arXiv subjects

Romain Grasset

Publications and source records attributed to Romain Grasset.

At least 19 recordsLinked to original sources

arXiv2026

Reaching Quantum Critical Point by Adding Non-magnetic Disorder in Single Crystals of Superconductor $(\text{Ca}_x\text{Sr}_{1-x})_3\text{Rh}_4\text{Sn}_{13}$

The Remeika series superconductor, $(\text{Ca}_x\text{Sr}_{1-x})_3\text{Rh}_4\text{Sn}_{13}$, shows a rare nonmagnetic quantum critical point (QCP) associated with the continuous charge-density wave (CDW) and structural transition under the ``dome'' of superconductivity achieved by tuning composition and applying pressure. Here we use a nonmagnetic point-like disorder induced by 2.5 MeV electron irradiation to suppress the CDW and drive the system to and even beyond the QCP. This conclusion is based on a clear evolution of temperature-dependent resistivity, $\rho\left(T\right)$, from the Fermi liquid to the non-Fermi liquid regime with increasing amount of disorder. Starting on the CDW side, below the suggested QCP concentration of $x_c=0.9$, added disorder resulted in a progressively larger linear term and a reduced quadratic term in $\rho\left(T\right)$. Nearly perfect $T-$linear dependence is observed at the dose at which long-range CDW order is suppressed to $T=$0, consistent with the expectations. We refine the QCP location in this system and place it in the interval between $x=$0.75 and 0.85. Our results strongly support the concept that the disorder can tune the system to the quantum critical regime and even beyond. It follows from the argument by Imry and Ma that any ordered phase is unstable toward quenched disorder. Introduced in a controlled way, this disorder becomes a novel non-thermal tuning parameter likely applicable to a variety of different systems.

cond-mat.supr-con↗

arXiv2026

Tuning of anomalous magnetotransport properties in half-Heusler topological semimetal GdPtBi

Half-Heusler compounds from the $RE$PtBi family exemplify Weyl semimetals in which external magnetic field induce Weyl nodes. These materials exceptionally host topologically non-trivial states near the Fermi level and their manifestation can be clearly seen in the magnetotransport properties. In this study, we tune the Fermi level of the archetypal half-Heusler Weyl semimetal GdPtBi through high-energy electron irradiation, moving it away from the Weyl nodes to investigate the resilience of the contribution of topologically non-trivial states to magnetotransport properties. Remarkably, we observe that the negative longitudinal magnetoresistance, which is a definitive indicator of the chiral magnetic anomaly occurring in topological semimetals, persists even when the Fermi level is shifted by 100\,meV from its original position in the pristine sample. Additionally, the anomalous Hall effect shows complex variations as the Fermi level is altered, attributed to the energy-dependent nature of the Berry curvature, which arises from avoided band crossing. Our findings show the robust influence of Weyl nodes on the magneto-transport properties of GdPtBi, irrespective of the Fermi level position, a behaviour likely applicable to many half-Heusler Weyl semimetals.

cond-mat.mtrl-sci↗

arXiv2025

Plasmonic metamaterial time crystal

Periodically driven optical materials and metamaterials have recently emerged as a promising platform for realizing photonic time crystals (PTCs), which are systems whose optical properties are strongly and periodically modulated on timescales comparable to the optical cycle of light. These time-varying structures are the temporal counterparts of spatial photonic crystals (SPCs), for which a large and periodic dielectric contrast is achieved spatially on wavelength scales. Just as SPCs have revolutionized control over light-matter interactions by engineering the photonic density of states in space, PTCs promise comparable breakthroughs from a fundamentally new perspective: a temporal one. However, harnessing such phenomena all-optically poses severe experimental challenges, as it requires order-unity modulation depths of a material's optical properties on ultrafast timescales comparable to the light cycle, a regime that has remained elusive to date. Here, we demonstrate the first all-optical realization of a photonic time crystal, achieved with a surface plasmon cavity metamaterial operating at Terahertz (THz) frequencies. We demonstrate strong (near-unity) and coherent (sub-optical cycle) periodic driving of the plasmonic metamaterial enabled by field-induced dynamical modulation of the carriers' kinetic energy and effective mass - reaching up to 80% of their rest mass. Our spectroscopic measurements reveal a transition into the PTC regime mediated by an exceptional point, at which two Floquet-driven optical eigenmodes coalesce. In the PTC regime, emergent gain is shown to reduce plasmonic losses by more than 50% and we predict plasmonic lasing to be within experimental reach. These results pave the way for temporal engineering of losses and light-matter interactions in plasmonic systems, and establish a robust new platform for time-domain photonics.

physics.optics↗

arXiv2025

Disorder-induced suppression of superconductivity in infinite-layer nickelates

The pairing symmetry of superconducting infinite-layer nickelates is a fundamental yet experimentally challenging question. We employ high-energy electron irradiation to induce disorder in superconducting Nd$_{0.825}$Sr$_{0.175}$NiO$_2$ thin films and examine the impact of pair-breaking defects on superconductivity and elucidate the nature of the superconducting gap. Our measurements reveal a complete suppression of superconductivity with increasing disorder, suggesting an unconventional, sign-changing order parameter.

cond-mat.supr-con↗

arXiv2024

Chiral TeraHertz surface plasmonics

Chiral engineering of TeraHertz (THz) light fields and the use of the handedness of light in THz light-matter interactions promise many novel opportunities for advanced sensing and control of matter in this frequency range. Unlike previously explored methods, this is achieved here by leveraging the chiral properties of highly confined THz surface plasmon modes. More specifically, we design ultrasmall surface plasmonic-based THz cavities and THz metasurfaces that display significant and adjustable chiral behavior under modest magnetic fields (B<500mT). For such a prototypical example of non-hermitian and dispersive photonic system, we demonstrate the capacity to magnetic field-tune both the poles and zeros of cavity resonances, the two fundamental parameters governing their resonance properties. Alongside the observed handedness-dependent cavity frequencies, this highlights the remarkable ability to engineer chiral and tunable radiative couplings for THz resonators and metasurfaces. The extensive tunability offered by the surface plasmonic approach paves the way for the development of agile and multifunctional THz metasurfaces as well as the realization of ultrastrong chiral light-matter interactions at low energy in matter with potential far-reaching applications for the design of material properties.

physics.optics↗

arXiv2024

Single-gap Isotropic $s-$wave Superconductivity in Single Crystals $\text{AuSn}_4$

London, $λ_L (T)$, and Campbell, $λ_{C} (T)$, penetration depths were measured in single crystals of a topological superconductor candidate $\text{AuSn}_4$. At low temperatures, $λ_L (T)$ is exponentially attenuated and, if fitted with the power law, $λ(T) \sim T^n$, gives exponents $n>4$, indistinguishable from the isotropic single $s-$wave gap Bardeen-Cooper-Schrieffer (BCS) asymptotic. The superfluid density fits perfectly in the entire temperature range to the BCS theory. The superconducting transition temperature, $T_c = 2.40 \pm 0.05\:\text{K}$, does not change after 2.5 MeV electron irradiation, indicating the validity of the Anderson theorem for isotropic $s-$wave superconductors. Campbell penetration depth before and after electron irradiation shows no hysteresis between the zero-field cooling (ZFC) and field cooling (FC) protocols, consistent with the parabolic pinning potential. Interestingly, the critical current density estimated from the original Campbell theory decreases after irradiation, implying that a more sophisticated theory involving collective effects is needed to describe vortex pinning in this system. In general, our thermodynamic measurements strongly suggest that the bulk response of the $\text{AuSn}_4$ crystals is fully consistent with the isotropic $s-$wave weak-coupling BCS superconductivity.

cond-mat.supr-con↗

arXiv2024

Reaching quantum critical point by adding nonmagnetic disorder in single crystals of (Ca$_{x}$Sr$_{1-x}$)$_{3}$Rh$_{4}$Sn$_{13}$ superconductor

The quasi-skutterudites (Ca$_{x}$Sr$_{1-x}$)$_{3}$(Rh, Ir)$_{4}$Sn$_{13}$ show a rare nonmagnetic quantum critical point associated with the second-order charge-density-wave (CDW) and structural distortion transition extended under the superconducting "dome". So far, the non-thermal tuning parameters for accessing the QCP included changing stoichiometry, pressure, and a magnetic field. Here we add another parameter -- a nonmagnetic point-like disorder induced by 2.5 MeV electron irradiation. The non-Fermi liquid regime was inferred from the analysis of the temperature-dependent resistivity, $ρ\left(T\right)$, in single crystals of (Ca$_{x}$Sr$_{1-x}$)$_{3}$Rh$_{4}$Sn$_{13}$. Starting at compositions below the known QCP concentration of $x_c=0.9$, added disorder resulted in a progressively larger linear term and a reduced quadratic term in $ρ\left(T\right)$. This behavior is supported by theoretical analysis based on the idea of superconducting fluctuations encompassing the crossover from quantum to thermal regimes. Our results strongly support the concept that the nonmagnetic disorder can drive the system toward the quantum critical regime.

cond-mat.supr-con↗

arXiv2024

Anisotropic Paramagnetic Peak Effect in Reversible Magnetization of Crystalline Miassite Superconductor $\text{Rh}_{17}\text{S}_{15}$

We report an unusual anisotropic paramagnetic peak effect observed in reversible magnetization of a single crystalline nodal superconductor $\text{Rh}_{17}\text{S}_{15}$. Both temperature- and field-dependent magnetization measurements reveal a distinct novel vortex state above approximately 1 T. This peak effect is most pronounced when the magnetic field, $H$, is applied parallel to the $\left[111\right]$ direction, whereas it diminishes for $H\parallel\left[110\right]$. Intriguingly, for $H\parallel\left[100\right]$, instead of a peak, we observe a step-like decrease in $M(T)$, with the step amplitude increasing in larger applied magnetic fields. This behavior is opposite to the expectations of conventional Meissner expulsion. The magnitude of the peak effect, expressed in terms of dimensionless volume susceptibility, is on the order of $Δχ=10^{-5}$ (with full diamagnetic screening corresponding to $χ=-1$). The observed anisotropic paramagnetic vortex response is unusual considering the cubic symmetry of $\text{Rh}_{17}\text{S}_{15}$. We propose that in this distinct vortex phase, a small but finite attractive interaction between vortices below $H_{c2}$ may be responsible for this unusual phenomenon. Furthermore, the vortices seem to prefer aligning along the $\left[111\right]$ direction, rotating toward it when the magnetic field is applied in other directions. Our findings add another item to the list of unusual properties of $\text{Rh}_{17}\text{S}_{15}$ that attracted recent attention as the first unconventional superconductor that has a mineral analog, miassite, found in nature.

cond-mat.supr-con↗

arXiv2024

Creep-enhanced vortex pinning revealed through nonmonotonic relaxation of the Campbell length

We study the effects of flux creep on the linear AC response of the vortex lattice in single crystals Ca$_3$Ir$_4$Sn$_{13}$ by measuring the Campbell penetration depth, $λ_{\rm \scriptscriptstyle C}(T,H,t)$. Thermal fluctuations release vortices from shallow pinning sites, only for them to become re-trapped by deeper potential wells, causing an initial increase of the effective Labusch parameter, which is proportional to the pinning well curvature. This effect cannot be detected in conventional magnetic relaxation measurements but is revealed by our observation of a nonmonotonic time evolution of $λ_{\rm \scriptscriptstyle C}(T,H,t)$, which directly probes the average curvature of the occupied pinning centers. The time evolution of $λ_{\rm \scriptscriptstyle C}(T,H,t)$ was measured at different temperatures in samples with different densities of pinning centers produced by electron irradiation. The curves can be collapsed together when plotted on a logarithmic time scale $t \to T\ln{(t/t_0)}$ confirming that the time evolution is driven by flux creep. The $λ_{\rm \scriptscriptstyle C}(T,H,t)$ is hysteretic with a noticeable nonmonotonic relaxation in the presence of a vortex density gradient (after zero-field cooling), but is monotonic after field cooling, where the vortex density is uniform. This result quantitatively corroborates the novel picture of vortex creep based on the strong pinning theory.

cond-mat.supr-con↗

arXiv2024

Controllable suppression of the unconventional superconductivity in bulk and thin-film Sr$_{2}$RuO$_{4}$ via high-energy electron irradiation

In bulk Sr$_{2}$RuO$_{4}$, the strong sensitivity of the superconducting transition temperature $T_{\text{c}}$ to nonmagnetic impurities provides robust evidence for a superconducting order parameter that changes sign around the Fermi surface. In superconducting epitaxial thin-film Sr$_{2}$RuO$_{4}$, the relationship between $T_{\text{c}}$ and the residual resistivity $ρ_0$, which in bulk samples is taken to be a proxy for the low-temperature elastic scattering rate, is far less clear. Using high-energy electron irradiation to controllably introduce point disorder into bulk single-crystal and thin-film Sr$_{2}$RuO$_{4}$, we show that $T_{\text{c}}$ is suppressed in both systems at nearly identical rates. This suggests that part of $ρ_0$ in films comes from defects that do not contribute to superconducting pairbreaking, and establishes a quantitative link between the superconductivity of bulk and thin-film samples.

cond-mat.supr-con↗

arXiv2023

Dynamics of electronic states in the insulating Intermediate surface phase of 1T-TaS$_2$

This article reports a comparative study of bulk and surface properties in the transition metal dichalcogenide 1T-TaS$_2$. When heating the sample, the surface displays an intermediate insulating phase that persists for $\sim 10$ K on top of a metallic bulk. The weaker screening of Coulomb repulsion and stiffer Charge Density Wave (CDW) explain such resilience of a correlated insulator in the topmost layers. Both time resolved ARPES and transient reflectivity are employed to investigate the dynamics of electrons and CDW collective motion. It follows that the amplitude mode is always stiffer at the surface and displays variable coupling to the Mott-Peierls band, stronger in the low temperature phase and weaker in the intermediate one.

cond-mat.str-el↗

arXiv2023

Anisotropic multiband superconductivity in 2M-WS$_{2}$ probed by controlled disorder

The intrinsically superconducting Dirac semimetal 2M-WS$_{2}$ is a promising candidate to realize proximity-induced topological superconductivity in its protected surface states. A precise characterization of the bulk superconducting state is essential for understanding the nature of surface superconductivity in the system. Here, we perform a detailed experimental study of the temperature and nonmagnetic disorder dependence of the London penetration depth $λ$, the upper critical field $H_{c2}$, and the superconducting transition temperature $T_c$ in 2M-WS$_{2}$. We observe a power-law dependence $λ(T) - λ(0) \propto T^{3}$ at temperatures below $0.35~T_c$, which is remarkably different from the expected exponential attenuation of a fully gapped isotropic $s$-wave superconductor. We then probe the effect of controlled nonmagnetic disorder induced by 2.5 MeV electron irradiation at various doses and find a significant $T_c$ suppression rate. Together with the observed increase of the slope $dH_{c2}/dT|_{T=T_c}$ with irradiation, our results reveal a strongly anisotropic $s^{++}$ multiband superconducting state that takes the same sign on different Fermi sheets. Our results have direct consequences for the expected proximity-induced superconductivity of the topological surface states.

cond-mat.supr-con↗

arXiv2023

Electronic dispersion, correlations and stacking in the photoexcited state of 1T-TaS$_2$

Here we perform angle and time-resolved photoelectron spectroscopy on the commensurate Charge Density Wave (CDW) phase of 1T-TaS$_2$. Data with different probe pulse polarization are employed to map the dispersion of electronic states below and above the chemical potential. Upon photoexcitation, the fluctuations of CDW order erase the band dispersion near to the chemical potential and halve the charge gap size. This transient phase sets within half a period of the coherent lattice motion and is favored by strong electronic correlations. The experimental results are compared to Density-Functional Theory (DFT) calculations with a self-consistent evaluation of the Coulomb repulsion. Our simulations indicate that the screening of Coulomb repulsion depends on the stacking order of the TaS$_2$ layers. The entanglement of such degrees of freedom suggest that both the structural order and electronic repulsion are locally modified by the photoinduced CDW fluctuations.

cond-mat.str-el↗

arXiv2023

Investigation of Planckian behavior in a high-conductivity oxide: PdCrO$_2$

The layered delafossite metal PdCrO$_2$ is a natural heterostructure of highly conductive Pd layers Kondo coupled to localized spins in the adjacent Mott insulating CrO$_2$ layers. At high temperatures $T$ it has a $T$-linear resistivity which is not seen in the isostructural but non-magnetic PdCoO$_2$. The strength of the Kondo coupling is known, as-grown crystals are extremely high purity and the Fermi surface is both very simple and experimentally known. It is therefore an ideal material platform in which to investigate 'Planckian metal' physics. We do this by means of controlled introduction of point disorder, measurement of the thermal conductivity and Lorenz ratio and studying the sources of its high temperature entropy. The $T$-linear resistivity is seen to be due mainly to elastic scattering and to arise from a sum of several scattering mechanisms. Remarkably, this sum leads to a scattering rate within 10$\%$ of the Planckian value of $k_BT/$$\hbar$.

cond-mat.str-el↗

arXiv2021

Terahertz pulse-driven collective mode in the nematic superconducting state of Ba$_{1-x}$K$_x$Fe$_2$As$_2$

We investigate the iron-based superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$ using intense terahertz (THz) light. In the superconducting state a THz Kerr signal is observed and assigned to non-linear THz coupling to superconducting degrees of freedom. The polarization dependence of the THz Kerr signal is remarkably sensitive to the coexistence of a nematic order. In the absence of nematic order the $C_4$ symmetric polarization dependence of the THz Kerr signal is consistent with a coupling to the Higgs amplitude mode of the superconducting condensate. In the coexisting nematic and superconducting state the signal becomes purely nematic with a vanishing $C_4$ symmetric component, signaling the emergence of a new superconducting collective mode activated by nematicity.

cond-mat.supr-con↗

arXiv2021

Spin injection efficiency at metallic interfaces probed by THz emission spectroscopy

Terahertz (THz) spin-to-charge conversion has become an increasingly important process for THz pulse generation and as a tool to probe ultrafast spin interactions at magnetic interfaces. However, its relation to traditional, steady state, ferromagnetic resonance techniques is poorly understood. Here we investigate nanometric trilayers of Co/X/Pt (X=Ti, Au or Au0:85W0:15) as a function of the 'X' layer thickness, where THz emission generated by the inverse spin Hall effect is compared to the Gilbert damping of the ferromagnetic resonance. Through the insertion of the 'X' layer we show that the ultrafast spin current injected in the non-magnetic layer defines a direct spin conductance, whereas the Gilbert damping leads to an effective spin mixing-conductance of the trilayer. Importantly, we show that these two parameters are connected to each other and that spin-memory losses can be modeled via an effective Hamiltonian with Rashba fields. This work highlights that magneto-circuits concepts can be successfully extended to ultrafast spintronic devices, as well as enhancing the understanding of spin-to-charge conversion processes through the complementarity between ultrafast THz spectroscopy and steady state techniques.

cond-mat.mes-hall↗

arXiv2019

Colossal electromagnon excitation in the non-cycloidal phase of TbMnO3 under pressure

The magnetoelectric coupling, i.e., cross-correlation between electric and magnetic orders, is a very desirable property to combine functionalities of materials for next-generation switchable devices. Multiferroics with spin-driven ferroelectricity presents such a mutual interaction concomitant with magneto- and electro-active excitations called electromagnons. TbMnO3 is a paradigmatic material in which two electromagnons have been observed in the cycloidal magnetic phase. However, their observation in TbMnO3 is restricted to the cycloidal spin phase and magnetic ground states that can support the electromagnon excitation are still under debate. Here, we show by performing Raman spectroscopy measurements under pressure that the lower-energy electromagnon (4 meV) disappears when the ground state enters from a cycloidal phase to an antiferromagnetic phase (E-type). On the contrary, the magnetoelectric activity of the higher-energy electromagnon (8 meV) increases in intensity by one order of magnitude. Using microscopic model calculations, we demonstrate that the lowerenergy electromagnon, observed in the cycloidal phase, originates from a higher harmonic of the magnetic cycloid, and we determine that the symmetric exchange-striction mechanism is at the origin of the higher-energy electromagnon which survives even in the E-type phase. The colossal enhancement of the electromagnon activity in TbMnO3 paves the way to use multiferroics more efficiently for generation, conversion and control of spin waves in magnonic devices.

cond-mat.str-el↗

arXiv2018

Pressure induced collapse of the charge density wave and Higgs mode visibility in 2H-TaS$_2$

The pressure evolution of the Raman active electronic excitations of the transition metal dichalcogenides 2H-TaS$_2$ is followed through the pressure phase diagram embedding incommensurate charge-density-wave and superconducting states. At high pressure, the charge-density-wave is found to collapse at 8.5~GPa. In the coexisting charge-density-wave and superconducting orders, we unravel a strong in-gap superconducting mode, attributed to a Higgs mode, coexisting with the expected incoherent Cooper-pair breaking signature. The latter remains in the pure superconducting state reached above 8.5~GPa. Our report constitutes the first observation of such Raman active Higgs mode since the longstanding unique case 2H-NbSe$_2$.

cond-mat.supr-con↗