arXiv subjects

Shiki Ogata

Publications and source records attributed to Shiki Ogata.

8 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↗

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 $Δ(0)/k_{B}T_{c} \sim 1.48$, 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

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

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

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↗

arXiv2024

Investigation of the hyperfine coupling constant of locally noncentrosymmetric heavy-fermion superconductor CeRh$_{2}$As$_{2}$

We performed $^{75}$As-NMR measurements in $H\parallel ab$ to investigate the normal-state magnetic properties of CeRh$_2$As$_2$, a recently-discovered heavy-fermion superconductor. We compared the NMR Knight shift $K$ with the magnetic susceptibility $χ_{ab}$, and estimated the hyperfine coupling constant $A_{\mathrm{hf}}$ from the slope of the $K-χ$ plot. We observed that the magnitude of $A_{\mathrm{hf},ab}$ at the As(1) site changes at around 20 K owing to emerging the heavy-fermion state, which was also observed in $A_{\mathrm{hf}}$ at the As(2) site and in $H\parallel c$. The sign of $A_{\mathrm{hf},ab}$ at the As(1) site is negative in low temperature. These are important for the analysis of the NMR results of CeRh$_2$As$_2$ in the superconducting state.

cond-mat.supr-con↗

arXiv2023

Parity transition of spin-singlet superconductivity using sub-lattice degrees of freedom

Recently, a superconducting (SC) transition from low-field (LF) to high-field (HF) SC states was reported in CeRh$_2$As$_2$, indicating the existence of multiple SC states. It has been theoretically noted that the existence of two Ce sites in the unit cell, the so-called sub-lattice degrees of freedom owing to the local inversion symmetry breaking at the Ce sites, can lead to the appearance of multiple SC phases even under an interaction inducing spin-singlet superconductivity. CeRh$_2$As$_2$ is considered as the first example of multiple SC phases owing to this sub-lattice degree of freedom. However, microscopic information about the SC states has not yet been reported. In this study, we measured the SC spin susceptibility at two crystallographically inequivalent As sites using nuclear magnetic resonance for various magnetic fields. Our experimental results strongly indicate a spin-singlet state in both SC phases. In addition, the antiferromagnetic phase, which appears within the SC phase, only coexists with the LF SC phase; there is no sign of magnetic ordering in the HF SC phase. The present work reveals unique SC properties originating from the locally noncentrosymmetric characteristics.

cond-mat.supr-con↗

arXiv2021

Observation of antiferromagnetic order as odd-parity multipoles inside the superconducting phase in CeRh$_{2}$As$_{2}$

Spatial inversion symmetry in crystal structures is closely related to the superconducting (SC) and magnetic properties of materials. Recently, several theoretical proposals that predict various interesting phenomena caused by the breaking of the local inversion symmetry have been presented. However, experimental validation has not yet progressed owing to the lack of model materials. Here we present evidence for antiferromagnetic (AFM) order in CeRh$_{2}$As$_{2}$ (SC transition temperature $T_{\rm SC} \sim 0.37$~K), wherein the Ce site breaks the local inversion symmetry. The evidence is based on the observation of different extents of broadening of the nuclear quadrupole resonance spectrum at two crystallographically inequivalent As sites. This AFM ordering breaks the inversion symmetry of this system, resulting in the activation of an odd-parity magnetic multipole. Moreover, the onset of antiferromagnetism $T_{\rm N}$ within an SC phase, with $T_{\rm N} < T_{\rm SC}$, is quite unusual in systems wherein superconductivity coexists or competes with magnetism. Our observations show that CeRh$_{2}$As$_{2}$ is a promising system to study how the absence of local inversion symmetry induces or influences unconventional magnetic and SC states, as well as their interaction.

cond-mat.supr-con↗