperature incommensurate (I) charge density wave (CDW) phase of 1T-TaS2 can be photoinduced from the lower temperature, nearly commensurate (NC) CDW state. Here we report a time-resolved x-ray diffraction study of the growth process of the photoinduced I-CDW domains. The layered nature of the material results in a marked anisotropy in the size of the photoinduced domains of the I-phase. These are found to grow self-similarly, their shape remaining unchanged throughout the growth process. The photoinduced dynamics of the newly formed I-CDW phase was probed at various stages of the growth process using a double pump scheme, where a first pump creates I-CDW domains and a second pump excites the newly formed I-CDW state. We observe larger magnitudes of the coherently excited I-CDW amplitude mode in smaller domains, which suggests that the incommensurate lattice distortion is less stable for smaller domain sizes.

cond-mat.str-el↗

arXiv2017

Watching the birth of a charge density wave order: diffraction study on nanometer-and picosecond-scales

Femtosecond time-resolved X-ray diffraction is used to study a photo-induced phase transition between two charge density wave (CDW) states in 1T-TaS$_2$, namely the nearly commensurate (NC) and the incommensurate (I) CDW states. Structural modulations associated with the NC-CDW order are found to disappear within 400 fs. The photo-induced I-CDW phase then develops through a nucleation/growth process which ends 100 ps after laser excitation. We demonstrate that the newly formed I-CDW phase is fragmented into several nanometric domains that are growing through a coarsening process. The coarsening dynamics is found to follow the universal Lifshitz-Allen-Cahn growth law, which describes the ordering kinetics in systems exhibiting a non-conservative order parameter.

cond-mat.str-el↗

arXiv2014

First-Order Insulator-to-Metal Mott Transition in the Paramagnetic 3D System GaTa4Se8

The nature of the Mott transition in the absence of any symmetry braking remains a matter of debate. We study the correlation-driven insulator-to-metal transition in the prototypical 3D Mott system GaTa4Se8, as a function of temperature and applied pressure. We report novel experiments on single crystals, which demonstrate that the transition is of first order and follows from the coexistence of two states, one insulating and one metallic, that we toggle with a small bias current. We provide support for our findings by contrasting the experimental data with calculations that combine local density approximation with dynamical mean-field theory, which are in very good agreement.

cond-mat.str-el↗

arXiv2014

Non thermal and purely electronic resistive transition in narrow gap Mott insulators

Mott insulator to metal transitions under electric field are currently the subject of numerous fundamental and applied studies. This puzzling effect, which involves non-trivial out-of-equilibrium effects in correlated systems, is indeed at play in the operation of a new class of electronic memories, the Mott memories. However the combined electronic and thermal effects are difficult to disentangle in Mott insulators undergoing such transitions. We report here a comparison between the properties under electric field of a canonical Mott insulator and a model built on a realistic 2D resistor network able to capture both thermal effects and electronic transitions. This comparison made specifically on the family of narrow gap Mott insulators AM4Q8, (A = Ga or Ge; M=V, Nb or Ta, and Q = S or Se) unambiguously establishes that the resistive transition experimentally observed under electric field arises from a purely electronic mechanism.

cond-mat.str-el↗

arXiv2014

Amplitude `Higgs' mode in 2H-NbSe2 Superconductor

We report experimental evidences for the observation of the superconducting amplitude mode, so-called `Higgs' mode in the charge density wave superconductor 2H-NbSe2 using Raman scattering. By comparing 2H-NbSe2 and its iso-structural partner 2H-NbS2 which shows superconductivity but lacks the charge density wave order, we demonstrate that the superconducting mode in 2H-NbSe2 owes its spectral weight to the presence of the coexisting charge density wave order. In addition, temperature dependent measurements in 2H-NbSe2 show a full spectral weight transfer from the charge density wave mode to the superconducting mode upon entering the superconducting phase. Both observations are fully consistent with a superconducting amplitude mode or Higgs mode.

cond-mat.supr-con↗

arXiv2013

Pressure dependence of superconducting critical temperature and upper critical field of 2H-NbS2

We present measurements of the superconducting critical temperature Tc and upper critical field Hc2 as a function of pressure in the transition metal dichalcogenide 2H-NbS2 up to 20 GPa. We observe that Tc increases smoothly from 6K at ambient pressure to about 8.9K at 20GPa. This range of increase is comparable to the one found previously in 2H-NbSe2. The temperature dependence of the upper critical field Hc2(T) of 2H-NbS2 varies considerably when increasing the pressure. At low pressures, Hc2(0) decreases, and at higher pressures both Tc and Hc2(0) increase simultaneously. This points out that there are pressure induced changes of the Fermi surface, which we analyze in terms of a simplified two band approach.

cond-mat.supr-con↗

arXiv2013

Scanning tunneling spectroscopy of layers of superconducting 2H-TaSe$_\textbf{2}$: Evidence for a zero bias anomaly in single layers

We report a characterization of surfaces of the dichalcogenide TaSe$_2$ using scanning tunneling microscopy and spectroscopy (STM/S) at 150 mK. When the top layer has the 2H structure and the layer immediately below the 1T structure, we find a singular spatial dependence of the tunneling conductance below 1 K, changing from a zero bias peak on top of Se atoms to a gap in between Se atoms. The zero bias peak is additionally modulated by the commensurate $3a_0 \times 3a_0$ charge density wave of 2H-TaSe$_2$. Multilayers of 2H-TaSe$_2$ show a spatially homogeneous superconducting gap with a critical temperature also of 1 K. We discuss possible origins for the peculiar tunneling conductance in single layers.

cond-mat.supr-con↗

arXiv2013

Ultrafast filling of an electronic pseudogap in an incommensurate crystal

We investigate the quasiperiodic crystal (LaS)1.196(VS2) by angle and time resolved photoemission spectroscopy. The dispersion of electronic states is in qualitative agreement with band structure calculated for the VS2 slab without the incommensurate distortion. Nonetheless, the spectra display a temperature dependent pseudogap instead of quasiparticles crossing. The sudden photoexcitation at 50 K induces a partial filling of the electronic pseudogap within less than 80 fs. The electronic energy flows into the lattice modes on a comparable timescale. We attribute this surprisingly short timescale to a very strong electron-phonon coupling to the incommensurate distortion. This result sheds light on the electronic localization arising in aperiodic structures and quasicrystals.

cond-mat.str-el↗

arXiv2012

Temperature driven Vanadium clusterization and band gap enlargement in the layered misfit compound (LaS)$_{1.196}$VS$_2$

Intriguing properties of the misfit layered chalcogenide (LaS)$_{1.196}$VS$_2$ crystals were investigated by transport, optical measurements, angle-resolved photoemission (ARPES) and x-ray diffraction. Although no clear anomaly is found in transport properties as a function of temperature, a large spectral weight transfer, up to at least 1 eV, is observed by both optical and photoemission spectroscopies. ARPES reveals that a nearly filled band with negative curvature, close enough from the Fermi level at 300K to produce metallic-like behaviour as observed in optical conductivity spectra. At low temperature, the band structure is strongly modified, yielding to an insulating state with a optical gap of 120 meV. An accurate (3+1)D analysis of x-ray diffraction data shows that, although a phase transition does not occur, structural distortions increase as temperature is decreased, and vanadium clusterization is enhanced. We found that the changes of electronic properties and structure are intimately related. This indicates that structural distorsion play a major role in the insulating nature of (LaS)$_{1.196}$VS$_2$ and that electronic correlation may not be important, contrary to previous belief. These results shed a new light on the mechanism at the origin of non-linear electric properties observed in (LaS)$_{1.196}$VS$_2$.

cond-mat.str-el↗

arXiv2011

Chiral charge order in the superconductor 2H-TaS2

We find chiral charge order in the superconductor 2H-TaS2 using Scanning Tunneling Microscopy and Spectroscopy (STM/S) at 0.1 K. Topographic images show hexagonal atomic lattice and charge density wave (CDW) with clockwise and counterclockwise charge modulations. Tunneling spectroscopy reveals the superconducting density of states, disappearing at Tc = 1.75 K and showing a wide distribution of values of the superconducting gap, centered around Δ=0.28 meV.

cond-mat.supr-con↗

arXiv2010

Specific-heat measurements of superconducting NbS2 single crystal in an external magnetic field: Study on the energy gap structure

The heat capacity of a 2H-NbS2 single crystal has been measured by a highly sensitive ac technique down to 0.6 K and in magnetic fields up to 14 T. At very low temperatures data show excitations over an energy gap (2DS/kBTc \approx 2.1) much smaller than the BCS value. The overall temperature dependence of the electronic specific heat Ce can be explained either by the existence of a strongly anisotropic single-energy gap or within a two-gap scenario with the large gap about twice bigger than the small one. The field dependence of the Sommerfeld coefficient shows a strong curvature for both principal-field orientations, parallel and perpendicular to the c axis of the crystal, resulting in a magnetic field dependence of the superconducting anisotropy. These features are discussed in comparison to the case of MgB2 and to the data obtained by scanning-tunneling spectroscopy. We conclude that the two-gap scenario better describes the gap structure of NbS2 than the anisotropic s-wave model.

cond-mat.supr-con↗

arXiv2009

Electric Pulse Induced Resistive Switching, Electronic Phase Separation, and Possible Superconductivity in a Mott insulator

Metal-insulator transitions (MIT) belong to a class of fascinating physical phenomena, which includes superconductivity, and colossal magnetoresistance (CMR), that are associated with drastic modifications of electrical resistance. In transition metal compounds, MIT are often related to the presence of strong electronic correlations that drive the system into a Mott insulator state. In these systems the MIT is usually tuned by electron doping or by applying an external pressure. However, it was noted recently that a Mott insulator should also be sensitive to other external perturbations such as an electric field. We report here the first experimental evidence of a non-volatile electric-pulse-induced insulator-to-metal transition and possible superconductivity in the Mott insulator GaTa4Se8. Our Scanning Tunneling Microscopy experiments show that this unconventional response of the system to short electric pulses arises from a nanometer scale Electronic Phase Separation (EPS) generated in the bulk material.

cond-mat.str-el↗

arXiv2008

Superconducting density of states and vortex cores of 2H-NbS2

Scanning tunneling microscopy and spectroscopy (STM/S) measurements in the superconducting dichalcogenide 2H-NbS2 show a peculiar superconducting density of states with two well defined features at 0.97 meV and 0.53 meV, located respectively above and below the value for the superconducting gap expected from single band s-wave BCS model (D=1.76kBTc=0.9 meV). Both features have a continuous temperature evolution and disappear at Tc = 5.7 K. Moreover, we observe the hexagonal vortex lattice with radially symmetric vortices and a well developed localized state at the vortex cores. The sixfold star shape characteristic of the vortex lattice of the compound 2H-NbSe2 is, together with the charge density wave order (CDW), absent in 2H-NbS2.

cond-mat.supr-con↗