We report successive anomalies at low temperature in the magnetic field dependence of the thermoelectric signal in the heavy fermion compound UPd_2Al_3 inside the antiferromagnetic state up to the metamagnetic transition at H_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_M in addition to the appearance of large quantum oscillations in the thermoelectric power above H_M indicate a strong Fermi surface reconstruction at the metamagnetic transition due to the unfolding of the electronic bands.

We report resistivity and Hall effect measurements in the chiral antiferromagnet EuPtSi. Depending on the magnetic field orientation with respect to the crystallographic axes, EuPtSi presents different topological magnetic phases below the N & eacute;el temperature TN = 4.05 K. In particular, for a field H [111], it exhibits the well-known skyrmion lattice A phase inside the conical phase between T = 0.45 K and TN in the field range 0.8-1.4 T. Further outstanding features of EuPtSi are the nanoscale size of its skyrmions (placing EuPtSi in the less common regime of nonadiabatic electron-skyrmion scattering) and its substantial topological Hall effect. Remarkably, the skyrmion phase can be extended down to very low temperatures (lower than 0.1 K) through field-cooling regardless of the cooling rate and of the magnetic history. Similarly the metastability of the A' and B phases (H [100]) at low temperature is evidenced by our measurements. These results suggest that EuPtSi is a peculiar example where the competition between topological stability and thermal agitation can lead to a metastable quantum skyrmion state.

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.

The heavy-fermion superconductor UTe2 is unique in that, at ambient pressure, it exhibits three distinct superconducting phases, two of which are induced by magnetic field. When the field is applied along the crystallographic b axis in the orthorhombic structure, the field-induced phase SC2 develops above approximately 20 T and persists up to the metamagnetic transition at Hm about 34 T. When the magnetic field is tilted towards the c axis, another superconducting phase, SC3, emerges at very high fields above about 40 T over a certain angular range. The origin of this exotic phase remains under debate. One of the key open questions regarding the origin of SC3 is whether it is confined to the spin-polarized state above Hm, or whether it already develops at lower fields. Here, we report magnetoresistance measurements performed on a high-quality single crystal of UTe2 in static magnetic fields up to 42 T applied in the (bc) plane at temperatures down to 0.35 K. At this temperature, we find that the SC3 phase first appears at an angle of 20 deg from the b axis. At larger angles, the onset of the SC3 phase, defined by a maximum in resistivity, occurs below Hm. However, zero resistivity is reached only above Hm throughout the entire angular range investigated. These results are summarized in the resulting field-angle phase diagram. Furthermore, we find that at 21 deg the SC3 phase is rapidly suppressed with increasing temperature, whereas at 24 deg it becomes considerably more robust and persists up to about 1 K. Finally, we observe Shubnikov de Haas (SdH) oscillations in the vicinity of the c axis. The observed oscillation frequencies are in good agreement with our previous results. The field dependence of the strongest SdH frequency and of the effective mass is discussed.

Kagome metals AV   3 Sb   5 (A = K, Rb, Cs) are renowned for their intricate electronic band structures, providing a rich platform for investigating topological states and electronic correlations. Within the AV   3 Sb   5 family, the detailed electronic structures of CsV   3 Sb   5 and KV   3 Sb   5 have been well established, both in their charge-density-wave (CDW) phase or in the pristine metallic phase. Yet, the electronic structure of RbV   3 Sb   5 remains under-explored. In this manuscript, we present a detailed study of the electronic structure of RbV   3 Sb   5 revealed by Shubnikov-de Haas oscillation in both the CDW phase (9 kbar), and where the CDW phase is just fully suppressed (22 kbar). The greatly simplified Fast Fourier transform spectrum at 22 kbar implies the absence of Fermi surface reconstruction caused by the CDW order, and the observation of enhanced quasi-particle effective masses near the CDW boundary indicates enhanced quantum fluctuations. Furthermore, the mobilities of charge carriers from the CDW phase to the metallic pristine phase are studied using mobility spectrum analysis (MSA). Our MSA results reveal that high-mobility carriers ( ≈ 10 , 000 cm   2 /Vs) coexist with the non-monotonic Hall effect near-zero field at 9 kbar. As the pressure increases to 22 kbar and 30 kbar, this non-monotonic feature is suppressed, concomitant with the disappearance of the high-mobility carriers. By summarizing the MSA results across the AV   3 Sb   5 family, we observed that the superconducting behavior appears to be positively correlated with carrier mobility and number. This correlation suggests that high-mobility carriers may play a crucial role in the underlying superconducting pairing mechanism.

The existence of multiple superconducting phases induced by either pressure or magnetic field is one of the most striking features of superconductivity of UTe2, 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 high-field superconducting phase at ambient pressure continuously evolves under pressure into the zero-field high-temperature superconducting phase, which occurs above 0.2 GPa.

Results of magnetotransport measurements are reported on KV3Sb5 thin flakes under pressure. The zero-field electrical resistance reveals an additional anomaly emerging under pressure (p), marking a previously unidentified phase boundary T*(p). Together with the established TCDW(p) and Tc(p), denoting the charge-density-wave transition and a superconducting transition, respectively, the temperature-pressure phase diagram of KV3Sb5 features a rich interplay among multiple phases. The Hall coefficient evolves reasonably smoothly when crossing the T* phase boundary compared with the variation when crossing TCDW, indicating the preservation of the pristine electronic structure. The mobility spectrum analysis provides further insights into distinguishing different phases. Finally, the high-pressure quantum oscillation studies up to 31 T combined with the density functional theory calculations further demonstrate that the new phase does not reconstruct the Fermi surface, confirming that the translational symmetry of the pristine metallic state is preserved.

We report on the synthesis of single crystals of the kagome compound UV6Sn6, and present the results of magnetization, electrical resistivity, heat capacity, x-ray, and neutron diffraction experiments to characterize the structure and magnetic properties. UV6Sn6 crystallizes in a large supercell of the HfFe6Ge6 parent structure with an hexagonal symmetry in which some of the U atoms are shifted by c/2 in an ordered fashion. Below TN1 29 K, an incommensurate magnetic structure with a temperature-dependent wave vector (0, 0, kz) is observed. Below TN2 = 23.5 K, the wave vector locks in to (0,0,0.5), forming an antiferromagnetic ground state. The U moments align along the c axis retaining a large magnetic anisotropy. These findings highlight the role of the 5 f orbitals from uranium in this structural family in driving both magnetic ordering and structural modulation and distinguish UV6Sn6 from its lanthanide-based analogs.

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 wave vector 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 modeling 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-electron magnetism in UTe2 will be a key for describing its unconventional superconducting phases.

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 features in the atomic lattice and its interaction with the electronic band structure that facilitate charge ordering. However, CDWs are rarely observed in 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 for a refined identification of the nesting wavevectors in the electronic bandstructure. Although these wavevectors have no specific influence on the bulk properties, for example on antiferromagnetic fluctuations, they cause the interactions leading to the CDW at the surface. The heavy fermion hybridization pattern is spatially modulated specifically at the nesting wavevectors, suggesting that surface induced modifications in the U 5f electron valence enable a novel form of purely electron-driven charge ordering.

Details of the electronic band structure in unconventional superconductors are key to the understanding of their fundamental ground state. The potential spin-triplet superconductor UTe_2, with T_c≈ 2.1K, has attracted attention recently. Its main Fermi surface consists of weakly corrugated, two-dimensional Fermi-surface cylinders that run along the crystallographic c axis. In addition, there is evidence for the presence of an additional small three-dimensional band. This has been discussed controversially as it may be essential for the realization of superconductivity in UTe_2. Here, we investigate the angle-resolved magnetoresistance and Hall effect in bulk crystalline samples with current along the c axis in fields up to 60T. We observe low-frequency magnetic quantum oscillations with light effective masses that are most pronounced for magnetic field applied along the a axis. Two distinct frequencies indicate two separate changes in the Fermi-surface topology, likely connected with Lifshitz transitions. We discuss the origin of these oscillations in terms of magnetic breakdown, quantum interference, and other potential mechanisms.

Lifshitz transitions are being increasingly recognised as significant in a wide variety of strongly correlated and topological materials, and understanding the origin and influence of Lifshitz transitions is leading to deeper understanding of key aspects of magnetic, transport or quantum critical behavior. In the ferromagnetic superconductor UCoGe, a magnetic field applied along the c-axis has been shown to induce a series of anomalies in both transport and thermopower that may be caused by Lifshitz transitions. The need to understand the subtleties of the relationship between magnetism, superconductivity and a heavy electron Fermi surface in the ferromagnetic superconductors makes it important to explore if and why a series of magnetic-field-induced Lifshitz transitions occurs in UCoGe. Here we report magnetic susceptibility measurements of UCoGe, performed at temperatures down to 45 mK and magnetic fields (B ||c) up to 30 T. We observe a series of clearly-defined features in the susceptibility, and multiple sets of strongly field-dependent de Haas-van Alphen oscillations, from which we extract detailed field-dependence of the quasiparticle properties. We complement our experimental results with density functional theory bandstructure calculations, and include a simple model of the influence of magnetic field on the calculated Fermi surface. By comparing experimental and calculated results, we determine the likely shape of the Fermi surface and identify candidate Lifshitz transitions that could correspond to two of the features in susceptibility. We connect these results to the development of magnetization in the system.

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.

A structural orthorhombic-to-tetragonal phase transition was recently discovered in the heavy-fermion compound UTe 2 at a pressure p * <^>-center dot 3 - 8 GPa [Honda et al. , J. Phys. Soc. Jpn. 92 , 044702 (2023); Huston et al. , Phys. Rev. Mater. 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 0 H = 58 T combined with pressures up to p = 6 GPa. The field was applied in a direction tilted by 30 degrees from b to c in the orthogonal structure, which is identified as the direction 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 0 H x 1 = 24 T and mu 0 H x 2 = 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 .

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.

Abstract The potential spin-triplet heavy-fermion superconductor UTe2 exhibits signatures of multiple distinct superconducting phases. For field aligned along the b axis, a metamagnetic transition occurs at μ 0 H m ≈ 35 T. It is associated with magnetic fluctuations that may be beneficial for the field-reinforced superconductivity surviving up to H m. Once the field is tilted away from the b towards the c axis, a reentrant superconducting phase emerges just above H m. In order to better understand this remarkably field-resistant superconducting phase, we conducted magnetic-torque and magnetotransport measurements in pulsed magnetic fields. We determine the record-breaking upper critical field of μ 0 H c2 ≈ 73 T and its evolution with angle. Furthermore, the normal-state Hall effect experiences a drastic suppression indicative of a reduced band polarization above H m in the angular range around 30° caused by a partial compensation between the applied field and an exchange field. This promotes the Jaccarino-Peter effect as a likely mechanism for the reentrant superconductivity above H m.

A structural orthorhombic-to-tetragonal phase transition was recently discovered in the heavy-fermion compound UTe_2 at a pressure p^*≃3-8 GPa [Honda et al., J. Phys. Soc. Jpn. 92, 044702 (2023); Huston et al., Phys. Rev. Mat. 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 μ_0H=58 T combined with pressures up to p = 6 GPa. The field was applied in a direction tilted by 30 ^∘ from b to c in the orthogonal structure, which is identified as the direction 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 μ_0H_x1=24 T and μ_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.

The kagome metal CsV3Sb5 is an ideal platform to study the interplay between topology and electron correlation. To understand the fermiology of CsV3Sb5, intensive quantum oscillation (QO) studies at ambient pressure have been conducted. However, due to the Fermi surface reconstruction by the complicated charge density wave (CDW) order, the QO spectrum is exceedingly complex, hindering a complete understanding of the fermiology. Here, we directly map the Fermi surface of the pristine CsV3Sb5 by measuring Shubnikov-de Haas QOs up to 29 T under pressure, where the CDW order is completely suppressed. The QO spectrum of the pristine CsV3Sb5 is significantly simpler than the one in the CDW phase, and the detected oscillation frequencies agree well with our density functional theory calculations. In particular, a frequency as large as 8,200 T is detected. Pressure-dependent QO studies further reveal a weak but noticeable enhancement of the quasiparticle effective masses on approaching the critical pressure where the CDW order disappears, hinting at the presence of quantum fluctuations. Our high-pressure QO results reveal the large, unreconstructed Fermi surface of CsV3Sb5, paving the way to understanding the parent state of this intriguing metal in which the electrons can be organized into different ordered states.

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.

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.