arXiv subjects

G. Lapertot

Publications and source records attributed to G. Lapertot.

At least 19 recordsLinked to original sources

arXiv2026

Fermi-liquid versus non-Fermi-liquid/'strange-metal' fits to the electrical resistivity in the quantum critical magnetic regime of an unconventional superconductor

The question of a possible quantum critical point lying inside of a superconducting phase is central for understanding unconventional superconductivity. In various unconventional superconductors, non-Fermi-liquid/'strange-metal' $T^{n}$ variations, with $n<2$, of the electrical resistivity have been identified as the signature of magnetic quantum criticality. However, a difficulty is to prove experimentally that a non-Fermi-liquid/'strange-metal' law identified at temperatures above the superconducting temperature is the signature of an intrinsic zero-temperature quantum critical regime. In the heavy-fermion paramagnet UTe$_2$, unconventional superconductivity develops in the vicinity of a metamagnetic quantum phase transition induced by a magnetic field, and the quantum critical magnetic properties are suspected to play a role for the superconducting mechanism. In this work, we present a comparative analysis of electrical resistivity data collected on two UTe$_2$ samples of different qualities, in magnetic fields tilted by angles $\theta\simeq35-40$~$^\circ$ from $\mathbf{b}$ to $\mathbf{c}$. Fits to the data have been performed either with a Fermi-liquid function $\rho=\rho_0+AT^{2}$ or with a non-Fermi-liquid/'strange-metal' function $\rho=\rho_0+A_nT^n$. Near to a superconducting phase induced beyond 40~T, non-physical residual resistivities $\rho_0<0$ are extracted from the $T^n$ fits, revealing that a 'hidden' Fermi-liquid $T^2$ regime may be ultimately recovered at low temperature. The results obtained here highlight the importance to investigate high-quality samples with low residual resistivity to confirm - or not - the presence of a suspected 'hidden' quantum critical behavior masked by superconductivity.

cond-mat.supr-con↗

arXiv2026

Quantitative thermodynamic study of superconducting and normal states in UTe2 under pressure

We report a quantitative calorimetric study of UTe2 under pressure with a direct measurement of the Sommerfeld gamma coefficient, showing a three-fold enhancement of electronic effective mass when approaching the critical pressure where superconductivity is suppressed and ordered states occur. We analyse the evolution of gamma with the amplitude of the jumps in the specific heat at the two superconducting transitions, and the superconducting critical temperature with pressure. This analysis would suggest that the high pressure superconducting phase nucleates only on a fraction of the Fermi surface. It also points to the possible major role of a quantum critical point of the unidentified phase that has been called weak magnetic order, rather than to the critical pressure of the antiferromagnetic phase. Just at the border of long-range antiferromagnetic order, where superconductivity emerges from the weak magnetic order phase, a significant increase in the specific heat jump for both superconducting transitions is found, accompanied by a noticeable change of their shapes.

cond-mat.str-el↗

arXiv2025

Bulk signatures of re-entrant superconductivity in UTe$_2$ from ultrasound measurements

We report bulk ultrasound measurements up to 80 T and down to 0.5 K of the field re-entrant superconducting phase of the unconventional superconductor UTe$_2$. Clear bulk signatures of superconductivity are observed in the longitudinal elastic mode $c_{11}$ for fields applied at a tilt angle of $\theta_{b-c} =30^\circ$ from $b$-axis. We confirm an upper critical field of $H_{\rm c2}\approx65$ T at 0.5 K and bulk superconductivity which survives up to $T\approx 2$ K for fields above the metamagnetic transition. The $c_{11}$ mode has propagation and displacement vectors along the $a$-axis, and for fields applied at a tilt angle of $\theta_{b-c} =30^\circ$, this mode is sensitive to the elasticity of the vortex lattice. The anomalies observed in $c_{11}$ are in part reminiscent of superconducting vortices pinned to lattice defects. Nonetheless, an excess attenuation, with respect to the normal state, is observed throughout the entire superconducting phase, suggesting unusual vortex dynamics and pinning in the field re-entrant superconducting phase of UTe$_2$.

cond-mat.supr-con↗

arXiv2025

Experimental evidence of the topological obstruction in twisted bilayer graphene

The rich physics of magic angle twisted bilayer graphene (TBG) results from the Coulomb interactions of electrons in flat bands of non-trivial topology. While the bands' dispersion is well characterized, accessing their topology remains an experimental challenge. Recent measurements established the local density of states (LDOS) as a topological observable. Here, we use scanning tunnelling microscopy to investigate the LDOS of TBG near a defect. We observe characteristic patterns resulting from the Dirac cones having the same chirality within a moir\'e valley. At higher energies, we observe the Lifshitz transition associated with the Dirac cones mixing. Our measurements provide a full characterization of TBG's band structure, confirming the main features of the continuum model including the renormalization of the Fermi velocity, the role of emergent symmetries and the topological obstruction of the wavefunctions.

cond-mat.mes-hall↗

arXiv2025

Boosted magnetic fluctuations at the onset of superconductivity in UTe$_2$ beyond 40 T

Several unconventional superconducting phases have been discovered close to a metamagnetic transition in the heavy-fermion compound UTe$_2$. Although suspected to be of magnetic nature, the mechanisms stabilizing these superconducting phases remain mysterious. Here, we present electrical-resistivity measurements on UTe$_2$, with a current $\mathbf{I}\parallel\mathbf{a}$ and under pulsed magnetic fields up to 60~T rotating in the ($\mathbf{b}$,$\mathbf{c}$) plane. We find that the maximum of the Fermi-liquid coefficient $A$ at the metamagnetic transition is enhanced under magnetic fields tilted by $30-40~^\circ$ from $\mathbf{b}$ to $\mathbf{c}$. The enhancement of $A$ coincides with the stabilization of superconductivity in the polarized paramagnetic regime beyond the metamagnetic field $\mu_0H_m\gtrsim40$~T. It is the signature of a boosted quantum-critical magnetic-fluctuation mode probably in play for the mechanism of this superconducting phase. This result appeals for descriptions of the interplay between magnetic-field-induced superconductivity and quantum critical magnetic properties.

cond-mat.str-el↗

arXiv2024

Origin of the suppression of magnetic order in MnSi under hydrostatic pressure

We experimentally study the evolution of the magnetic moment $m$ and exchange interaction $J$ as a function of hydrostatic pressure in the zero-field helimagnetic phase of the strongly correlated electron system MnSi. The suppression of magnetic order at $\approx 1.5$~GPa is shown to arise from the $J$ collapse and not from a quantum fluctuations induced reduction of $m$. Our work provides benchmarks for first principles theories that are challenged by the presence of strong correlations and the possible role of Hund's coupling. In addition, our experimental data are consistent with a reorientation of the magnetic propagation wavevector recently evidenced above $\approx 1.2$~GPa. This result calls for a thorough investigation of the crystal structure in this pressure range.

cond-mat.str-el↗

arXiv2024

Connecting High-Field and High-Pressure Superconductivity in UTe2

The existence of multiple superconducting phases induced by either pressure or magnetic field is one of the most striking features of superconductivity of UTe$_2$, among the many unusual superconducting properties of this system. Here we report thermodynamic measurements of the superconducting phase diagram combining pressure and magnetic fields up to 30 T. We show that the ambient pressure, high-field, superconducting phase evolves continuously with pressure to join the high-pressure, zero-field superconducting phase. This proves that these two phases are one and the same, and must have the same order parameter.

cond-mat.str-el↗

arXiv2024

Metamagnetism in the high-pressure tetragonal phase of UTe$_2$

A structural orthorhombic-to-tetragonal phase transition was recently discovered in the heavy-fermion compound UTe$_2$ at a pressure $p^*\simeq3-8$~GPa [Honda \textit{et al.}, J. Phys. Soc. Jpn. \textbf{92}, 044702 (2023); Huston \textit{et al.}, Phys. Rev. Mat. \textbf{6}, 114801 (2022)]. In the high-pressure tetragonal phase, a phase transition at $T_x=235$~K and a superconducting transition at $T_{sc}=2$~K have been revealed. In this work, we present an electrical-resistivity study of UTe$_2$ in pulsed magnetic fields up to $\mu_0H=58$~T combined with pressures up to $p$ = 6 GPa. The field was applied in a direction tilted by 30~$^\circ$~from \textbf{b} to \textbf{c} in the orthogonal structure, which is identified as the direction \textbf{c} of the tetragonal structure. In the tetragonal phase, the presence of superconductivity is confirmed and signatures of metamagnetic transitions are observed at the fields $\mu_0H_{x1}=24$~T and $\mu_0H_{x2}=34$~T and temperatures smaller than $T_x$. We discuss the effects of uniaxial pressure and we propose that a magnetic ordering drives the transition at $T_x$.

cond-mat.str-el↗

arXiv2024

Experimental determination of the spin Hamiltonian of the cubic chiral magnet MnSi

A thorough description of the physics of a magnetic compound requires the validation of its microscopic spin Hamiltonian. Here, from the analysis of muon-spin rotation spectra recorded in the magnetically ordered state at low temperature in zero and finite magnetic fields, we determine the minimal Hamiltonian for the chiral binary intermetallic magnet MnSi, consistent with its high-temperature nonsymmorphic cubic space group P$2_1$3. The model provides constraints for the orientation of the Moriya vector characterizing the microscopic Dzyaloshinskii-Moriya interaction, with respect to the Mn nearest-neighbor bonds. Small twist and canting of the magnetic structure are revealed. Our result indicates that, within experimental uncertainties, the magnetoelastic coupling is not strong enough to lower the paramagnetic crystal symmetry in the magnetically ordered state. Additional implications from our work are discussed and complementary studies are suggested.

cond-mat.str-el↗

arXiv2023

Incommensurate antiferromagnetism in UTe2 under pressure

The discovery of multiple superconducting phases in UTe2 boosted research on correlated-electron physics. This heavy-fermion paramagnet was rapidly identified as a reference compound to study the interplay between magnetism and unconventional superconductivity with multiple degrees of freedom. The proximity to a ferromagnetic quantum phase transition was initially proposed as a driving force to triplet-pairing superconductivity. However, we find here that long-range incommensurate antiferromagnetic order is established under pressure. The propagation vector km = (0.07,0.33,1) of the antiferromagnetic phase is close to a wavevector where antiferromagnetic fluctuations have previously been observed at ambient pressure. These elements support that UTe2 is a nearly-antiferromagnet at ambient pressure. Our work appeals for theories modelling the evolution of the magnetic interactions and electronic properties, driving a correlated paramagnetic regime at ambient pressure to a long-range antiferromagnetic order under pressure. A deeper understanding of itinerant-f-electrons magnetism in UTe2 will be a key for describing its unconventional superconducting phases.

cond-mat.str-el↗

arXiv2023

c axis electrical transport at the metamagnetic transition in the heavy-fermion superconductor UTe2 under pressure

The electrical resistivity of the unconventional superconductor UTe$_2$ shows very anisotropic behavior in the normal state depending on the current direction. In the present paper we show that the maximum in the resistivity $\rho_c$ for current applied along the $c$ axis at $T^{\rm max}_{\rho_c} \approx 14.75$~K follows the minimum in the thermal expansion $T_\alpha^\star$ along $b$ axis. Under a magnetic field applied along the $b$ axis, $T^{\rm max}_{\rho_c}$ can be tracked up to the critical point of the first order metamagnetic transition, which is located near 6~K and 34.5~T. Surprisingly, at the metamagnetic field $H_m$ the resistivity $\rho_c$ shows a steplike decrease while the resistivities $\rho_a$ and $\rho_b$, for current along the $a$ and $b$ axis, respectively, show a steplike increase. Under hydrostatic pressure $T^{\rm max}_{\rho_c}$ and $H_m$ decrease significantly up to the critical pressure $p_c$ at which superconductivity is suppressed and a long range antiferromagnetic order appears. We show that the phase diagram at different pressures can be scaled by $T^{\rm max}_{\rho_c}$ in field and temperature suggesting that this temperature scale is governing the main interactions in the normal state.

cond-mat.str-el↗

arXiv2022

Anisotropic signatures of the electronic correlations in the electrical resistivity of UTe$_2$

Multiple unconventional superconducting phases are suspected to be driven by magnetic fluctuations in the heavy-fermion paramagnet UTe$_2$, and a challenge is to identify the signatures of the electronic correlations, including the magnetic fluctuations, in the bulk physical quantities. Here, we investigate thoroughly the anisotropy of the electrical resistivity of UTe$_2$ under intense magnetic fields up to 70~T, for different electrical-current and magnetic-field configurations. Two characteristic temperatures and an anisotropic low-temperature Fermi-liquid-like coefficient $A$, controlled by the electronic correlations, are extracted. Their critical behavior near the metamagnetic transition induced at $μ_0H_m\simeq35$~T for $\mathbf{H}\parallel\mathbf{b}$ is characterized. Anisotropic scattering processes are evidenced and magnetic fluctuations are proposed to contribute, via a Kondo hybridization, to the electrical resistivity. Our work appeals for a microscopic modeling of the anisotropic contributions to the electrical resistivity as a milestone for understanding magnetically-mediated superconductivity in UTe$_2$.

cond-mat.str-el↗

arXiv2021

Magnetic reshuffling and feedback on superconductivity in UTe2 under pressure

The discovery of superconductivity in the heavy-fermion paramagnet UTe$_2$ has attracted a lot of attention, particularly due to the reinforcement of superconductivity near pressure- and magnetic-field-induced magnetic quantum phase transitions. A challenge is now to characterize the effects of combined pressure and magnetic fields applied along variable directions in this strongly anisotropic paramagnet. Here, we present an investigation of the electrical resistivity of UTe$_2$ under pressure up to 3~GPa and pulsed magnetic fields up to 58~T along the hard magnetic crystallographic directions $\mathbf{b}$ and $\mathbf{c}$. We construct three-dimensional phase diagrams and show that, near the critical pressure, a field-enhancement of superconductivity coincides with a boost of the effective mass related to the collapse of metamagnetic and critical fields at the boundaries of the correlated paramagnetic regime and magnetically-ordered phase, respectively. Beyond the critical pressure, field-induced transitions precede the destruction of the magnetically-ordered phase, suggesting an antiferromagnetic nature. By bringing new elements about the interplay between magnetism and superconductivity, our work appeals for microscopic theories describing the anisotropic properties of UTe$_2$ under pressure and magnetic field.

cond-mat.supr-con↗

arXiv2021

Feedback of superconductivity on the magnetic excitation spectrum of UTe$_{2}$

We investigate the spin dynamics in the superconducting phase of UTe$_{2}$ by triple-axis inelastic neutron scattering on a single crystal sample. At the wave-vector $\bf{k_1}$=(0, 0.57, 0), where the normal state antiferromagnetic correlations are peaked, a modification of the excitation spectrum is evidenced, on crossing the superconducting transition, with a reduction of the relaxation rate together with the development of an inelastic peak at $Ω$ $\approx$ 1 meV. The low dimensional nature and the the $a$-axis polarization of the fluctuations, that characterise the normal state, are essentially maintained below $T_{sc}$. The high ratio $Ω/k_{B}T_{sc}$ $\approx$ 7.2 contrasts with the most common behaviour in heavy fermion superconductors.

cond-mat.str-el↗

arXiv2021

Superconductivity in an extreme strange metal

Some of the highest-transition-temperature superconductors across various materials classes exhibit linear-in-temperature `strange metal' or `Planckian' electrical resistivities in their normal state. It is thus believed by many that this behavior holds the key to unlock the secrets of high-temperature superconductivity. However, these materials typically display complex phase diagrams governed by various competing energy scales, making an unambiguous identification of the physics at play difficult. Here we use electrical resistivity measurements into the micro-Kelvin regime to discover superconductivity condensing out of an extreme strange metal state -- with linear resistivity over 3.5 orders of magnitude in temperature. We propose that the Cooper pairing is mediated by the modes associated with a recently evidenced dynamical charge localization-delocalization transition, a mechanism that may well be pertinent also in other strange metal superconductors.

cond-mat.supr-con↗

arXiv2021

Low-dimensional antiferromagnetic fluctuations in the heavy-fermion paramagnetic ladder UTe$_2$

Inelastic-neutron-scattering measurements were performed on a single crystal of the heavy-fermion paramagnet UTe$_2$ above its superconducting temperature. We confirm the presence of antiferromagnetic fluctuations with the incommensurate wavevector $\mathbf{k}_1=(0,0.57,0)$. A quasielastic signal is found, whose momentum-transfer dependence is compatible with fluctuations of magnetic moments $μ\parallel\mathbf{a}$, with a sine-wave modulation of wavevector $\mathbf{k}_1$ and in-phase moments on the nearest U atoms. Low dimensionality of the magnetic fluctuations, consequence of the ladder structure, is indicated by weak correlations along the direction $\mathbf{c}$. These fluctuations saturate below the temperature $T_1^*\simeq15$~K, in possible relation with anomalies observed in thermodynamic, electrical-transport and nuclear-magnetic-resonance measurements. The absence or weakness of ferromagnetic fluctuations, in our data collected at temperatures down to 2.1 K and energy transfers from 0.6 to 7.5 meV, is emphasized. These results constitute constraints for models of magnetically-mediated superconductivity in UTe$_2$.

cond-mat.str-el↗

arXiv2021

Anomalous Anisotropy of the Lower Critical Field and Meissner Effect in UTe2

We report on low temperature susceptibility and magnetization measurements made on single crystals of the recently discovered heavy-fermion superconductor UTe$_2$ and compare the results with the two ambient pressure ferromagnetic superconductors URhGe and UCoGe. Hysteresis curves in the superconducting phase show a familiar diamond shape superimposed on a large paramagnetic background. The Meissner state was measured by zero field cooling in small fields of a few Oe as well as ac susceptibility measurements in small fields and resulted in 100\% shielding, with a sharp transition. However the field cooling Meissner-Ochsenfeld effect (expulsion of flux) was negligible in fields greater than just a few Oe, but becomes nearly 30\% of the perfect diamagnetic signal when the field was reduced to 0.01~Oe. The critical current due to flux pinning was studied by ac susceptibility techniques. Over the range in fields and temperature of this study, no signature of a ferromagnetic transition could be discerned. The lower critical field $H_{\rm c1}$ has been measured along the three crystalographic axes, and surprisingly, the anisotropy of $H_{\rm c1}$ contradicts that of the upper critical field. We discuss this discrepancy and show that it may provide additional support for a magnetic field-dependent pairing mediated by ferromagnetic fluctuations in UTe$_2$.

cond-mat.str-el↗

arXiv2020

Comparison of two superconducting phases induced by a magnetic field in UTe2

Superconductivity induced by a magnetic field near metamagnetism is a striking manifestation of magnetically-mediated superconducting pairing. After being observed in itinerant ferromagnets, this phenomenon was recently reported in the orthorhombic paramagnet UTe$_2$. Under a magnetic field applied along the hard magnetization axis b, superconductivity is reinforced on approaching metamagnetism at $μ_0H_m$ = 35 T, but it abruptly disappears beyond $H_m$. On the contrary, field-induced superconductivity was reported beyond $μ_0H_m$ = 40-50 T in a magnetic field tilted by $\simeq25-30°$ from b in the (b,c) plane. Here we explore the phase diagram of UTe2 under these two magnetic-field directions. Zero-resistance measurements permit to confirm unambiguously that superconductivity is established beyond Hm in the tilted-field direction. While superconductivity is locked exactly at fields either smaller (for a H || b), or larger (for H tilted by $\simeq27°$ from b to c), than Hm, the variations of the Fermi-liquid coefficient in the electrical resistivity and of the residual resistivity are surprisingly similar for the two field directions. The resemblance of the normal states for the two field directions puts constraints for theoretical models of superconductivity and implies that some subtle ingredients must be in play.

cond-mat.supr-con↗