arXiv subjects

Kenji Ishida

Publications and source records attributed to Kenji Ishida.

At least 19 recordsLinked to original sources

arXiv2026

Incommensurate modulation with $Q=0$ A-type Antiferromagnetic Order in CeRh$_2$As$_2$ revealed by NQR studies

We performed $^{75}$As nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) measurements on a higher-quality single-crystalline CeRh$_2$As$_2$, a heavy-fermion superconductor exhibiting multiple superconducting (SC) phases under magnetic fields along the $c$ axis. This SC multiphase is believed to originate from staggered Rashba spin-orbit coupling associated with locally broken inversion symmetry. In addition to superconductivity, CeRh$_2$As$_2$ exhibits phase I below $T_0\sim0.5$ K and an antiferromagnetic (AFM) state below $T_{N}\sim 0.25$ K in the early-stage samples. In the higher-quality sample, the AFM transition becomes more pronounced, and $T_{N}$ increases to nearly coincide with $T_{SC}$. The NQR spectra at the As(1) site imply an internal field with an incommensurate distribution, indicating a two-dimensional incommensurate modulation of the magnetic structure superimposed on a $Q=0$ A-type AFM component. Moreover, a pronounced decrease in the NQR intensity at $T_0$ well-above $T_{N}$ and an abrupt increase in the internal field at $T_{N}$ suggest the emergence of a slowly fluctuating AFM order at $T_0$ which becomes static at $T_{N}$.

cond-mat.supr-con↗

arXiv2026

Intrinsic Magnetic Excitations and Heavy-Fermion Formation in the Frustrated Mn Pyrochlore System YMn$_{2+\delta}$Zn$_{20-x}X_x$ ($X$ = In and Al) Revealed by Nuclear Magnetic Resonance and Nuclear Quadrupole Resonance Measurements

We performed nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the microscopic electronic states of the $d$-electron heavy-fermion candidates $\mathrm{YMn_{2+\delta}Zn_{20-x}In_x}$ and $\mathrm{YMn_{2+\delta}Zn_{20-x}Al_x}$. In these compounds, magnetic fluctuations of the Mn pyrochlore lattice are expected to play an important role in heavy-fermion formation; however, excess Mn atoms complicate the interpretation of the physical properties. Our spectral analysis reveals that In substitution exhibits much higher site selectivity and introduces significantly less disorder in local structure than Al substitution. The temperature dependence of the nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ measured by $^{55}$Mn-NQR shows a clear enhancement at low temperatures, indicating the development of low-energy excitations associated with heavy-fermion formation. However, its absolute magnitude is approximately 20 times smaller than that in the related compound YMn$_2$, which hosts stronger antiferromagnetic correlations, indicating that the magnetic interactions are substantially weakened by the enlarged Mn-Mn distance. These results demonstrate that the heavy-fermion state in this system arises from the Mn pyrochlore network and is more closely associated with frustration-induced magnetic excitations with low energy than with conventional antiferromagnetic quantum-critical fluctuations.

cond-mat.str-el↗

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.

cond-mat.supr-con↗

arXiv2026

Microscopic Determination of the c-axis-Oriented Antiferromagnetic Structure in LaMnSi by $^{55}$Mn and $^{139}$La NMR

We report a microscopic investigation of the magnetic structure and electronic properties of LaMnSi in its antiferromagnetic (AFM) state using nuclear magnetic resonance (NMR). Field-swept $^{55}$Mn- and $^{139}$La-NMR spectra, as well as zero-field 55Mn-NMR (ZFNMR) spectra, reveal that the Mn ordered moments are parallel to the tetragonal c axis, consistent with the C-type AFM structure and the realization of an odd-parity multipole order. The internal field at the Mn site is determined to be 19.64 T at 4.2 K, corresponding to a hyperfine coupling constant of Ahf = 6.0 T/uB. Nuclear spin-lattice relaxation rate 1/T1 exhibits a characteristic behavior of itinerant antiferromagnetism, showing metallic behavior at low temperatures and magnon-induced enhancement upon approaching the Neel temperature (TN = 295 K). These results show LaMnSi as an ideal compound to study 3d electron magnetism and odd-parity multipole order in the RT Si (R = rare-earth, T = transition metal) system, free of the complexities of 4f electrons.

cond-mat.str-el↗

arXiv2025

Conventional $s$-wave Superconductivity in LaRh$_2$As$_2$; the Analog without the 4$f$ Electrons of CeRh$_2$As$_2$

Superconductor LaRh$_2$As$_2$ has the same crystal structures as CeRh$_2$As$_2$, which exhibits superconducting (SC) multiphase in the $c$-axis magnetic field. Although the SC transition temperatures $T_c$ are similar, around 0.3 K, LaRh$_2$As$_2$ shows conventional type-II superconductivity with a small upper critical field $H_{c2}\sim$ 10 mT. At present, the SC properties of LaRh$_2$As$_2$ have not been clarified yet. We performed $^{75}$As-nuclear quadrupole resonance (NQR) measurements on LaRh$_2$As$_2$ to investigate the SC properties and gap structure. $1/T_1$ shows a clear coherence peak just below $T_c$ and an exponential decrease at lower temperatures, suggesting full-gap $s$-wave superconductivity. The numerical calculations based on an $s$-wave SC model reveal an SC gap size of $\Delta(0)/k_{B}T_{c} \sim 1.48$, consistent with the weak-coupling $s$-wave superconductivity. These results suggest that the 4$f$ electrons in CeRh$_2$As$_2$ not only enhance the orbital limiting field but also contribute to the formation of unconventional superconductivity with SC multiphase.

cond-mat.supr-con↗

arXiv2025

Electronic state of superconductivity in line nodal material CaSb2 under pressure up to 4.2 GPa

We report the results of resistance measurements under pressure up to $4.2\,\mathrm{GPa}$ on single-crystalline $\mathrm{CaSb}_2$ , which shows the maximum of superconducting transition temperature $T_\mathrm{c}$ at $3.1\,\mathrm{GPa}$. At room temperature, $R(P)$ shows a subtle anomaly at $3.1\,\mathrm{GPa}$. However, Bloch-Gr\"uneisen analysis of $R(T)$ indicates that the electronic state does not change significantly across $3.1\,\mathrm{GPa}$.

cond-mat.supr-con↗

arXiv2025

Uniform electronic states and $s$-wave superconductivity in a strongly disordered high-entropy compound (RuRhPdIr)$_{0.6}$Pt$_{0.4}$Sb

High-entropy compounds, where multiple elements occupy a single crystallographic site in a highly disordered manner, challenge conventional understandings of electronic structure based on periodicity and well-defined band dispersion. Here, we report a detailed nuclear magnetic resonance study of the high-entropy superconductor (RuRhPdIr)$_{0.6}$Pt$_{0.4}$Sb, revealing a spatially homogeneous electronic environment in the normal state, in stark contrast to its crystallographically disordered lattice. The superconducting state exhibits a small but solid Hebel-Slichter coherence peak followed by a significant decrease in the nuclear spin-lattice relaxation rate, providing compelling evidence for fully gapped $s$-wave pairing. Our findings not only deepen the understanding of superconductivity in highly disordered quantum materials but also open a new pathway for exploring novel superconducting states in entropy-stabilized systems.

cond-mat.supr-con↗

arXiv2025

Antiferromagnetic Order and Magnetic Frustration in the Honeycomb Heavy-Fermion System Ce(Pt$_{1-x}$Pd$_{x}$)$_6$Al$_3$: $^{27}$Al and $^{195}$Pt NMR Studies

Heavy-fermion systems with magnetic frustration offer a rich platform for investigating the interplay among Kondo screening, magnetic frustration, and quantum criticality. We report comprehensive $^{27}$Al and $^{195}$Pt nuclear magnetic resonance measurements on polycrystalline Ce(Pt$_{1-x}$Pd$_{x}$)$_6$Al$_3$ ($x = 0$, 0.1, 0.2, and 0.3). For $x = 0$, the Knight shift, linewidth, and nuclear spin-lattice relaxation rate reveal a paramagnetic heavy-fermion ground state persisting down to 0.1~K, characterized by a coherence temperature $T_{\mathrm{coh}} \simeq 15$~K. Substituting Pd induces antiferromagnetic order at $T_{\mathrm{N}} \simeq 3.5$~K, while suppressing $T_{\mathrm{coh}}$. Comparison between $x = 0.1$ and $x = 0.3$ reveals a crossover from itinerant spin-density-wave antiferromagnetism to more localized-moment antiferromagnetism, indicating a shift toward the localized side of the Doniach phase diagram. These findings establish Ce(Pt$_{1-x}$Pd$_{x}$)$_6$Al$_3$ as a tunable platform to explore the competition between Kondo screening and magnetic frustration.

cond-mat.str-el↗

arXiv2025

Superconductivity emerging from the N${\'e}$el state in ${\it infinite}$-${\it stage}$ single-layer cuprate La$_2$CuO$_{4+\delta}$

In copper oxides (cuprates) with single CuO$_2$ layer such as La$_{2-x}$Ba(Sr)$_x$CuO$_4$, antiferromagnetism coexists with superconductivity at small doping levels $x$, where chemical disorders are significant. Here, we report that superconductivity occurs in a uniform and fully ordered N${\'e}$el state in a single-layer cuprate La$_2$CuO$_{4+\delta}$ with a small amount of excess oxygen $(\delta = 0.015)$ as demonstrated by the $^{139}$La nuclear quadrupole resonance measurement. A uniform oxygen distribution in the crystal is crucial for achieving microscopic phase coexistence and overcoming the miscibility gap associated with the staging instability; self-organized periodic oxygen arrangement driven by mobile oxygen atoms. This finding prompts the reconsideration of superconductivity in cuprates, highlighting that it can emerge in a robust N${\'e}$el state that retains sizable magnetic moments and hosts only a small carrier density.

cond-mat.supr-con↗

arXiv2025

Magnetic Field Dependence of the Spin Susceptibility on Conventional s-wave Superconductor LaRu$_4$P$_{12}$ Revealed by $^{31}$P-NMR and $^{139}$La-NMR

The magnetic field dependence of the spin part of Knight shift, which is proportional to the superconducting-state spin susceptibility, was investigated at two nuclear sites, $^{31}$P and $^{139}$La in a conventional s-wave superconductor LaRu$_4$P$_{12}$. After the analyses, we confirmed that the superconducting-state spin susceptibility is proportional to magnetic field, and connects to the normal-state spin susceptibility smoothly. This is a textbook example, when the superconductivity is broken with the orbital pair-breaking effect.

cond-mat.supr-con↗

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.

cond-mat.supr-con↗

arXiv2025

Subtle Structural Anomaly under Compression in Line-Nodal CaSb$_2$

We report X-ray diffraction patterns and calculated electronic band structures of the Dirac line-nodal material CaSb$_2$ under pressure. Its superconducting transition temperature ($T_{\mathrm{c}}=1.7$ K) increases under pressure and reaches a maximum at 3.4 K at around 3 GPa. We observed subtle anomalies in lattice parameters accompanied by a jump in bulk modulus without any change in crystal symmetry at around 3 GPa. First-principles calculations revealed that the distorted lattice of Sb(1) site deforms in the pressure range of 0-3 GPa. Those results suggest the existence of a first-order structural transition and arouse expectations for unusual phononic properties affecting the superconducting state. The calculated pressure dependence of the electronic density of states (DOS) confirms that it is not the change in the DOS that governs the variations in $T_{\mathrm{c}}$.

cond-mat.mtrl-sci↗

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↗

arXiv2025

Magnetic-Field Dependence of Paramagnetic Properties Investigated by 63/65Cu-NMR on the Yb Zigzag-Chain Semiconductor YbCuS2

To investigate the paramagnetic properties of YbCuS2 under magnetic fields, we have performed the 63/65Cu-nuclear magnetic resonance (NMR) measurements. The NMR spectra can be reproduced by the simulations of the three-dimensional powder pattern and the additional two-dimensional powder pattern, indicating the partial sample orientation due to the anisotropy of the magnetic properties. These simulations suggest that the ac plane is the easy plane in YbCuS2. The Knight shift K is proportional to the bulk magnetic susceptibility and field-independent. The broad maximum of the nuclear spin-lattice relaxation rate 1/T1 at Tmax ~ 50 K (50 K anomaly) observed at zero magnetic field is quickly suppressed by the magnetic fields. This indicates that the 50 K anomaly is field-dependent. Furthermore, an anomalous enhancement of 1/T1 at low temperatures was observed above 3 T. This field seemingly corresponds to the magnetic field at which a field-induced phase transition occurs below the antiferromagnetic transition temperature TN ~ 1 K. The changes in 1/T1 observed in the paramagnetic state suggest the presence of the complex quantum phenomena under magnetic fields in YbCuS2.

cond-mat.str-el↗

arXiv2025

Highly Uniform Magnetic and Electronic Environment in Non-Centrosymmetric Superconductor LaRhGe$_3$

We report the results of $^{139}$La NMR measurements in the non-centrosymmetric superconductor LaRhGe$_3$. This material crystallizes in a tetragonal structure without inversion symmetry and exhibits type-I superconductivity below 385 mK. We observed remarkably sharp NMR signals, indicating that the magnetic and electronic properties of the sample are extremely uniform in LaRhGe$_3$ despite the complex crystal structure. Our NMR results indicate that LaRhGe$_3$ is a weakly correlated semimetal in the normal state.

cond-mat.supr-con↗

arXiv2024

Lu/Se Substitution Effect on Magnetic Properties of Yb-Based Zigzag Chain Semiconductor YbCuS2

We have investigated the changes in the magnetic properties on YbCuS2 by Lu or Se substitutions from a microscopic perspective. In general, it is expected that nonmagnetic Lu substitution dilutes the magnetic Yb3+ concentration, and that Se substitution induces the negative pressure in YbCuS2. The 63/65Cu-nuclear quadrupole resonance (NQR) measurements on polycrystalline Yb0.9Lu0.1CuS2 and YbCu(S0.9Se0.1)2 revealed that the Se substitution leads to larger lattice expansion compared to the Lu substitution. The antiferromagnetic transition temperature TN decreased from 0.95 K in the unsubstituted system to 0.75 K in both substituted systems. Furthermore, the T-linear behavior of the 63Cu-NQR spin-lattice relaxation rate 1/T1, suggesting the presence of the gapless quasiparticle excitations, was observed even in the substituted systems, and the value of 1/T1T increases. These results under the chemical substitutions are opposite to those under pressure as previously reported, and are consistent with the expectation. Our systematic study indicates a clear relationship between the magnetic ordered states and the quasiparticle excitations in YbCuS2.

cond-mat.str-el↗

arXiv2024

Pressure Evolution of Magnetic Structure and Quasiparticle Excitations in Anisotropic Frustrated Zigzag Chains

Frustrated magnetic systems with anisotropic exchange interactions have been recognized as key platforms for discovering exotic quantum states and quasiparticles. In this study, we report the pressure evolution of magnetic structures and quasiparticle excitations in the frustrated semiconductor YbCuS2, characterized by Yb3+ zigzag chains with competing exchange interactions. At ambient pressure, YbCuS2 exhibits a magnetic transition at TN ~ 0.95 K, forming an incommensurate helical magnetic order. Under hydrostatic pressure of 1.6 GPa, TN increases to 1.17 K, and the magnetic structure changes to a commensurate one, which can be regarded as an odd-parity magnetic multipole order. Remarkably, pressure suppresses the gapless quasiparticle excitations. These findings suggest that pressure alters the exchange interactions between the Yb ions, affecting both the magnetic ground state and the quasiparticle excitations. Our results highlight the pivotal role of anisotropic interactions in one-dimensionality to stabilize the complex quantum phases, offering insights into the interplay among frustration, dimensionality, multipoles and emergent quasiparticles.

cond-mat.str-el↗

arXiv2024

Appearance of $c$-axis magnetic moment in odd-parity antiferromagnetic state in CeRh$_2$As$_2$ revealed by $^{75}$As-NMR

CeRh$_2$As$_2$ shows the superconducting (SC) multiphase under the $c$-axis magnetic field, which is considered to originate from local inversion symmetry breaking at the Ce site. We reported that the antiferromagnetic (AFM) order is inside the SC phase and that the AFM state disappears at the transition field to the high-field SC phase. However, the magnetic structure in the AFM state has not been clarified yet. In this study, we performed $^{75}$As-NMR measurements in the SC phase in $H\parallel [110]$ to identify the magnetic structure. Comparing the NMR linewidth with $H \parallel c$, we found that the internal magnetic field is oriented to the $c$ axis. This suggests a $q = 0$ $A$-type AFM with the moments parallel to the $c$ axis. We also observed the reduction of the spin susceptibility, which indicates spin-singlet superconductivity in the low-field SC phase. This study provides an important clue to clarify the correlation between the SC multiphase, magnetism, and local inversion symmetry breaking.

cond-mat.supr-con↗