arXiv subjects

Y. Kohama

Publications and source records attributed to Y. Kohama.

At least 19 recordsLinked to original sources

arXiv2026

Defying Diamagnetism: Anomalous Redshift and Robust Chiral Spin Mixing in 2D Perovskites under High Magnetic Fields up to 50 T

We report circularly polarized magneto-photoluminescence (MPL) measurements on the enantiomeric pair of two-dimensional chiral perovskites R- and S-(C4H9NH3)2PbI4 in pulsed magnetic fields up to 50 T at 4.2 K. The R-form exhibits strong preferential emission into the RC channel (I_RC/I_LC = 10:1), with the S-form showing the opposite sense, robust against the applied field. Three key observations are reported: (i) an anomalous redshift of all exciton transition energies with increasing field-contrasting with the normal diamagnetic blueshift of achiral perovskites-with a non-monotonic initial blueshift to 7.6 T in the dominant RC channel of the R-form; (ii) complete invariance of PL peak positions upon reversal of the field direction, consistent with Rashba-induced mixing of the bright exciton with a nearby m_x = 0 dark state; and (iii) field-induced increase in PL intensity and narrowing of the linewidth, attributed to suppression of disorder scattering as the cyclotron orbit contracts. These results are interpreted in the framework of Rashba spin-splitting and polaron formation in the chiral two-dimensional perovskite lattice.

cond-mat.mtrl-sci↗

arXiv2025

Quintuplet condensation in the skyrmionic insulator Cu2OSeO3 at ultrahigh magnetic fields

We report ultrahigh magnetic field Faraday rotation results on the chiral helimagnet Cu2OSeO3, the first Mott insulator showing skyrmion lattice phases and a linear magnetoelectric effect. Between 180 and 300 T, we find signatures of a Bose-Einstein condensation (BEC) of magnons, which can be described as a canted XY ferrimagnet. Due to the magnetoelectric coupling, the transverse magnetic order of the indivual Cu2+ spins is accompanied by a characteristic dome-like electric polarization which is crucial for the observation of the condensate via the Faraday rotation effect.

cond-mat.str-el↗

arXiv2025

High-field NMR study of field-induced states in Pb(TiO)Cu$_4$(PO$_4$)$_4$

The square cupola antiferromagnet Pb(TiO)Cu$_4$(PO$_4$)$_4$ exhibits the intriguing magnetoelectric responses arising from the consecutive change in the magnetic quadrupolar-type configuration of magnetic moments under external magnetic fields higher than 15 T. To clarify the high-field magnetic structures in Pb(TiO)Cu$_4$(PO$_4$)$_4$, an NMR measurement was performed in pulsed fields up to 32.2 T significantly extending the field range accessible by superconducting magnets. The double-peak structure of NMR spectra emerging above 29 T applied along the [001] direction evidences the successive magnetic transitions. The field dependence of NMR spectra was analyzed on the basis of cluster mean-field theory, which allows us to propose possible magnetic structures for the high-field magnetic states.

cond-mat.str-el↗

arXiv2025

Triplon Bose-Einstein condensation and proximate magnetism in dimerized antiferromagnets

Dimerized quantum magnets provide a useful arena for novel quantum states and phases transitions with the singlet-triplet type of triplon excitations. Here we study the triplon physics and the Bose-Einstein condensation in two isostructural dimerized antiferromagnets $A$Cu(SeO$_3$)$_2$ ($A$ = Hg, Cd). With the systematic measurements, we demonstrate a dimer singlet ground state in HgCu(SeO$_3$)$_2$ with a triplon gap $\sim$ 7.9 K and a triplon Bose-Einstein condensation with an antiferromagnetic order in CdCu(SeO$_3$)$_2$ below 4.4 K. We further adopt the bond-operator technique and show that the elemental replacement preserves the Hamiltonian and allows the study in a unified theoretical framework with tunable interdimer and intradimer interactions on the opposite sides of the quantum critical point. With the peculiar Cu$_2$O$_8$ dimer configuration and effective ferromagnetic interdimer interaction, $A$Cu(SeO$_3$)$_2$ is distinguished from other $S$ = 1/2 dimerized antiferromagnets. Our results represent a global understanding of the magnetic ground states as well as the magnetic transitions in the dimerized magnets of this unusual crystal structure.

cond-mat.str-el↗

arXiv2025

XRD study of the magnetization plateau above 40 T in the frustrated helimagnet CuGaCr$_{4}$S$_{8}$

CuGaCr$_{4}$S$_{8}$, which contains a chromium breathing pyrochlore network, exhibits diverse magnetic phases, including an incommensurate helical state below 31 K and a 1/2-magnetization plateau above 40 T, owing to the interplay between magnetic frustration and spin-lattice coupling. Here, we perform a single-shot powder x-ray diffraction experiment on CuGaCr$_{4}$S$_{8}$ in a pulsed high magnetic field of 55 T, revealing an orthorhombic-to-cubic (or pseudocubic) structural transition upon entering the 1/2-magnetization plateau phase at low temperatures. This observation suggests the emergence of a commensurate ferrimagnetic order, where a 3-up-1-down spin configuration is realized in each small tetrahedron, and the all-up or all-down in each large tetrahedron. We propose two types of 16-sublattice magnetic structures, which are degenerate within exchange interactions between the first, second, and third nearest neighbors.

cond-mat.mtrl-sci↗

arXiv2025

Ultrasound measurement technique for the single-turn-coil magnets

Ultrasound is a powerful means to study numerous phenomena of condensed-matter physics as acoustic waves couple strongly to structural, magnetic, orbital, and charge degrees of freedom. In this paper, we present such technique combined with single-turn coils (STC) which generate magnetic fields beyond 100 T with the typical pulse duration of 6 us. As a benchmark of this technique, the ultrasound results for MnCr2S4, Cu6[Si6O18]6H2O, and liquid oxygen are shown. The resolution for the relative sound-velocity change in the STC is estimated as Delta v/v~10^-3, which is sufficient to study various field-induced phase transitions and critical phenomena.

cond-mat.other↗

arXiv2024

Gapless spin excitations in a quantum spin liquid state of S=1/2 perfect kagome antiferromagnet

Quantum spin liquids (QSLs) represent an exotic quantum many-body state characterized by the suppression of long-range magnetic order due to strong quantum fluctuations. The kagome spin-1/2 antiferromagnet (AFM) is a prime candidate for realizing QSLs, but its ground state remains an unresolved conundrum. Here we investigate the recently discovered perfect kagome AFM YCu$_3$(OH)$_{6.5}$Br$_{2.5}$ to elucidate two central enigmas surrounding the kagome AFM. Ultra-sensitive torque magnetometry experiments reveal that the intrinsic magnetic susceptibility arising from the kagome layer remains nearly temperature-independent down to exceedingly low temperatures. This observation seemingly implies the emergence of gapless fermionic spin excitations akin to Pauli paramagnetism in metals. However, most strikingly, these results stand in stark contrast to the conspicuous absence of a temperature-linear contribution to the specific heat. These findings appear irreconcilable with the widely-discussed theoretical frameworks assuming fermionic quasiparticles (QPs), instead suggesting a transition of bosonic QPs into a superfluid state with a gapless Goldstone mode. Furthermore, magnetocaloric measurements evince an entropy anomaly, constituting thermodynamic evidence that magnetic fields instigate the opening of a spin gap, driving a quantum phase transition into a 1/9 magnetization plateau state. These results shed light on the nature of the low-energy excitations in zero and strong magnetic fields, providing crucial insights into the long-standing unresolved issues of the ground state of the kagome AFM.

cond-mat.str-el↗

arXiv2023

Rhenium oxyhalides: a showcase for anisotropic-triangular-lattice quantum antiferromagnets

The spin-1/2 Heisenberg antiferromagnet on an anisotropic triangular lattice (ATL) is an archetypal spin system hosting exotic quantum magnetism and dimensional crossover. However, the progress in experimental research on this field has been limited due to the scarcity of ideal model materials. Here, we show that rhenium oxyhalides $A_{3}$ReO$_{5}X_{2}$, where spin-1/2 Re$^{6+}$ ions form a layered structure of ATLs, allow for flexible chemical substitution in both cation $A^{2+}$ ($A$ = Ca, Sr, Ba, Pb) and anion $X^{-}$ ($X$ = Cl, Br) sites, leading to seven synthesizable compounds. By combining magnetic susceptibility and high-field magnetization measurements with theoretical calculations using the orthogonalized finite-temperature Lanczos method, we find that the anisotropy $J'/J$ ranges from 0.25 to 0.45 depending on the chemical composition. Our findings demonstrate that $A_{3}$ReO$_{5}X_{2}$ is an excellent platform for realizing diverse effective spin Hamiltonians that differ in the strength of the anisotropy $J'/J$ as well as the relevance of perturbation terms such as the Dzyaloshinskii-Moriya interaction and interlayer exchange coupling.

cond-mat.mtrl-sci↗

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

Signatures of a magnetic superstructure phase induced by ultrahigh magnetic fields in a breathing pyrochlore antiferromagnet

The mutual coupling of spin and lattice degrees of freedom is ubiquitous in magnetic materials and potentially creates exotic magnetic states in response to the external magnetic field. Particularly, geometrically frustrated magnets serve as a fertile playground for realizing magnetic superstructure phases. Here, we observe an unconventional two-step magnetostructural transition prior to a half-magnetization plateau in a breathing pyrochlore chromium spinel by means of state-of-the-art magnetization and magnetostriction measurements in ultrahigh magnetic fields available up to 600 T. Considering a microscopic magnetoelastic theory, the intermediate-field phase can be assigned to a magnetic superstructure with a three-dimensional periodic array of 3-up-1-down and canted 2-up-2-down spin molecules. We attribute the emergence of the magnetic superstructure to a unique combination of the strong spin-lattice coupling and large breathing anisotropy.

cond-mat.mtrl-sci↗

arXiv2023

Radio Frequency Electrical Resistance Measurement under Destructive Pulsed Magnetic Fields

We developed a resistance measurement using radio frequency reflection to investigate the electrical transport characteristics under destructive pulsed magnetic fields above 100 T. A homemade flexible printed circuit for a sample stage reduced the noise caused by the induced voltage from the pulsed magnetic fields, improving the accuracy of the measurements of the reflected waves. From the obtained reflectance data, the absolute value of the magnetoresistance was successfully determined by using a phase analysis with admittance charts. These developments enable more accurate and comprehensive measurements of electrical resistance in pulsed magnetic fields.

physics.app-ph↗

arXiv2023

Unveiling new quantum phases in the Shastry-Sutherland compound SrCu2(BO3)2up to the saturation magnetic field

Under magnetic fields, quantum magnets often undergo exotic phase transitions with various kinds of order. The discovery of a sequence of fractional magnetization plateaus in the Shastry-Sutherland compound SrCu2(BO3)2 has played a central role in the high-field research on quantum materials, but so far this system could only be probed up to half the saturation value of the magnetization. Here, we report the first experimental and theoretical investigation of this compound up to the saturation magnetic field of 140 T and beyond. Using ultrasound and magnetostriction techniques combined with extensive tensor-network calculations (iPEPS), several spin-supersolid phases are revealed between the 1/2 plateau and saturation (1/1 plateau). Quite remarkably, the sound velocity of the 1/2 plateau exhibits a drastic decrease of -50%, related to the tetragonal-to-orthorhombic instability of the checkerboard-type magnon crystal. The unveiled nature of this paradigmatic quantum system is a new milestone for exploring exotic quantum states of matter emerging in extreme conditions.

cond-mat.str-el↗

arXiv2023

Quantum oscillations in a centrosymmetric skyrmion-hosting magnet GdRu2Si2

We have performed magnetic torque and resistivity measurements on a centrosymmetric skyrmion-host GdRu2Si2, in which the dominant magnetic interaction leading to skyrmion formation is under debate. We observe both the de Haas-van Alphen and Shubnikov-de Haas oscillations in the forced ferromagnetic phase. The angular dependence of the quantum oscillation frequencies can be reproduced by the ab-initio calculation. The de Haas-van Alphen oscillation is also observed in the double-Q phase with a different frequency to that in the forced ferromagnetic phase, indicating a Fermi surface reconstruction due to the coupling between localized spins and conduction electrons. Based on these experimental findings, the magnetic interactions in this system are discussed.

cond-mat.str-el↗

arXiv2023

Nonreciprocal Phonon Propagation in a Metallic Chiral Magnet

The phonon magnetochiral effect (MChE) is the nonreciprocal acoustic and thermal transports of phonons caused by the simultaneous breaking of the mirror and time-reversal symmetries. So far, the phonon MChE has been observed only in a ferrimagnetic insulator Cu2OSeO3, where the nonreciprocal response disappears above the Curie temperature of 58 K. Here, we study the nonreciprocal acoustic properties of a room-temperature ferromagnet Co9Zn9Mn2 for unveiling the phonon MChE close to the room temperature. Surprisingly, the nonreciprocity in this metallic compound is enhanced at higher temperatures and observed up to 250 K. This clear contrast between insulating Cu2OSeO3 and metallic Co9Zn9Mn2 suggests that metallic magnets have a mechanism to enhance the nonreciprocity at higher temperatures. From the ultrasound and microwave-spectroscopy experiments, we conclude that the magnitude of the phonon MChE of Co9Zn9Mn2 mostly depends on the magnon bandwidth, which increases at low temperatures and hinders the magnon-phonon hybridization. Our results suggest that the phonon nonreciprocity could be further enhanced by engineering the magnon band of materials.

cond-mat.mtrl-sci↗

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

Dimensional reduction and incommensurate dynamic correlations in the $S=\frac{1}{2}$ triangular-lattice antiferromagnet Ca$_3$ReO$_5$Cl$_2$

The observation of spinon excitations in the $S=\frac{1}{2}$ triangular antiferromagnet Ca$_3$ReO$_5$Cl$_2$ reveals a quasi-one-dimensional (1D) nature of magnetic correlations, in spite of the nominally 2D magnetic structure. This phenomenon is known as frustration-induced dimensional reduction. Here, we present high-field electron spin resonance spectroscopy and magnetization studies of Ca$_3$ReO$_5$Cl$_2$, allowing us not only to refine spin-Hamiltonian parameters, but also to investigate peculiarities of its low-energy spin dynamics. We argue that the presence of the uniform Dzyaloshinskii-Moriya interaction (DMI) shifts the spinon continuum in momentum space and, as a result, opens a zero-field gap at the $Γ$ point. We observed this gap directly. The shift is found to be consistent with the structural modulation in the ordered state, suggesting this material as a perfect model triangular-lattice system, where a pure DMI-spiral ground state can be realized.

cond-mat.str-el↗

arXiv2021

Physical properties of liquid oxygen under ultrahigh magnetic fields

We studied the acoustic properties of liquid oxygen up to 90 T by means of ultrasound measurements. We observed a monotonic decrease of the sound velocity and an asymptotic increase of the sound attenuation when applying magnetic fields. The unusual attenuation, twenty times as large as the zero-field value, suggests strong fluctuations of the local molecular arrangement. We point out that the observed fluctuations are related to a liquid-liquid transition or crossover, from a small-magnetization to a large-magnetization liquid, which is characterized by a local-structure rearrangement. To investigate higher-field properties of liquid oxygen, we performed single-turn-coil experiments up to 180 T by means of the acoustic, dilatometric, magnetic, and optical techniques. We observed only monotonic changes of these properties, reflecting the absence of the proposed liquid-liquid transition in our experimental conditions.

cond-mat.soft↗

arXiv2021

Higher magnetic-field generation by a mass-loaded single-turn coil

Single-turn coil (STC) technique is a convenient way to generate ultrahigh magnetic fields of more than 100 T. During the field generation, the STC explosively destructs outward due to the Maxwell stress and Joule heating. Unfortunately, the STC does not work at its full potential because it has already expanded when the maximum magnetic field is reached. Here, we propose an easy way to delay the expansion and increase the maximum field by using a mass-loaded STC. By loading clay on the STC, the field profile drastically changes, and the maximum field increases by 4 %. This method offers an access to higher magnetic fields for physical property measurements.

cond-mat.mtrl-sci↗