arXiv subjects

Dai Aoki

Publications and source records attributed to Dai Aoki.

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.

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arXiv2026

Unusual upper critical field in UTe2 revealed by magnetotransport measurements up to 42 T

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.

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arXiv2026

Carrier-doping effect and anomalous transport properties in Ni-doped CeCoIn5 investigated by Hall resistivity measurements

We investigated the effects of Ni doping on carrier density and anomalous electrical transport properties in CeCo$_{1-x}$Ni$_x$In$_5$ ($x \leq 0.3$) by performing Hall resistivity measurements. The carrier density, estimated from the Hall coefficient $R_{\rm H}$ at a temperature of 0.5 K in high magnetic fields, increases linearly with $x$, indicating that the doped Ni ions act as electron dopants. In CeCoIn$_5$, the magnitude of $-R_{\rm H}$ is strongly enhanced at magnetic fields near the superconducting upper critical field $H_{c2}$ and in the low-field region above the superconducting transition temperature $T_c$. However, these anomalies are found to be significantly suppressed by Ni doping. Possible origins of this suppression in $-R_{\rm H}$ are discussed.

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arXiv2026

Electron-Hole Scattering Dichotomy and Anisotropic Warping in Quasi-Two-Dimensional Fermi Surfaces of UTe2

We present a combined experimental and theoretical study of the detailed Fermi-surface (FS) geometry of UTe2, a heavy-fermion superconductor that has recently attracted considerable attention as a promising candidate for spin-triplet pairing. Using angle-dependent magnetoresistance oscillations, a bulk- and low-energy-sensitive transport probe for quasi-two-dimensional (Q2D) electronic structures, we directly determine the in-plane FS geometry. We found that the Q2D FS exhibits a rectangular cross-sectional shape with strongly anisotropic warping, originating from the hybridization of two orthogonal quasi-one-dimensional bands. Through a quantitative comparison between experiment and theoretical calculations, we further reveal a large electron-hole scattering dichotomy: the quasiparticle lifetime on the electron FS is substantially shorter than that on the hole FS. This dichotomy is naturally explained by anisotropic, low-dimensional antiferromagnetic fluctuations, which selectively enhance scattering on the electron FS. This suggests a dominant role of the electron pockets for the emergence of superconductivity. Our results clarify a direct relation between FS geometry, magnetic fluctuations, and momentum-dependent quasiparticle lifetimes, and thus providing a crucial basis for the microscopic understanding of pairing mechanism, and impose stringent constraints on the gap symmetry of spin-triplet superconductivity in UTe2.

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arXiv2026

Hall effect on nontrivial quadrupole order in quasi-kagome compound URhSn

This study focuses on the transport properties of the quasi-kagome compound URhSn, which exhibits successive phase transitions at TC =16 K (ferromagnetic phase) and TO =54 K (intermediate phase). A large anomalous Hall component is present along the easy-magnetization axis (H|| [0001]), and the Hall resistivity shows very complex temperature- and field-dependence, with a sign reversal at low temperatures. The Hall resistivity exhibits a nonlinear and unusual field-dependence. Interestingly, there exists an unusual Hall component that is not proportional to the magnetic susceptibility for H || [0001] in both the intermediate and ferromagnetic states. These results reveal unconventional transport properties of URhSn, providing important insights into nontrivial multipolar phases in 5f- electron systems.

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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.

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arXiv2026

Intimate relationship between spin configuration in the triplet pair and superconductivity in UTe$_2$

Spin-triplet superconductivity is an intriguing quantum coherent state with both spin and orbital degrees of freedom, which holds significant potential for future applications in quantum technology. However, how the spin of the triplet pairs responds to an external magnetic field remains poorly understood. This is mainly due to the absence of suitable spin-triplet superconductors. Here, we report results of Knight-shift and ac-susceptibility measurements on UTe$_2$. We demonstrate that the spin susceptibility, which slightly decreases compared to the normal-state value below the superconducting (SC) transition temperature $T_{\rm c}$, is rapidly restored and nearly recovers to the normal-state values around 5 T, well below the SC upper critical field $H_{c2}$ when the magnetic field is applied along the $c$ axis ($H \parallel c$). In addition, we found that $H_{\rm c2}$ of superconductivity becomes larger when the SC spin aligns with the magnetic field. By considering the results on $H \parallel b$, our results suggest the presence of a close relationship between the spin configuration of the triplet pair and $H_{\rm c2}$, as well as the anisotropic pinning interaction acting on the triplet pairs. These phenomena, which have never been observed in spin-singlet superconductors, represent characteristic features unique to spin-triplet superconductors. We discuss the similarities between superconductivity in UTe$_2$ and superfluid $^3$He, focusing on their spin-triplet pairing states.

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arXiv2026

Nodal Superconductivity of UTe$_2$ Probed by Field-Angle-Resolved Specific Heat on a Crystal with $T_{\rm c}=2.1$ K

Field-angle-resolved specific-heat measurements were performed on a clean single crystal of a spin-triplet superconductor UTe$_2$ with $T_{\rm c}=2.1$ K and a low residual electronic specific heat. At low temperatures, the specific heat exhibits a linear dependence on the magnetic field when the field is applied precisely along the $b$ axis, in stark contrast to its rapid increase at low fields for other orientations. This pronounced anisotropy suggests the presence of nodal quasiparticle excitations with the Fermi velocity predominantly aligned along the $b$ axis. Considering the characteristic field-angle dependences of both the specific heat and the upper critical field, these observations are broadly compatible with theoretical models that assume a superconducting gap structure featuring either point nodes consistent with $B_{\rm 2u}$ symmetry, allowed in the infinitely strong spin-orbit coupling scheme, or line nodes confined to flat regions of the quasi-two-dimensional Fermi surface, consistent with $^3B_{\rm 3u}$ symmetry in the finite spin-orbit classification scheme. These results yield crucial hints for resolving the pairing symmetry of UTe$_2$, paving the way for a deeper understanding of its spin-triplet superconductivity.

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arXiv2025

Metastability of the Topological Magnetic Orders in the Chiral Antiferromagnet EuPtSi

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\'eel temperature $T_N=4.05$K. In particular, for a field $H \parallel $ [111], it exhibits the well known skyrmion lattice A-phase inside the conical phase between $T=0.45$K and $T_N$ in the field range from 0.8T to 1.4T. Remarkably, the skyrmion lattice state in EuPtSi, composed of nanoscale skyrmions, can be extended down to very low temperature (lower than 0.1K) through field-cooling regardless of the cooling rate and of the magnetic history. Similarly the metastability of the A'- and B-phases ($H \parallel $ [100]) at low temperature is evidenced by our measurements. These results suggest that EuPtSi is a peculiar example where the competition between the topological stability and the thermal agitation can lead to metastable quantum skyrmion state.

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arXiv2025

Successive Phase Transitions in the Quasi-Kagome Lattice System URhSn Studied by Resonant X-ray Scattering

Successive phase transitions in the quasi-kagome compound URhSn were investigated by resonant X-ray scattering (RXS) at the uranium $M_4$ edge. In the high-temperature phase between 16 K and 54 K, an additional RXS signal was detected superposed onto fundamental reflections in both $\pi$-$\sigma'$ and $\pi$-$\pi'$ polarization channels. Upon cooling below 16 K, reported as a ferromagnetic phase along $c$, substantial enhancements were observed again in the both polarization channels at the 300 reflection, demonstrating a simultaneous emergence of in-plane spin alongside the $c$-axis ferromagnetic components. The observed behavior can be interpreted by an antiferro-quadrupole (AFQ) order of $O_{yz}$ or $O_{zx}$ characterized by a propagation vector $q = 0$ in the intermediate phase, which then coexists with a ferromagnetic component below 16 K. The resulting ground state structure breaks the mirror symmetry perpendicular to the kagome plane, identifying the formation of a unique AFQ order with either chirality or polarity in URhSn.

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arXiv2025

$b$-axis and $c$-axis Knight shift measurements in the superconducting state on ultraclean UTe$_2$ with $T_c$ = 2.1 K

Knight shifts along the $b$ and $c$ axes ($K_b$ and $K_c$) at two crystallographically distinct Te sites were measured down to 70 mK using $^{125}$Te nuclear magnetic resonance (NMR) on an ultraclean UTe$_2$ single crystal with a superconducting (SC) transition temperature $T_{\mathrm{c}}$ = 2.1 K. This was carried out to determine the $\boldsymbol{d}$-vector components, which are the order parameter in the spin-triplet pairing. Although the decrease in $K_b$ and $K_c$ is comparable to the theoretical estimation of the SC diamagnetic shielding effect, it is confirmed, by taking the difference between two Knight shifts at the distinct Te sites, that the spin susceptibility along the $b$ and $c$ axes decreases in the SC state. Taking into account the large decrease in $K_a$ in the SC state, we conclude that the $\boldsymbol{d}$ vector has components along all three crystal axes.

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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.

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arXiv2025

Novel Easy-Axis Switching through Metamagnetism in CeSb$_2$

A novel magnetic field-induced switching of the magnetization easy axis has been discovered in the layered compound CeSb$_2$, which crystallizes in an orthorhombic structure with nearly identical lattice constants along the a- and b-axes, giving it a tetragonal-like appearance. When a magnetic field is applied along an orthorhombic in-plane axis at 4.2 K, magnetization increases abruptly around 34 T, followed by a hysteresis loop upon decreasing the field. Subsequent measurements reveal a significantly enhanced magnetization, indicative of a switch of the magnetization easy axis. Conversely, the other orthorhombic in-plane axis becomes the magnetization hard axis. Surprisingly, the easy axis is switchable by changing the field direction to the other orthorhombic in-plane axis. Moreover, this memory effect is stable up to room temperature. Polarized light microscope images have visually revealed domain formation of the as-cast samples and domain rearrangement by magnetic fields. This intriguing axis-conversion phenomenon is a novel magnetic shape memory effect for heat cycles up to room temperature. It is attributed to the specific in-plane Ce-pantograph networks in CeSb$_2$.

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arXiv2025

Slow magnetic quantum oscillations in the c-axis magnetoresistance of UTe$_2$

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 $T_\mathrm{c}\approx 2.1\,$K, 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 $60\,$T. 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.

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arXiv2025

Intrinsic low-temperature magnetic properties on the ultra-clean UTe$_2$ with $T_{\rm c}$ = 2.1 K revealed by $^{125}$Te NMR

To investigate the intrinsic magnetic properties of UTe$_2$, we performed $^{125}$Te-NMR measurements on the ultra-clean single-crystalline UTe$_2$ with superconducting transition temperature $T_{\rm c}$ = 2.1~K and compared the results with those of the $T_{\rm c}$ = 1.6~K sample. The broadening of the linewidth of the NMR spectrum in the $a$-axis magnetic field and the low-temperature magnetic fluctuations observed in the 1.6~K sample are suppressed in the ultra-clean sample, indicating that such magnetic properties originate from a tiny amount of U deficiency. The present results suggest that the magnetic properties in UTe$_2$ are sensitive to the U deficiency. We also observed a peculiar angular dependence of the NMR quantities due to large magnetic anisotropy with the $a$-axis as the magnetic easy axis.

cond-mat.supr-con↗

arXiv2024

Clear Reduction in Spin Susceptibility and Superconducting Spin Rotation for $H \parallel a$ in the Early-Stage Sample of Spin-Triplet Superconductor UTe$_2$

We report the re-measurement of the $a$-axis spin susceptibility component in an early-stage sample of the spin-triplet superconductor UTe$_2$ with the transition temperature of $T_{\rm SC}$ = 1.6 K. Using Knight-shift measurements along the $b$ axis and at a 10-degree tilt from the $b$ axis towards the $a$ axis, we accurately determined the $a$-axis component without directly measuring the $a$-axis Knight shift. Our results reveal a decrease of approximately 3\% in the $a$-axis spin susceptibility in the superconducting state under $a$-axis magnetic field $\mu_0 H_a \sim 0.1$ T, indicating that the spin susceptibility decreases similarly in both early-stage and ultraclean samples with $T_{\rm SC}$ = 2.1 K. The previously reported absence of the reduction in Knight shift is attributed to the missing of signal from the superconducting region and to the detection of residual signals from the non-superconducting region instead. We also found that the decrease in the $a$-axis spin susceptibility is immediately suppressed with increasing the $a$-axis magnetic field and is estimated to be completely suppressed at around 1.5 T due to superconducting spin rotation.

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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.

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arXiv2024

Magnetic Properties of YbBe13 Probed by Neutron Scattering and Thermodynamic Measurements

We examined the magnetic properties of YbBe13, which exhibits an antiferromagnetic order below TN = 1.2 K. Unlike other MBe13 compounds (M = rare earth/actinide elements), based on elastic neutron scattering, we observed an incommensurate magnetic propagation vector {\tau} = (0,0,{\tau}z), where {\tau}z = 0.5385 is in the reciprocal lattice unit. Additionally, we constructed a precise magnetic phase diagram for YbBe13. We observed non-trivial magnetic anomalies in YbBe13, which cannot be understood based on a simple helical order. Our results for YbBe13 provide an opportunity to reconsider the electron state of UBe13 and present an important step toward a comprehensive understanding of magnetic correlations in MBe13 series.

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