arXiv subjects

Yusei Shimizu

Publications and source records attributed to Yusei Shimizu.

At least 19 recordsLinked to original sources

arXiv2026

Competition between Geometrical Frustration and the Kondo Effect in CePdAl Revealed by High-Resolution Magnetization

CePdAl is a heavy-fermion compound with a quasi-kagome structure, where geometrical frustration competes with the Kondo effect. Using a high-sensitivity magnetometer, we observe clear first-order metamagnetic transitions without magnetization plateaus in CePdAl at 80 mK, indicating a lifting of frustration through the suppression of the Kondo effect and spin flips of ordered moments under fields. An anomaly in the nonlinear magnetic susceptibility at 4.3 T suggests nondipolar correlations in the polarized paramagnetic state. Furthermore, we found no evidence of non-Fermi-liquid behavior in the high-field region, where the antiferromagnetic order is completely suppressed. These findings establish the essential thermodynamic constraints for understanding the field-induced spin-liquid state in CePdAl.

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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

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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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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arXiv2025

Field-induced anomaly in the anisotropic non-Fermi-liquid normal state of UBe$_{13}$

We report the results of high-resolution dc magnetization and specific-heat measurements at very low temperatures for a single crystal \color{black} of UBe$_{13}$ in magnetic fields applied along the [001] and [111] directions, in both the normal and superconducting states. In the normal state, magnetic susceptibility $\chi(T) = M/H$ exhibits a logarithmic temperature dependence over a wide temperature range (1-20 K). However, with increasing field, this non-Fermi-liquid (NFL) behavior of $\chi(T) $ at low temperatures is suppressed. Moreover, a susceptibility maximum occurs below 4 T, whereas Fermi-liquid coherence is recovered above 8 T. In addition, thermodynamic anomalies ($T_{\rm A}$ and $H_{\rm A}$) occur in both magnetic susceptibility and specific heat at intermediate fields (6--10 T) along the [111] direction. Furthermore, a nontrivial fifth-order nonlinear susceptibility is observed in the normal-state magnetization of UBe$_{13}$. These results suggest a close relationship between the field-induced multipolar correlations of $5f$-electron degrees of freedom and the Fermi-surface reconstruction accompanying the crossover from the NFL state to the Fermi-liquid state in UBe$_{13}$.

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arXiv2025

Signatures of field-induced multi-color kagome spin liquids in the dipole-octupole pyrochlore $\mathrm{Ce_2Hf_2O_7}$

We report low-temperature magnetization and magnetostriction measurements on the dipole-octupole pyrochlore $\mathrm{Ce_2Hf_2O_7}$, revealing an unconventional field response for $\mathbf B\parallel[111]$. The magnetization shows no kagome-ice plateau; instead it evolves continuously and exhibits two rapid changes in slope near 0.35~T and 1.2~T, accompanied by magnetostriction features at the same field scales. Classical Monte Carlo simulations, exact diagonalization, and ground-state analysis of the Hamiltonian show that a representative QSI-compatible parameter set captures the data. For this parameter set, the lower-field anomaly marks a closely spaced transition sequence from a two-color kagome spin liquid (KSL) through a narrow three-color KSL into a mixed two-/three-color KSL, while the upper-field anomaly marks the transition from the mixed KSL into a nearly polarized state. These results identify $\mathrm{Ce_2Hf_2O_7}$, and dipole-octupole pyrochlore magnets more broadly, as promising platforms for exotic KSLs beyond conventional spin ice.

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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 $π$-$σ'$ and $π$-$π'$ 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

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

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.

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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 $μ_0 H_a \sim 0.1$ 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

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 τ = (0,0,τz), where τz = 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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arXiv2024

Geometric frustration and Dzyaloshinskii-Moriya interactions in a quantum star lattice hybrid copper sulfate

We study the magnetism of a layered, spin-$\frac12$ organic-inorganic copper sulfate, which is a close realization of the star lattice antiferromagnet, one of the playgrounds of geometric frustration and resonating valence bond physics in two spatial dimensions. Our thermodynamic measurements show no ordering down to 0.1 K and a characteristic field-induced entropic shift, revealing the presence of an infinite number of competing states down to very low energy scales. The response to external magnetic fields shows, in addition, a peculiar anisotropy, reflected in the formation of a 1/3 magnetization plateau (stable up to full saturation around 105 T) and a paramagnetic, Curie-like susceptibility for one direction of the field (${\bf H}\parallel{\bf c}$), and a completely different response in other field directions. Our first-principles density functional theory calculations and exact diagonalizations show that these experimental puzzles are distinctive signatures of a strong interplay between geometric frustration and sizable Dzyaloshinskii-Moriya interactions, and the emergence of a continuous U(1) symmetry at low energy scales.

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arXiv2023

Impact of the Ce $4f$ states in the electronic structure of the intermediate-valence superconductor CeIr$_3$

The electronic structure of the $f$-based superconductor $\mathrm{CeIr_3}$ was studied by photoelectron spectroscopy. The energy distribution of the $\mathrm{Ce}~4f$ states were revealed by the $\mathrm{Ce}~3d-4f$ resonant photoelectron spectroscopy. The $\mathrm{Ce}~4f$ states were mostly distributed in the vicinity of the Fermi energy, suggesting the itinerant character of the $\mathrm{Ce}~4f$ states. The contribution of the $\mathrm{Ce}~4f$ states to the density of states (DOS) at the Fermi energy was estimated to be nearly half of that of the $\mathrm{Ir}~5d$ states, implying that the $\mathrm{Ce}~4f$ states have a considerable contribution to the DOS at the Fermi energy. The $\mathrm{Ce}~3d$ core-level and $\mathrm{Ce}~3d$ X-ray absorption spectra were analyzed based on a single-impurity Anderson model. The number of the $\mathrm{Ce}~4f$ states in the ground state was estimated to be $0.8-0.9$, which is much larger than the values obtained in the previous studies (i.e., $0-0.4$).

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arXiv2023

Superconducting-Spin Reorientation in Spin-Triplet Multiple Superconducting Phases of UTe2

Superconducting (SC) state has spin and orbital degrees of freedom, and spin-triplet superconductivity shows multiple SC phases due to the presence of these degrees of freedom. However, the observation of spin-direction rotation occurring inside the SC state (SC spin rotation) has hardly been reported. UTe2, a recently discovered topological superconductor, exhibits various SC phases under pressure: SC state at ambient pressure (SC1), high-temperature SC state above 0.5 GPa (SC2), and low-temperature SC state above 0.5 GPa (SC3). We performed nuclear magnetic resonance and AC susceptibility measurements on single-crystal UTe2. The b-axis spin susceptibility remains unchanged in SC2, unlike in SC1, and decreases below the SC2-SC3 transition with spin modulation. These unique properties in SC3 arise from the coexistence of two SC order parameters. Our NMR results confirm the spin-triplet superconductivity with SC spin parallel to b in SC2, and unveil the remaining of spin degrees of freedom in superconducting UTe2.

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arXiv2023

Emergence of Elastic Softening Featuring Ultra-Slow Dynamics Around Magnetic Critical Endpoint in UCoAl

We conducted an investigation on the temperature and magnetic field dependence of the elastic properties of UCoAl. The longitudinal elastic stiffness, $C_{33}$, exhibits significant softening as the system approaches the critical endpoint (CEP). This softening is indicative of an ultrasonic dispersion phenomenon, where the anomaly in the elastic constants diminishes with increasing measurement frequency. Fine structures were observed near the CEP in higher frequencies. The magnetic field dependence of $C_{33}$ can be explained by assuming a specific field dependence of the relaxation time. Remarkably, we recorded a relaxation time of 3.5$\times$10$^{-8}$ s in the vicinity of the CEP, which is the longest observed value among solids. These peculiar ultrasonic properties cannot be explained solely by Ising-like ferromagnetic fluctuations, suggesting the involvement of the quadrupole (orbital) degree of freedom in the formation of the CEP. We discussed the origin of these observed phenomena in relation to the magnetic ground state of the UCoAl system.

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