arXiv subjects

Jacques Flouquet

Publications and source records attributed to Jacques Flouquet.

At least 19 recordsLinked to original sources

arXiv2026

Successive Electronic Topological Transitions in the Antiferromagnet UPd$_2$Al$_3$

We report successive anomalies at low temperature in the magnetic field dependence of the thermoelectric signal in the heavy fermion compound UPd$_{2}$Al$_{3}$ inside the antiferromagnetic state up to the metamagnetic transition at $H_\text{M} =18$~T. Based on renormalisation perturbation theory and the partitioning of the $f$ orbitals into localized and delocalized parts, our analysis attributes these anomalies to complex topological changes of the Fermi surface driven by Zeeman effect. The observation of a sudden change of sign both in the thermoelectric power and in the Hall coefficient at $H_\text{M}$ in addition to the appearance of large quantum oscillations in the thermoelectric power above $H_\text{M}$ indicate a strong Fermi surface reconstruction at the metamagnetic transition due to the unfolding of the electronic bands.

cond-mat.str-el↗

arXiv2026

Bridging atomic and mesoscopic length scales with Replica Scanning Tunneling Microscopy: Visualizing the atomic lattice of UTe$_2$ and the atomic scale superconducting gap modulations of FeSe close to micron length scales

Scanning Tunneling Microscopy is a cornerstone technique for visualizing the electronic density of states with atomic resolution (typically below 0.1 nm). While the field of view of most STM setups extends up to a few microns, obtaining atomic resolution over these large areas is often impractical and excessively time-consuming. This is due to the need to acquire maps with a point number reaching $10^7$ or more with a full current or conductance vs voltage curve at each point. The standard procedure is to make large scale maps and then select small regions to zoom-in for high-resolution atomic scale analysis. However, this approach fails to address a question which is often critical: Does a specific atomic-scale modulation of the electronic density of states persist over much larger, mesoscopic length scales? Here we present a new method: Replica STM (R-STM), that overcomes this limitation, allowing the study of atomic-scale phenomena up to micron length scales. We obtained new large-area STM tunneling conductance maps in UTe$_2$ and FeSe, spanning areas over 200 nm in size. In these large scale maps we discovered periodic signals with wavelengths significantly exceeding interatomic distances. We show that these large-wavelength periodic signals are replicas of the underlying atomic-scale density of states modulations. R-STM leverages these replica signals to efficiently track atomic-scale features over large areas. We discuss the influence of phase slips, disorder and defects in the replicas. Our results suggest that atomic scale modulations of the superconducting density of states could persist over large length scales in FeSe. R-STM provides a new capability for STM to compare atomic scale with micrometer scale phenomena. The proof of principle of R-STM can be extended to any other scanning probe microscopy experiment where a periodic signal is traced as a function of position.

cond-mat.supr-con↗

arXiv2025

Surface charge density wave in UTe2

The spatially uniform electronic density characteristic of a metal can become unstable at low temperatures, leading to the formation of charge density waves (CDWs). These CDWs, observed in dichalcogenides, cuprates and pnictides, arise from the interplay between the crystal lattice and the electronic structure, which can facilitate charge ordering. However, CDWs are rarely observed in the presence of Kondo screening and heavy fermion quasiparticles. The heavy fermion topological superconductor candidate UTe$_2$ presents a notable exception, exhibiting a CDW whose origin remains elusive. Here we report high resolution scanning tunneling microscopy (STM) experiments that reveal the primitive wavevectors of the CDW in UTe$_2$. This allows us to identify hot spots in the electronic band structure that are connected to the CDW. Although the corresponding wavevectors have apparently no specific influence on some bulk properties, for example on antiferromagnetic fluctuations, we find that they lead to a spatial modulation of the heavy fermion hybridization pattern. We propose that surface induced modifications in the U 5f electron valence enable a novel form of purely electron-driven charge ordering.

cond-mat.str-el↗

arXiv2024

High Field Superconducting Phases of Ultra Clean Single Crystal UTe2

We report the magnetoresistance of high-quality single crystals of UTe2 with Tc=2.1K in high magnetic fields up to 36T, with the field direction between the b and c-axes. From the angular dependence of the upper critical field Hc2, we found that the field-reentrant superconducting phase near H // b-axis extends up to a field angle (24 deg) from the b to c-axis, where another field-reentrant superconducting phase begins to appear above the metamagnetic transition field, Hm. Our results suggest that the field-reentrant superconductivity below Hm near the b-axis is closely related to the superconductivity above Hm when the field is tilted toward the c-axis. Superconductivity appears to be robust when the field direction is maintained perpendicular to the magnetization easy axis, implying that fluctuations boosting superconductivity may persist. At first glance, these findings resemble the field-reentrant (reinforced) superconductivity observed in ferromagnetic superconductors URhGe and UCoGe, where Ising-type ferromagnetic fluctuations play a crucial role. However, in UTe2, the fluctuations are more complex. The angular dependence of the upper critical field Hc2 contrasts with that of the initial slope of Hc2 near Tc, revealing the anisotropic field response of fluctuations. Thanks to the high-quality samples, quantum oscillations were detected for field directions close to the c-axis using magnetoresistance (Shubnikov-de Haas effect) and torque (de Haas-van Alphen effect) measurements. The angular dependence of frequencies is in good agreement with those observed previously using the field-modulation technique, confirming quasi-two-dimensional Fermi surfaces.

cond-mat.str-el↗

arXiv2023

de Haas-van Alphen Oscillations for the Field Along c-axis in UTe2

We performed de Haas-van Alphen (dHvA) experiments in the spin-triplet superconductor UTe2 for magnetic field along the c-axis above 15T. Three fundamental dHvA frequencies, named alpha1, alpha2 and beta corresponding to the cross sections of cylindrical Fermi surfaces (FSs) with large cyclotron effective masses (33-43 m0) were detected. No other fundamental dHvA frequencies were detected at high frequency range, suggesting a cylindrical-shaped electron FS without connecting at the Z point of the Brillouin zone. However, the existence of small pocket FSs associated with extremely heavy masses cannot be fully excluded.

cond-mat.str-el↗

arXiv2023

Pressure-induced structural phase transition and new superconducting phase in UTe2

We report on the crystal structure and electronic properties of the heavy fermion superconductor UTe2 at high pressure up to 11 GPa, as investigated by X-ray diffraction and electrical resistivity experiments. The X-ray diffraction measurements under high pressure using a synchrotron light source reveal anisotropic linear compressibility of the unit cell up to 3.5 GPa, while a pressure-induced structural phase transition is observed above 3.5-4GPa at room temperature, where the body-centered orthorhombic crystal structure with the space group Immm changes into a body-centered tetragonal structure with the space group I4/mmm. The molar volume drops abruptly at the critical pressure, while the distance between the first-nearest neighbor of U atoms increases, implying a switch from the heavy electronic states to the weakly correlated electronic states. Surprisingly, a new superconducting phase at pressures higher than 7 GPa was detected at Tsc above 2K with a relatively low upper-critical field, Hc2(0). The resistivity above 3.5GPa, thus, in the high-pressure tetragonal phase, shows a large drop below 230 K, which may also be related to a considerable change from the heavy electronic states to the weakly correlated electronic states.

cond-mat.str-el↗

arXiv2023

Field-induced tuning of the pairing state in a superconductor

The recently discovered superconductor UTe$_2$, with a T$_c$ between 1.5~K and 2~K, is attracting much attention due to strong suspicion of spin-triplet and topological superconductivity. Its properties under magnetic field are also remarkable, with field-reinforced and field-induced superconducting phases. Here, we report the first complete thermodynamic determination of the phase diagram for fields applied along the three crystallographic directions. Measurements were performed up to 36~T along the hard magnetisation $b$~axis in order to follow the superconducting transition up to the metamagnetic transition at $H_{m} = 34.75$~T. They reveal the existence of a phase transition line within the superconducting phase, and drastic differences occurring between these two phases. Detailed analysis supports a different spin state between the two phases, implying a low-field spin-triplet to high-field spin-singlet transition, a unique case among superconductors, giving insight on the mechanisms leading to spin-triplet superconductivity.

cond-mat.supr-con↗

arXiv2022

Lattice Instability of UTe$_2$ Studied by Ultrasonic Measurements

The elastic constants of an unconventional superconductor, UTe$_2$, were investigated using ultrasound. In this paper, we report the elastic response of the normal state at temperatures down to 2 K and up to 14 T for $H \parallel b$ at ambient pressure. The transverse ultrasonic mode $C_{55}$, which corresponds to the strain susceptibility of $\varepsilon_{zx}$, shows softening with decreasing temperature, whereas the $C_{44}$ and $C_{33}$ modes do not show such softening. This clear mode dependence strongly suggests that UTe$_2$ has a lattice instability for this specific symmetry.

cond-mat.supr-con↗

arXiv2022

Quantum-well states at the surface of the heavy-fermion superconductor URu$_2$Si$_2$

Electrons can form a two-dimensional electron gas at metal surfaces, where lateral confinement leads to quantum-well states. Such states have been observed for highly itinerant electrons, but it remains an open question whether quantum-well states can be formed from strongly correlated electrons. Here we study atomically flat terraces on surfaces of the heavy-fermion superconductor URu$_2$Si$_2$ using millikelvin scanning tunneling spectroscopy. We observe two-dimensional heavy fermions (2DHF) with an effective mass 17 times the free electron mass that form quantized states separated by a fraction of a meV. Superconductivity at the surface is induced by the bulk into the 2DHF. Our results provide a new route to realize quantum well states in correlated quantum materials.

cond-mat.str-el↗

arXiv2022

First Observation of de Haas-van Alphen Effect and Fermi Surfaces in Unconventional Superconductor UTe2

We report the first observation of the de Haas-van Alphen (dHvA) effect in the novel spin-triplet superconductor UTe2 using high quality single crystals with the high residual resistivity ratio (RRR) over 200. The dHvA frequencies, named alpha and beta, are detected for the field directions between c and a-axes. The frequency of branch beta increases rapidly with the field angle tilted from c to a-axis, while branch alpha splits, owing to the maximal and minimal cross-sectional areas from the same Fermi surface. Both dHvA branches, alpha and beta reveal two kinds of cylindrical Fermi surfaces with a strong corrugation at least for branch alpha. The angular dependence of the dHvA frequencies is in very good agreement with that calculated by the generalized gradient approximation (GGA) method taking into account the on-site Coulomb repulsion of U = 2 eV, indicating the main Fermi surfaces are experimentally detected. The detected cyclotron effective masses are large in the range from 32 to 57 m0 . They are approximately 10-20 times lager than the corresponding band masses, consistent with the mass enhancement obtained from the Sommerfeld coefficient and the calculated density of states at the Fermi level. The local density approximation (LDA) calculations of ThTe2 assuming U4+ with the 5f^2 localized model are in less agreement with our experimental results, in spite of the prediction for two cylindrical Fermi surfaces, suggesting a mixed valence states of U4+ and U3+ in UTe2.

cond-mat.str-el↗

arXiv2022

Magnetovolume Effect on the First-Order Metamagnetic Transition in UTe$_2$

The link between the metamagnetic transition and novel spin-triplet superconductivity of UTe$_2$ was discussed thermodynamically through magnetostriction measurements in a pulsed-magnetic field. We revealed a discontinuous magnetostriction across the metamagnetic transition at $μ_0H_{\rm m}\approx 35$~T for the applied magnetic fields along the crystallographic $b$ axis in the orthorhombic structure. The resultanting volume magnetostriction of $ΔV/V \approx-5.9\times 10^{-4}$ gives the initial pressure dependence of $H_{\rm m}$ by employing the Clausius-Clapeyron's equation, which agrees with previous pressure experiments. Further, significant anisotropic magnetostriction (AMS), derived by subtracting the averaged linear magnetostriction, was revealed. Contrary to the weakly field-dependent AMS along the $a$ axis, those along the $b$ and $c$ axes show strong field dependences with a similar magnitude but with opposite signs, indicating its lattice instability. The relationship between characteristic energy scales of magnetic fields and temperatures was discussed in terms of the Grüneisen parameters compared to the other $f$-electron systems. The volume shrinkage in UTe$_2$ at $H_{\rm m}$, contrary to the volume expansion in typical heavy fermion metamagnets, pushes to invoke the link with the valence instability related to the itinerant-localized dual nature of the U magnetism.

cond-mat.str-el↗

arXiv2021

Enhancement and Discontinuity of Effective Mass through the First-Order Metamagnetic Transition in UTe$_2$

Metamagnetic transitions in the novel spin-triplet superconductor UTe$_2$ were investigated through the newly developed simultaneous measurements of magnetization and sample temperature for the field along the orthorhombic $b$-axis and close to the [011] direction, where reentrant superconductivity (RSC) is detected below and above the first-order metamagnetic transition field $H_{\rm m}$. Combining the Maxwell's relation and the Clausius-Clapeyron equation, we obtained the field dependence of Sommerfeld coefficient $γ$ through the first-order metamagnetic transition. A significant enhancement of the effective mass toward $H_{\rm m}$ were detected for both field directions. On the other hand, above $H_{\rm m}$, the effective mass discontinuously decreases for $H~||~b$, while it discontinuously increases for $H~||~\sim [011]$, which seems to make a crucial role for the RSC.

cond-mat.supr-con↗

arXiv2021

Magnetic Properties under Pressure in Novel Spin-Triplet Superconductor UTe2

We report the magnetic susceptibility and the magnetization under pressures up to 1.7GPa above the critical pressure, Pc ~ 1.5GPa, for H // a, b, c-axes in the novel spin triplet superconductor UTe2. The anisotropic magnetic susceptibility at low pressure with the easy magnetization a-axis changes to the quasi-isotropic behavior at high pressure, revealing a rapid suppression of the susceptibility for a-axis, and a gradual increase of the susceptibility for the b-axis. At 1.7GPa above Pc, magnetic anomalies are detected at T_MO ~ 3K and T_WMO ~ 10K. The former anomaly corresponds to long-range magnetic order, most likely antiferromagnetism, while the latter shows a broad anomaly, which is probably due to the development of short range order. The unusual decrease and increase of the susceptibility below T_WMO for H // a and b-axes, respectively, indicate the complex magnetic properties at low temperatures above Pc. This is related to the interplay between multiple fluctuations dominated by antiferromagnetism, ferroamgnetism, valence and Fermi surface instabilities.

cond-mat.str-el↗

arXiv2021

Field-Induced Superconductivity near the Superconducting Critical Pressure in UTe2

We report the magnetoresistance in the novel spin-triplet superconductor UTe2 under pressure close to the critical pressure Pc, where the superconducting phase terminates, for field along the three a, b and c-axes in the orthorhombic structure. The superconducting phase for H // a-axis just below Pc shows a field-reentrant behavior due to the competition with the emergence of magnetic order at low fields. The upper critical field Hc2 for H // c-axis shows a quasi-vertical increase in the H-T phase diagram just below Pc, indicating that superconductivity is reinforced by the strong fluctuations which persist even at high fields above 20T. Increasing pressure leads to the disappearance of superconductivity at zero field with the emergence of magnetic order. Surprisingly, field-induced superconductivity is observed at high fields, where a spin-polarized state is realized due to the suppression of the magnetic ordered phases; the spin-polarized state is favorable for superconductivity, whereas the magnetic ordered phase at low field seems to be unfavorable. The huge Hc2 in the spin-polarized state seems to imply a spin-triplet state. Contrary to the a- and c-axes, no field-reinforcement of superconductivity occurs for magnetic field along the b-axis. We compare the results with the field-reentrant superconductivity above the metamagnetic field, Hm for the field direction tilted by about 30 deg. from b to c-axis at ambient pressure as well as the field-reentrant (-reinforced) superconductivity in ferromagnetic superconductors, URhGe and UCoGe.

cond-mat.str-el↗

arXiv2020

One-dimensional moire charge density wave in the hidden order state of URu2Si2 induced by fracture

Moiré patterns can lead to fundamentally new electronic behavior when formed between two atomic lattices slightly shifted with respect to each other. A solid is however not just characterized by the atomic lattice, but also by charge or magnetic excitations that do not need to be commensurate to the lattice. This raises the question if one can obtain a moiré by combining periodic electronic modulations and the atomic lattice. Here we report on the discovery of a one-dimensional charge density wave (1D-CDW) which is a moiré pattern between the atomic lattice and a hot spot for electronic scattering in the bandstructure of the hidden order (HO) state of URu$_2$Si$_2$. The moiré is produced by fracturing the crystal at low temperatures. Our results suggest that charge interactions are among the most relevant features competing with HO in URu$_2$Si$_2$.

cond-mat.str-el↗

arXiv2020

Electronic nematicity in URu2Si2 revisited

The nature of the hidden-order (HO) state in URu2Si2 remains one of the major unsolved issues in heavy-fermion physics. Recently, torque magnetometry, x-ray diffraction and elastoresistivity data have suggested that the HO phase transition at THO = 17.5 K is driven by electronic nematic effects. Here, we search for thermodynamic signatures of this purported structural instability using anisotropic thermal-expansion, Youngś modulus, elastoresistivity and specific-heat measurements. In contrast to the published results, we find no evidence of a rotational symmetry-breaking in any of our data. Interestingly, our elastoresistivity measurements, which are in full agreement with published results, exhibit a Curie-Weiss divergence, which we however attribute to a volume and not to a symmetry-breaking effect. Finally, clear evidence for thermal fluctuations is observed in our heat-capacity data, from which we estimate the HO correlation length.

cond-mat.str-el↗

arXiv2020

Multiple Superconducting Phases and Unusual Enhancement of the Upper Critical Field in UTe2

We performed AC calorimetry and magnetoresistance measurements under pressure for H || a-axis (easy-magnetization axis) in the novel heavy-fermion superconductor UTe2. Thanks to the thermodynamic information, multiple superconducting phases have been revealed under pressure and magnetic field. The (H,T) phase diagram of superconductivity under pressure displays an abrupt increase of the upper critical field (Hc2) at low temperature and in the high field region, and a strong convex curvature of Hc2 at high temperature. This behavior of Hc2 and the multiple superconducting phases require a state for the superconducting order parameter more complex than a spin-triplet equal spin pairing. Above the superconducting critical pressure, Pc, we find strong indications that the possible magnetic order is closer to antiferromagnetism than to ferromagnetism.

cond-mat.str-el↗

arXiv2020

Evidence of Fermi-Surface Reconstruction at the Metamagnetic Transition of the Strongly Correlated Superconductor UTe2

Thermoelectric power ($S$) and Hall effect ($R_\mathrm{H}$) measurements on the paramagnetic superconductor UTe$_2$ with magnetic field applied along the hard magnetization $b$-axis are reported. The first order nature of the metamagnetic transition at $H_\mathrm{m}=H^b_\mathrm{c2}=35$~T leads to drastic consequences on $S$ and $R_\mathrm{H}$. In contrast to the field dependence of the specific heat in the normal state through $H_\mathrm{m}$, $S(H)$ is not symmetric with respect to $H_\mathrm{m}$. This implies a strong interplay between ferromagnetic (FM) fluctuations and a Fermi-surface reconstruction at $H_\mathrm{m}$. $R_\mathrm{H}$ is very well described by incoherent skew scattering above the coherence temperature $T_\mathrm{m}$ corresponding roughly to the temperature of the maximum in the susceptibility $T_{χ_\mathrm{max}}$ and coherent skew scattering at lower temperatures. The discontinuous field dependence of both, $S(H)$ and the ordinary Hall coefficient $R_0$, at $H_\mathrm{m}$ and at low temperature, provides evidence of a change in the band structure at the Fermi level.

cond-mat.str-el↗