arXiv subjects

Y. Tokunaga

Publications and source records attributed to Y. Tokunaga.

At least 19 recordsLinked to original sources

arXiv2026

Field-induced antiferromagnetic transition in CeIrIn$_5$

We report low-temperature $^{115}$In nuclear magnetic resonance (NMR) study of the prototypical heavy-fermion compound CeIrIn$_5$ in high magnetic fields applied close to the crystallographic $c$ axis. For this orientation, a field-induced transition was previously reported to take place at about 28 T. Although we do not observe any change of the NMR spectrum above the transition, the intensity of the NMR lines drastically decreases as a consequence of a considerable shortening of the $T_2$ relaxation time. In addition, $1/T_1$ shows a pronounced maximum at the transition. Taking into account previous high-field de Haas-van Alphen results in conjunction with band-structure calculations, our NMR results are most naturally explained by the field-induced transition into an antiferromagnetic state with the propagation vector $\mathbf{Q} = (1/2, 1/2, 0)$ and magnetic moments aligned antiferromagnetically along the $c$ axis. This makes CeIrIn$_5$ a unique case where the application of the magnetic field induces an ordered state with moments antiferromagnetically aligned along the field direction.

cond-mat.str-el↗

arXiv2026

Imaging antiferromagnetic domains in LiCoPO$_4$ via the optical magnetoelectric effect

Antiferromagnetic (AFM) materials are considered as promising building blocks of novel data storage devices, still, detecting and manipulating AFM domains have remained challenging. Here, we demonstrate that the two antiphase domains of the magnetoelectric antiferromagnet LiCoPO$_4$ can be distinguished by their light absorption difference. Using visible and infrared spectroscopy, we observed spontaneous non-reciprocal absorption, also termed as directional dichroism, at the crystal field excitations of Co$^{2+}$ ions coordinated by distorted oxygen octahedra. This absorption contrast is particularly pronounced near the telecommunication wavelength of 1550 nm. These findings allowed us to image the AFM domains in LiCoPO$_4$ using a simple transmission light microscopy setup. Our findings suggest that optical magnetoelectric effects offer promising routes for probing the AFM order parameter in non-centrosymmetric transition metal compounds.

cond-mat.mtrl-sci↗

arXiv2025

Anisotropic spin fluctuations in the triangular Kondo lattice compound CePtAl$_4$Ge$_2$ probed by site-selective $^{27}$Al NMR

A site-selective $^{27}$Al nuclear magnetic resonance (NMR) study is carried out on the Kondo lattice compound CePtAl$_4$Ge$_2$, which crystallizes in a rhombohedral lattice with quasi-two-dimensional Ce layers forming a triangular lattice network. Two inequivalent Al sites, Al(1) and Al(2), are unambiguously assigned by comparing measured nuclear quadrupole parameters with electric field gradients obtained from electronic structure calculations. Knight shift analysis yields distinct hyperfine coupling constants, revealing that they arise predominantly from RKKY-type transferred hyperfine fields through conduction electrons. Spin-lattice relaxation measurements reveal pronounced anisotropic spin fluctuations, and comparison of the relaxation rates between the two Al sites clarifies the momentum-space structure of these fluctuations. At low magnetic fields, $(T_1T)^{-1}$ is strongly enhanced on cooling toward the Néel temperature, indicating the growth of in-plane antiferromagnetic correlations in the paramagnetic state.

cond-mat.str-el↗

arXiv2025

Collinear, incommensurate antiferromagnetism in van der Waals magnet alpha-UTe3

alpha-UTe3, a van der Waals (vdW) actinide compound with a monoclinic ZrSe3-type structure, is a narrow-gap semiconductor with 5f moments. 125Te NMR reveals strongly anisotropic, layer-confined spin fluctuations below about 20 K, with the a-axis component enhanced, and a signal wipeout at the antiferromagnetic (AFM) transition at TN = 5 K. Single-crystal neutron diffraction finds q approx. (0.17, 0.5, 0) and a longitudinal sinusoidal modulation of a-axis moments (amplitude about 0.8 muB) with AFM stacking along b. A CEF singlet-singlet induced-moment framework accounts for the easy-axis anisotropy, the small heat-capacity anomaly at TN, the reduced ordered moment, and the exchange-driven selection of q in this localized 5f vdW magnet, establishing a constrained exchange geometry stabilizing this in-plane incommensurate state.

cond-mat.str-el↗

arXiv2025

Self-reconstruction of order parameter in spin-triplet superconductor UTe$_2$

We investigate the effect of easy-axis metamagnetic crossover on superconductivity in UTe$_2$ along the $a$-axis through measurements of AC susceptibility, magnetization, and the magnetocaloric effect. In ultra-clean single crystals, we identify a field-induced phase transition within the superconducting state at 5.6 T, driven by metamagnetism. This transition leads to a high-field superconducting state, significantly increasing the upper critical field to 12 T. A sudden increase in entropy at the transition suggests a self-reconstruction of the order parameter, enabling multi-component superconducting states to adapt to external perturbations.

cond-mat.supr-con↗

arXiv2025

Investigation of the Paramagnetic State of the Kagome Kondo Lattice Compound YbV$_6$Sn$_6$: a $^{51}$V Nuclear Magnetic Resonance Study

YbV$_6$Sn$_6$ is a recently discovered kagome-lattice metal that orders at $T_{\rm N}\approx0.4$~K. Its layered structure combines a triangular Kondo lattice of Yb$^{3+}$ ions with vanadium-based kagome planes, which may host an interplay between strong correlations and band topology. We report a $^{51}$V nuclear magnetic resonance (NMR) study of the paramagnetic state of YbV$_6$Sn$_6$. Detailed field-angular dependence of single-crystal NMR spectra determined the principal-axis directions of the electric field gradient tensor at the $^{51}$V sites, as well as their nuclear quadrupole frequency, $ν_{\rm Q}$, and asymmetry parameter, $η$. The Knight shift, $K$, was measured for different field orientations, and the analysis of $K$ against magnetic susceptibility to extract anisotropic hyperfine couplings. Accurate spectral assignments further enabled measurements of the nuclear spin-lattice relaxation rate, $1/T_1$, for both in-plane and out-of-plane field directions. The temperature dependence of $1/T_1$ shows that out-of-plane spin fluctuations are suppressed below $\sim$20~K, whereas in-plane fluctuations are markedly enhanced, which might be understood by thermal depopulation of the low-lying crystalline electric field excited state. The notable anisotropy in $1/T_1$ indicates that the paramagnetic state of YbV$_6$Sn$_6$ is strongly affected by in-plane spin dynamics.

cond-mat.str-el↗

arXiv2024

Reinforcement of superconductivity by quantum critical fluctuations of metamagnetism in UTe$_2$

The normal-conducting state of the superconductor UTe$_2$ is studied by entropy analysis for magnetic fields along the $b$-axis, obtained from magnetization using the relation $(\partial M/\partial T)_B=(\partial S/\partial B)_T$. We observe a strong increase in entropy with magnetic field due to metamagnetic fluctuations (spatially uniform, $Q=0$). The field dependence is well described by the Hertz-Millis-Moriya theory for quantum criticality of itinerant metamagnetism. Notably, the lower bound of the quantum-critical region coincides well with the position of the minimum in the superconducting transition temperature $T_c(B)$. Hence, our results suggest that $Q=0$ fluctuations reinforce the superconductivity.

cond-mat.supr-con↗

arXiv2023

Breathing pyrochlore magnet CuGaCr$_{4}$S$_{8}$: Magnetic, thermodynamic, and dielectric properties

We investigate the crystallographic and magnetic properties of a chromium-based thiospinel CuGaCr$_{4}$S$_{8}$. From a synchrotron x-ray diffraction experiment and structural refinement, Cu and Ga atoms are found to occupy the tetrahedral $A$-sites in an alternate way, yielding breathing pyrochlore Cr network. CuGaCr$_{4}$S$_{8}$ undergoes a magnetic transition associated with a structural distortion at 31 K in zero magnetic field, indicating that the spin-lattice coupling is responsible for relieving the geometrical frustration. When applying a pulsed high magnetic field, a sharp metamagnetic transition takes place at 40 T, followed by a 1/2-magnetization plateau up to 103 T. These phase transitions accompany dielectric anomalies, suggesting the presence of helical spin correlations in low-field phases. The density-functional-theory calculation reveals that CuGaCr$_{4}$S$_{8}$ is dominated by antiferromagnetic and ferromagnetic exchange couplings within small and large tetrahedra, respectively, in analogy with CuInCr$_{4}$S$_{8}$. We argue that $A$-site-ordered Cr thiospinels serve as an excellent platform to explore diverse magnetic phases along with pronounced magnetoelastic and magnetodielectric responses.

cond-mat.mtrl-sci↗

arXiv2023

Observation of field-induced single-ion magnetic anisotropy in a multiorbital Kondo alloy $\mathrm{(Lu,Yb)}\mathrm{Rh}_{2}\mathrm{Zn}_{20}$

We demonstrate field-induced single-ion magnetic anisotropy resulting from the multiorbital Kondo effect on the diluted ytterbium alloy $(\mathrm{Lu}_{1-x}\mathrm{Yb}_x)\mathrm{Rh}_2\mathrm{Zn}_{20}$. Single-ion anisotropic metamagnetic behavior is revealed in low-temperature regions where the local Fermi-liquid state is formed. Specific hea, low-field magnetic susceptibility, and resistivity indicate reproduction of the ground-state properties by the $\mathrm{SU}(N = 8)$ Kondo model with a relatively large $c$-$f$ hybridization of $T_{\mathrm{K}} = 60.9 \ \mathrm{K}$. Dynamical susceptibility measurements on $\mathrm{Yb}\mathrm{Rh}_2\mathrm{Zn}_{20}$ support realizing the multiorbital Kondo ground state in $(\mathrm{Lu}_{1-x}\mathrm{Yb}_x)\mathrm{Rh}_2\mathrm{Zn}_{20}$. The single-ion magnetic anisotropy becomes evident above $\sim5 \ \mathrm{T}$, which is lower than the isotropic Kondo crossover field of 22.7 T, verifying blurred low-lying crystal field states through the multiorbital Kondo effect.

cond-mat.str-el↗

arXiv2023

Field-induced magnetic structures in the chiral polar antiferromagnet Ni$_2$InSbO$_6$

We have performed $^{115}$In-NMR spectroscopy for Ni$_{2}$InSbO$_6$ with corundum-related crystal structure to reveal magnetic structures that develop in high magnetic fields. At low fields Ni$_{2}$InSbO$_6$ shows a helical magnetic order with a long wavelength because of its chiral and polar crystal structure. The field-induced magnetic state was not investigated by microscopic experiment because an extremely high magnetic field is required to modify the antiferromagnetically coupled helical structure. From the analysis of our $^{115}$In-NMR spectra obtained at high magnetic fields, we confirm that the canted antiferromagnetic structure appears in fields applied in the $[110]$ direction and the propagation vector of magnetic helix is rotated toward the field direction for fields in the $[001]$ direction. We discuss the effect of magnetic field that modifies the magnetic structure of an antiferromagnetic chiral magnet.

cond-mat.str-el↗

arXiv2023

Anomalous vortex dynamics in spin-triplet superconductor UTe$_2$

The vortex dynamics in the spin-triplet superconductor, UTe$_2$, are studied by measuring the DC electrical resistivity with currents along the $a$-axis under magnetic fields along the $b$-axis. Surprisingly, we have discovered an island region of low critical current deep inside the superconducting (SC) state, well below the SC upper critical field, attributed to a weakening of vortex pinning. Notably, this region coincides with the recently proposed intermediate-field SC state. We discuss the possibility of nonsingular vortices in the intermediate state, where SC order parameter does not vanish entirely in the vortex cores due to the mixing of multiple SC components.

cond-mat.str-el↗

arXiv2023

Change of superconducting character in UTe2induced by magnetic field

UTe2 is a recently discovered spin-triplet superconductor. One of the characteristic features of UTe2 is a magnetic field (H)-boosted superconductivity above 16 T when H is applied exactly parallel to the b axis. To date, this superconducting (SC) state has not been thoroughly investigated, and the SC properties as well as the spin state of this high-H SC (HHSC) phase are not well understood. In this study, we performed AC magnetic susceptibility and nuclear magnetic resonance (NMR) measurements and found that, up to 24.8 T, the HHSC state is intrinsic to UTe2 and quite sensitive to the H angle, and that its SC character is different from that in the low-H SC (LHSC) state. The dominant spin component of the spin-triplet pair is along the a axis in the LHSC state but is changed in the HHSC state along the b axis. Our results indicate that H-induced multiple SC states originate from the remaining spin degrees of freedom.

cond-mat.supr-con↗

arXiv2023

Complex magnetic phase diagram with a small phase pocket in a three-dimensional frustrated magnet CuInCr$_{4}$S$_{8}$

Frustrated magnets with a strong spin-lattice coupling can show rich magnetic phases and the associated fascinating phenomena. A promising platform is the breathing pyrochlore magnet CuInCr$_{4}$S$_{8}$ with localized $S=3/2$ Cr$^{3+}$ ions, which is proposed to be effectively viewed as an $S=6$ Heisenberg antiferromagnet on the face-centered-cubic lattice. Here, we unveil that CuInCr$_{4}$S$_{8}$ exhibits a complex magnetic phase diagram with a small phase pocket ($A$ phase) by means of magnetization, magnetostriction, magnetocapacitance, and magnetocaloric-effect measurements in pulsed high magnetic fields of up to 60 T. Remarkably, the appearance of $A$ phase is accompanied by anomalous magnetostrictive and magnetocapacitive responses, suggesting the emergence of helimagnetism in contrast to the neighboring commensurate magnetic phases. Besides, the high-entropy nature is confirmed in the high-temperature side of $A$ phase. These features are potentially related to a thermal fluctuation-driven multiple-$q$ state caused by the magnetic frustration, which has been theoretically predicted but yet experimentally undiscovered.

cond-mat.str-el↗

arXiv2022

Field Induced Multiple Superconducting Phases in UTe2 along Hard Magnetic Axis

The superconducting (SC) phase diagram in uranium ditelluride is explored under magnetic fields ($H$) along the hard magnetic b-axis using a high-quality single crystal with $T_{\rm c} = 2.1$ K. Simultaneous electrical resistivity and AC magnetic susceptibility measurements discern low- and high-field SC (LFSC and HFSC, respectively) phases with contrasting field-angular dependence. Crystal quality increases the upper critical field of the LFSC phase, but the $H^{\ast}$ of $\sim$15 T, at which the HFSC phase appears, is always the same through the various crystals. A phase boundary signature is also observed inside the LFSC phase near $H^{\ast}$, indicating an intermediate SC phase characterized by small flux pinning forces.

cond-mat.supr-con↗

arXiv2022

Enhancement of giant magnetoelectric effect in Ni-doped CaBaCo$_{4}$O$_{7}$

The polar magnet CaBaCo$_{4}$O$_{7}$ is known to exhibit the largest magnetic-field-driven electric polarization change ($ΔP$) associated with an antiferromagnetic (AFM)-ferrimagnetic (FIM) transition in a narrow temperature range between 62 and 69 K. In this work, we investigate the effect of Ni doping on its multiferroic properties, by means of magnetization, electric polarization, dielectric constant, and magnetostriction measurements on single crystals of CaBaCo$_{3.9}$Ni$_{0.1}$O$_{7}$ up to 50 T. In the doped material, two kinds of AFM phases appear below 78 K, accompanying negative $ΔP$. Upon the application of a magnetic field along any crystallographic axis, giant positive $ΔP$ of up to $11 \sim 12$ mC/m$^{2}$ is observed along with an AFM-FIM transition in the whole temperature range below 78 K. The giant magnetoelectric effect inherent in CaBaCo$_{4}$O$_{7}$ can be further enhanced just by a small amount of chemical substitution, in terms of (i) increasing the magnitude of $ΔP$ and (ii) expanding the temperature range in which giant $ΔP$ appears.

cond-mat.mtrl-sci↗

arXiv2022

Refining magnetic interactions from the magnetic field dependence of spin-wave excitations in magnetoelectric LiFePO$_4$

We investigated the spin excitations of magnetoelectric $\text{LiFePO}_4$ by THz absorption spectroscopy in magnetic fields up to 33$\,$T. By studying their selection rules, we found not only magnetic-dipole, but also electric-dipole active (electromagnons) and magnetoelectric resonances. The magnetic field dependence of four strong low-energy modes is reproduced well by our four-sublattice spin model for fields applied along the three orthorhombic axes. From the fit, we refined the exchange couplings, single-ion anisotropies, and the Dzyaloshinskii-Moriya interaction parameters. Additional spin excitations not described by the mean-field model are observed at higher frequencies. Some of them shows a strong shift with magnetic field, up to 4$\,\text{cm}^{-1}/\text{T}$, when the field is applied along the easy axis. Based on this field dependence, we attribute these high frequency resonances to excitation of higher spin multipoles and of two magnons, which become THz-active due to the low symmetry of the magnetically ordered state.

cond-mat.str-el↗

arXiv2022

Effect of uranium deficiency on normal and superconducting properties in unconventional superconductor UTe$_2$

Single crystals of the unconventional superconductor UTe$_2$ have been grown in various conditions which result in different superconducting transition temperature as well as normal state properties. Stoichiometry of the samples has been characterized by the single-crystal X-ray crystallography and electron microprobe analyses. Superconducting samples are nearly stoichiometric within an experimental error of about 1 \%, while non-superconducting sample significantly deviates from the ideal composition. The superconducting UTe$_2$ showed that the large density of states was partially gapped in the normal state, while the non-superconducting sample is characterized by the relatively large electronic specific heat as reported previously.

cond-mat.supr-con↗

arXiv2021

Unconventional Superconductivity in UTe2

The novel spin-triplet superconductor candidate UTe2 was discovered only recently at the end of 2018 and attracted enormous attention. We review key experimental and theoretical progress which has been achieved in different laboratories. UTe2 is a heavy-fermion paramagnet, but right after the discovery of superconductivity it has been expected to be close to a ferromagnetic instability showing many similarities to the U-based ferromagnetic superconductors, URhGe and UCoGe. The competition between different types of magnetic interactions and the duality between the local and itinerant character of the 5f Uranium electrons, as well as the shift of the U valence appear as key parameters in the rich phase diagrams discovered recently under extreme conditions like low temperature, high magnetic field, and pressure. We discuss macroscopic and microscopic experiments at low temperature to clarify the normal phase properties at ambient pressure. Special attention will be given to the occurrence of a metamagnetic transition at Hm = 35 T for a magnetic field applied along the hard magnetic axis b. Adding external pressure leads to strong changes in the magnetic and electronic properties with a direct feedback on superconductivity. Attention will be given on the possible evolution of the Fermi surface as a function of magnetic field and pressure. Superconductivity in UTe2 is extremely rich exhibiting various unconventional behaviors which will be highlighted. It shows an exceptionally huge superconducting upper critical field with a re-entrant behavior under magnetic field and the occurrence of multiple superconducting phases in the temperature field, pressure phase diagram. There is evidence for spin-triplet pairing. The different theoretical approaches will be described. Notably we discuss that UTe2 is a possible example for the realization of a fascinating topological superconductor.

cond-mat.str-el↗