arXiv subjects

Nobuyuki Kurita

Publications and source records attributed to Nobuyuki Kurita.

At least 19 recordsLinked to original sources

arXiv2026

Critical Berezinskii-Kosterlitz-Thouless dynamics in the archetypal two-dimensional spin system Ba$_2$CuSi$_2$O$_6$Cl$_2$

We study the spin dynamics in the quasi-2D spin-$1/2$ dimer compound Ba$_2$CuSi$_2$O$_6$Cl$_2$, which exhibits a magnetic field-induced Bose-Einstein condensate (BEC) of triplons. Using nuclear magnetic resonance spin-lattice relaxation rate ($T_1^{-1}$) measurements combined with large-scale quantum Monte Carlo (QMC) simulations, we investigate critical fluctuations across the field-temperature phase diagram. Bridging the behavior observed in 1D and 3D systems, the $T_1^{-1}$ relaxation rate shows a pronounced peak extending well above the Néel temperature $T_N$, indicating strong two-dimensional Berezinskii-Kosterlitz-Thouless (BKT)-type fluctuations. A quantitative match between experimental and theoretical BEC phase boundaries validates an effective XXZ model. The study determines the intrinsic BKT transition temperature $T_{\mathrm{BKT}}$ from QMC, revealing a nearly field-independent $T_{\mathrm{BKT}}/T_N \approx 0.74$. Scaling analysis of the relaxation rate shows critical exponents consistent with 2D universality, and a narrow temperature window is identified where 2D physics dominates. These findings establish Ba$_2$CuSi$_2$O$_6$Cl$_2$ as a model system for exploring BKT dynamics in quantum magnets.

cond-mat.str-el↗

arXiv2026

NMR study on equilateral triangular lattice antiferromagnet Ba2La2CoTe2O12

We report a 139La-NMR study of Ba2La2CoTe2O12, S = 1/2 equilateral triangular-lattice antiferromagnet with easy-plane anisotropy at low temperatures. This compound undergoes a magnetic phase transition at TN = 3.26 K into an ordered state with the 120 degree spin structure. Under magnetic fields above 3T, TN splits into TN1 and TN2, which correspond to the transitions from the paramagnetic phase to the up-up-down (uud) phase and from the uud phase to the triangular coplanar phase, respectively. The NMR spin-lattice relaxation rate 1/T1 exhibits a critical divergence at TN1, indicating the onset of long-range magnetic order. At TN2, the NMR-linewidth measured at 5.4 T exhibits an anomalous decrease, which we attribute to a change in the spin structure from the uud to the triangular coplanar phase.

cond-mat.stat-mech↗

arXiv2024

Dirac Magnon in Honeycomb Lattice Magnet NiTiO3

We performed inelastic neutron scattering experiments on single-crystal samples of the honeycomb lattice magnet, ilmenite NiTiO3. Below the Neel temperature of 22 K, spin wave excitations with a band energy of 3.7 meV were observed. The neutron energy spectra were well-reproduced by modeling the system as a ferromagnetic honeycomb lattice with antiferromagnetic interlayer coupling, using linear spin wave theory. Similar to another ilmenite CoTiO3, a crossing structure was observed at the K point, suggesting the resence of Dirac magnons in NiTiO3. Further calculations suggested the formation of Dirac nodal line.

cond-mat.str-el↗

arXiv2023

NiCrAl piston-cylinder cell for magnetic susceptibility measurements under high pressures in pulsed high magnetic fields

We developed a metallic pressure cell made of nickel-chromium-aluminum (NiCrAl) for use with a non-destructive pulse magnet and a magnetic susceptibility measurement apparatus with a proximity detector oscillator (PDO) in pulsed magnetic fields of up to 51 T under pressures of up to 2.1 GPa. Both the sample and sensor coil of the PDO were placed in the cell so that the magnetic signal from NiCrAl would not overlay the intrinsic magnetic susceptibility of the sample. A systematic investigation of the Joule heating originating from metallic parts of the pressure cell revealed that the temperature at the sample position remains at almost 1.4 K until approximately 80 $\%$ of the maximum applied magnetic field ($H_{\rm max}$) in the field-ascending process (e.g., 40 T for $H_{\rm max}$ of 51 T). The effectiveness of our apparatus was demonstrated, by investigating the pressure dependence of the magnetization process of the triangular-lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$.

cond-mat.mtrl-sci↗

arXiv2023

Possible Intermediate Quantum Spin Liquid Phase in $α$-RuCl$_3$ under High Magnetic Fields up to 100 T

Pursuing the exotic quantum spin liquid (QSL) state in the Kitaev material $α$-RuCl$_3$ has intrigued great research interest recently. A fascinating question is on the possible existence of field-induced QSL phase in this compound. Here we perform a high-field measurement of the magnetization process of $α$-RuCl$_3$ up to 102 T employing the non-destructive and destructive pulsed magnets. Under the out-of-plane field along the c$^*$ axis (i.e., perpendicular to the honeycomb plane), two quantum phase transitions are uncovered at respectively 36 T and about 83 T, between which there lies an intermediate phase as the predicted QSL. By measuring the magnetization data with fields tilted from the c$^*$ axis up to $90^\circ$ (i.e., in-plane direction), we obtain the field-angle phase diagram that contains the zigzag, paramagnetic, and QSL phases. Based on the accurate $K$-$J$-$Γ$-$Γ^{\prime}$ model of $α$-RuCl$_3$, we perform density matrix renormalization group simulations and reproduce the quantum phase diagram in excellent agreement with experiments.

cond-mat.str-el↗

arXiv2023

Magnetic phase diagram in three-dimensional triangular-lattice antiferromagnet Sr$_3$CoTa$_2$O$_9$ with small easy-axis anisotropy

We report the results of low-temperature magnetization and specific heat measurements of Sr$_3$CoTa$_2$O$_9$ powder, in which Co$^{2+}$ ions with effective spin-1/2 form a uniform triangular lattice in the $ab$ plane. It was found that Sr$_3$CoTa$_2$O$_9$ undergoes successive antiferromagnetic transitions at $T_{\rm N1}\,{=}\,0.97~{\rm K}$ and $T_{\rm N2}\,{=}\,0.79~{\rm K}$ at zero magnetic field. As the magnetic field increases, both $T_{\rm N1}$ and $T_{\rm N2}$ decrease monotonically. The obtained magnetic field vs temperature phase diagram together with a sharp magnetization anomaly at a saturation field of ${μ_0}H_{\rm s}\,{=}\,2.3~{\rm T}$ indicates that Sr$_3$CoTa$_2$O$_9$ is described as a spin-1/2 three-dimensional triangular-lattice antiferromagnet with a weak easy-axis anisotropy. We discuss the characteristics of the magnetic phase diagram, which approximates the phase diagram for the magnetic field perpendicular to the $c$ axis.

cond-mat.str-el↗

arXiv2022

Field-orientation dependence of quantum phase transitions in the S=1/2 triangular-lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$

Ba$_3$CoSb$_2$O$_9$ approximates the two-dimensional spin-1/2 triangular-lattice Heisenberg antiferromagnet. This compound displays magnetic-field-induced quantum phase transitions, including the 1/3-magnetization-plateau, but its magnetization processes for the magnetic field $H$ parallel and perpendicular to the $c$ axis are different due to the weak easy-plane anisotropy and the weak interlayer antiferromagnetic exchange interaction. To elucidate how the quantum phase transitions change between these two field directions, we measured the field-angle dependence of the magnetization process in Ba$_3$CoSb$_2$O$_9$ using pulsed high magnetic fields. We compared obtained magnetic field-field angle phase diagram with those obtained by the large-size cluster mean-field method combined with a scaling scheme and the semiclassical theory. We also found a narrow 1/3-magnetization plateau and a high-field transition with a small magnetization jump for $H\,{\parallel}\,c$, not observed in the previous studies.

cond-mat.str-el↗

arXiv2022

Structures of magnetic excitations in the spin-1/2 kagome-lattice antiferromagnets Cs$_2$Cu$_3$SnF$_{12}$ and Rb$_2$Cu$_3$SnF$_{12}$

We show the structures of magnetic excitations in spin-1/2 kagome-lattice antiferromagnets Cs$_2$Cu$_3$SnF$_{12}$ and Rb$_2$Cu$_3$SnF$_{12}$ investigated by inelastic neutron scattering in wide energy and momentum ranges. For Cs$_2$Cu$_3$SnF$_{12}$, four single-magnon excitation modes were observed. Low-energy three modes are assigned to be transverse modes and the high-energy fourth mode is suggested to be an amplitude mode. It was found that the broad excitation continuum without a marked structure spreads in a wide energy range from $0.15J$ to approximately $2.5J$ in contrast to the clearly structured excitation continuum observed in the spin-1/2 triangular-lattice Heisenberg antiferromagnet. These findings strongly suggest spinon excitations as elementary excitations in Cs$_2$Cu$_3$SnF$_{12}$. In Rb$_2$Cu$_3$SnF$_{12}$, singlet-triplet excitations from the pinwheel VBS state and their ghost modes caused by the enlargement of the chemical unit cell were clearly confirmed. It was found that the excitation continuum is structured in the low-energy region approximately below $J_{\mathrm{avg}}$ and the almost structureless high-energy excitation continuum extends to approximately $2.6J_{\mathrm{avg}}$. The characteristics of the high-energy excitation continuum are common to both Cs$_2$Cu$_3$SnF$_{12}$ and Rb$_2$Cu$_3$SnF$_{12}$, irrespective of their ground states. The experimental results strongly suggest that the spin liquid component remains in the ground state as quantum fluctuations in Cs$_2$Cu$_3$SnF$_{12}$ and Rb$_2$Cu$_3$SnF$_{12}$.

cond-mat.str-el↗

arXiv2022

Contrasting Magnetic Structures in SrLaCuSbO$_{6}$ and SrLaCuNbO$_{6}$: Spin-1/2 Quasi-Square-Lattice $J_1 {-} J_2$ Heisenberg Antiferromagnets

We report the magnetic properties of the double perovskites SrLaCuSbO$_6$ (SLCSO) and SrLaCuNbO$_6$ (SLCNO). The temperature dependence of the magnetic susceptibilities of both compounds shows a broad maximum characteristic of an $S\,=\,1/2$ square lattice Heisenberg antiferromagnet. Magnetic ordering occurs at $T_{\rm N}\,{=}\,13.6$ and 15.7 K for SLCSO and SLCNO, respectively. Neutron powder diffraction measurements reveal contrasting spin structures in both compounds. The spin structures of SLCSO and SLCNO below $T_{\rm N}$ are Néel antiferromagnetic and collinear antiferromagnetic, respectively. This result demonstrates that the nearest-neighbor interaction is dominant in SLCSO, whereas the next-nearest-neighbor interaction is dominant in SLCNO. The magnitude of the ordered moment was evaluated at 3.5 K to be $m\,{=}\,0.39(3)\,μ_{\rm B}$ for SLCSO and $0.37(1)\,μ_{\rm B}$ for SLCNO, which are significantly smaller than those calculated using linear spin wave theory. We infer that the small ordered moment is caused by the effect of exchange bond randomness arising from the site disorder of Sr and La ions.

cond-mat.str-el↗

arXiv2021

Magnons and Spinons in $\mathrm{Ba}_2\mathrm{CoTeO}_6 $: A Composite System of Isolated Spin-$1/2$ Triangular Heisenberg-like and Frustrated Honeycomb Ising-like Antiferromagnets

We report the neutron scattering results on magnetic orderings and excitations in $\mathrm{Ba}_2\mathrm{CoTeO}_6$ composed of two almost isolated subsystems A and B, which are described as an $S\,{=}\,1/2$ triangular Heisenberg-like antiferromagnet and a frustrated honeycomb Ising-like antiferromagnet, respectively. Stripy ordering of subsystem B was confirmed below $T_{\rm N1}\,{=}\,12.0$ K, whereas sharp streaks were observed along $(1/3, 1/3, L)$ and $(2/3, 2/3, L)$ at 0.3 K (${\ll}\,T_{\rm N2}\,{=}\,3.0$ K). This indicates the two-dimensional nature of ordering in subsystem A. It was found that the excitation spectra of both subsystems are well separated and independent of each other. The excitation spectrum of subsystem A is composed of two single-magnon branches with roton-like minima at the M point and a clearly structured intense continuum, as similarly observed in $\mathrm{Ba}_3\mathrm{CoSb}_2\mathrm{O}_9$, which is strongly indicative of spinon excitations. Dispersion curves for subsystem B can be described by linear spin wave theory within the third-neighbor exchange interaction.

cond-mat.str-el↗

arXiv2020

Coupled spin-1/2 antiferromagnetic chain Cs$_2$LiRuCl$_6$ with partially disordered crystal lattice

We determined the crystal structure of Cs$_2$LiRuCl$_6$, which was synthesized first in this work, and investigated its magnetic properties. Cs$_2$LiRuCl$_6$ has a hexagonal structure composed of linear chains of face-sharing RuCl$_6$ and LiCl$_6$ octahedra. In two-thirds of the structural chains, Ru$^{3+}$ and Li$^+$ sites are almost ordered, while in the other chains their sites are disordered. This situation is analogous to the ground state of the antiferromagnetic Ising model on a triangular lattice. Using electron paramagnetic resonance, we evaluated the $g$ factors of Ru$^{3+}$ with effective spin-1/2 as $g_c\,{=}\,2.72$ and $g_{ab}\,{=}\,1.50$ for magnetic fields $H$ parallel and perpendicular to the $c$ axis, respectively. Magnetization curves for $H\,{\parallel}\,c$ and $H\,{\perp}\,c$ are highly anisotropic. However, these magnetization curves approximately coincide when normalized by the $g$ factors. It was found from the magnetization and specific heat results that Cs$_2$LiRuCl$_6$ can be described as a coupled one-dimensional $S\,{=}\,1/2$ Heisenberg-like antiferromagnet with $J/k_{\rm B}\,{\simeq}\,3.7$ K. Three-dimensional ordering occurs at $T_{\rm N}\,{=}\,0.48$ K. A magnetic phase diagram for $H\,{\parallel}\,c$ is also presented.

cond-mat.str-el↗

arXiv2019

Successive phase transitions and quantum magnetization plateau in the spin-1 triangular-lattice antiferromagnet Ba$_2$La$_2$NiTe$_2$O$_{12}$

The crystal structure and magnetic properties of the spin-1 triangular-lattice antiferromagnet Ba$_2$La$_2$NiTe$_2$O$_{12}$ are reported. Its crystal structure is trigonal $R\bar{3}$, which is the same as that of Ba$_2$La$_2$NiW$_2$O$_{12}$ [Y. Doi et al., J. Phys.: Condens. Matter 29, 365802 (2017)]. However, the exchange interaction $J/k_{\mathrm{B}}\simeq19$ K is much greater than that observed in the tungsten system. At zero magnetic field, Ba$_2$La$_2$NiTe$_2$O$_{12}$ undergoes successive magnetic phase transitions at $T_{\mathrm{N}1}=9.8$ K and $T_{\mathrm{N}2}=8.9$ K. The ground state is accompanied by a weak ferromagnetic moment. These results indicate that the ground-state spin structure is a triangular structure in a plane perpendicular to the triangular lattice owing to the small easy-axis-type anisotropy. The magnetization curve exhibits the one-third plateau characteristic of a two-dimensional triangular-lattice Heisenberg-like antiferromagnet. Exchange constants are also evaluated using density functional theory (DFT). The DFT results demonstrate the large difference in the exchange constants between tellurium and tungsten systems and the good two-dimensionality of the tellurium system.

cond-mat.str-el↗

arXiv2019

Novel Excitations near Quantum Criticality in Geometrically Frustrated Antiferromagnet CsFeCl$_{3}$

Investigation of materials that exhibit quantum phase transition provides valuable insights into fundamental problems in physics. We present neutron scattering under pressure in a triangular-lattice antiferromagnet which has a quantum disorder in the low-pressure phase and a noncollinear structure in the high-pressure phase. The neutron spectrum continuously evolves through the critical pressure; a single mode in the disordered state becomes soft with the pressure, and it splits into gapless and gapped modes in the ordered phase. Extended spin-wave theory reveals that the longitudinal and transverse fluctuations of spins are hybridized in the modes because of the noncollinearity, and novel magnetic excitations are formed. We report a new hybridization of the phase and amplitude fluctuations of the order parameter in a spontaneously symmetry-broken state.

cond-mat.str-el↗

arXiv2019

Localized magnetic excitations in the fully frustrated dimerized magnet Ba$_2$CoSi$_2$O$_6$Cl$_2$

Magnetic excitations of the effective spin $S$=1/2 dimerized magnet Ba$_2$CoSi$_2$O$_6$Cl$_2$ have been probed directly via inelastic neutron scattering experiments at temperatures down to 4 K. We observed five types of excitation at 4.8, 5.8, 6.6, 11.4, and 14.0 meV, which are all dispersionless within the resolution limits. The scattering intensities of the three low-lying excitations were found to exhibit different $Q$-dependences. Detailed analysis has demonstrated that Ba$_2$CoSi$_2$O$_6$Cl$_2$ is a two-dimensional spin dimer system described only by a single dimer site, where the triplet excitations are localized owing to the almost perfect frustration of the interdimer exchange interactions and the undimerized spins, even in small concentration, make an essential contribution to the excitation spectrum.

cond-mat.str-el↗

arXiv2018

Quantum magnetic properties of the spin-1/2 triangular-lattice antiferromagnet Ba$_2$La$_2$CoTe$_2$O$_{12}$

We report the crystal structure of Ba$_2$La$_2$CoTe$_2$O$_{12}$ determined by Rietveld analysis using X-ray powder diffraction data. It was found from magnetic measurements that Ba$_2$La$_2$CoTe$_2$O$_{12}$ can be described as a spin-1\2 triangular-lattice antiferromagnet with easy-plane anisotropy at low temperatures. This compound undergoes a magnetic phase transition at $T_{\rm N}\,{=}\,3.26$ K to an ordered state with the $120^{\circ}$ structure. The magnetization curve exhibits the one-third plateau characteristic of triangular-lattice quantum antiferromagnets. The antiferromagnetic exchange interaction and the $g$ factors parallel and perpendicular to the $c$ axis were evaluated to be $J/k_{\rm B}\,{=}\,22$ K, $g_{\parallel}\,{=}\,3.5$ and $g_{\perp}\,{=}\,4.5$, respectively.

cond-mat.str-el↗

arXiv2018

Triplon band splitting and topologically protected edge states in the dimerized antiferromagnet

The search for topological insulators has been actively promoted in the field of condensed matter physics for further development in energy-efficient information transmission and processing. In this context, recent studies have revealed that not only electrons but also bosonic particles such as magnons can construct edge states carrying nontrivial topological invariants. Here we demonstrate topological triplon bands in the spin-1/2 two-dimensional dimerized quantum antiferromagnet Ba$_2$CuSi$_2$O$_6$Cl$_2$, which is closely related to a pseudo-one-dimensional variant of the Su-Schrieffer-Heeger (SSH) model, through inelastic neutron scattering experiments. The excitation spectrum exhibits two triplon bands and a clear band gap between them due to a small alternation in interdimer exchange interactions along the $a$-direction, which is consistent with the crystal structure. The presence of topologically protected edge states is indicated by a bipartite nature of the lattice.

cond-mat.str-el↗

arXiv2018

Valence-Bond-Glass State with Singlet Gap in the Spin-1/2 Square-Lattice Random $J_1$-$J_2$ Heisenberg Antiferromagnet Sr$_2$CuTe$_{1-x}$W$_x$O$_6$

The double-perovskite compounds Sr$_2$CuTeO$_6$ and Sr$_2$CuWO$_6$ are magnetically described as quasi-two-dimensional spin-1/2 square-lattice $J_1{-}J_2$ Heisenberg antiferromagnets with predominant $J_1$ and $J_2$ exchange interactions, respectively. We report the low-temperature magnetic properties of Sr$_2$CuTe$_{1-x}$W$_x$O$_6$ with randomness in the magnitudes of $J_1$ and $J_2$. It was found that the low-temperature specific heat for $0.1\leq x \leq 0.5$ has a large component proportional to the temperature $T$ above 1.2 K, although the low-temperature specific heat for the two parent systems is approximately proportional to $T^3$. With decreasing temperature below 1.2 K, the $T$-linear component decreases rapidly toward zero, which is insensitive to the magnetic field up to 9 T. This is suggestive of the singlet excitation decoupled from the magnetic field. The NMR spectrum for $x=0.2$ exhibits no long-range order down to 1.8 K. These results indicates that the ground state of Sr$_2$CuTe$_{1-x}$W$_x$O$_6$ is a valence-bond-glass state with singlet gaps.

cond-mat.str-el↗

arXiv2017

Experimental Verification of the Universality of Magnetic-Field-Induced Bose-Einstein Condensation of Magnons

CsFeBr$_3$ is an $S\,{=}\,1$ hexagonal antiferromagnet that has a singlet ground state owing to its large easy-plane single-ion anisotropy. The critical behavior of the magnetic-field-induced phase transition for a magnetic field parallel to the $c$ axis, which can be described by the Bose-Einstein condensation (BEC) of magnons under the $U(1)$ symmetry, was investigated via magnetization and specific heat measurements down to 0.1$\,$K. For the specific heat measurement, we have developed a method of effectively suppressing the torque acting on a sample with strong anisotropy that uses the spin dimer compound Ba$_2$CoSi$_2$O$_6$Cl$_2$ with large and anisotropic Van Vleck paramagnetism. The temperature dependence of the transition field $H_{\rm c}(T)$ was found to follow the power-law $H_{\rm c}(T)\,{-}\,H_{\rm c}\,{\propto}\,T^ϕ$ with a critical exponent of $ϕ\,{=}\,1.50\,{\pm}\,0.02$ and critical field of $H_{\rm c}\,{=}\,2.60$ T. This result verifies the universality of the three-dimensional BEC of magnons described by $ϕ_{\rm BEC}\,{=}\,3/2$.

cond-mat.str-el↗