arXiv subjects

Takayuki Goto

Publications and source records attributed to Takayuki Goto.

At least 19 recordsLinked to original sources

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↗

arXiv2025

$^{23}$Na-NMR study on the one-dimensional superoxide spin-chain compound NaO$_2$

We report $^{23}$Na-NMR study on a candidate one-dimensional quantum spin system NaO$_2$. The Knight shift, linewidth, and spin-lattice relaxation rate $1/T_1$ were investigated down to 0.3 K under fields up to 16 T. The results reveal the opening of a spin gap of $Δ(10.1 {\rm T}) \simeq$ 38 K below $T_{\rm S3} =$ 40 K, consistent with a spin-Peierls-like instability. The hyperfine coupling constant was found to drop sharply across the structural phase transition at $T_{\rm S2} =$ 215 K, highlighting the pronounced one-dimensional character of the system. These findings establish NaO$_2$ as a rare example of a $π$-orbital-based one-dimensional spin chain that exhibits a spin-gapped ground state.

cond-mat.str-el↗

arXiv2024

27Al-NMR Study on a square-Kagome lattice antiferromagnet

NMR study has been performed on an S = 1/2 antiferromagnet KCu6AlBiO4(SO4)5Cl on the square-Kagome lattice, which has three slightly inequivalent nearest-neighbor interactions. Because of the geometrical frustration inherited from triangles within the square kagome lattice and of the low dimensionality, a long range magnetic order is strongly suppressed; its absence has so far been confirmed in low temperatures down to dilution refrigerator region. 27Al-NMR spectra and the longitudinal relaxation time T1 were measured by a conventional pulsed spectrometer on powder sample under several magnetic fields between 3 and 10 T and in low temperatures down to 0.35 K. The NMR line width due to the inhomogeneous broadening increased with lowering temperatures and leveled off below 3 K, where FWHM reached the value as large as 0.1 T, implying that the ground state is magnetic one, consistent with previous reports. On the other hand, the longitudinal nuclear spin relaxation rate 1/T1 obeyed the Arrhenius law with the thermal activation energy Δ = 2K at low temperatures, suggesting that a small gap is formed in the spin excitation spectrum.

cond-mat.str-el↗

arXiv2024

Electron transfer channel in the sugar recognition system assembled on nano gold particle

Existence of 1D spin diffusion in the electrochemical sugar recognition system consisting of a nano-sized gold particle (GNP), a ruthenium complex and a phenylboronic acid was investigated by NMR and muSR. When sugar molecules are recognized by the phenylboronic site, the response of electrochemical voltammetry of the Ru site changes, enabling the system to work as a sensitive sugar-sensor. In this recognition process, the change in the electronic state at the boron site caused by sugar must be transferred to the Ru site via alkyl chains. We have utilized the muon-labelled electrons method and the proton NMR to find out a channel of the electron transfer from the phenylboronic acid site to the gold nano particle via the one dimensional alkyl chain. If this transfer is driven by diffusive spin channel, characteristic field dependence is expected in the longitudinal spin relaxation rate of muSR and 1H-NMR. We have observed significant decrease in the spin relaxation rates with increasing applied field. The result is discussed in terms of low dimensional spin diffusion.

cond-mat.stat-mech↗

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

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↗

arXiv2018

Low temperature magnetism of gold nano particles contained in electrochemical sugar recognition system

Low temperature magnetic properties were investigated on the gold nano particles (GNP) with an average size of 11.5 nm, assembled with molecules of ruthenium complex (Ru0) and phenylboronic acid (B0) by the proton nuclear resonance (1H-NMR) and susceptibility measurements. The temperature dependence of the NMR shift and the uniform susceptibility was described as the sum of the Curie-Weiss term and a positive constant term. From the former, the average number of Ru0 on each GNP was estimated to be 118, which is 23 % of the calculation. The finite positive constant term shows a clear contrast with the well-known fact that the bulk gold is diamagnetic. Finally, a disappearance of the motional narrowing effect in the proton NMR spectra below 60 K indicates that the wavering motion of Ru0 complexes on GNP at room temperature is frozen at low temperatures.

cond-mat.mes-hall↗

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

Rb-NMR study of the quasi-one dimensional competing spin-chain compound Rb2Cu2Mo3O12

Rb-NMR study has been performed on the quasi-one dimensional competing spin chain Rb2Cu2Mo3O12 with ferromagnetic and antiferromagnetic exchange interactions on nearest neighboring and next nearest neighboring spins, respectively. The system changes from a gapped ground state at zero field to the gapless state at H_C \simeq 2 T, where the existence of magnetic order below 1 K was demonstrated by a broadening of NMR spectrum, associated with a critical divergence of 1/T_1. In higher temperature region, 1/T_1 showed a power-law type temperature dependence, from which the field dependence of Luttinger parameter K was obtained and compared with theoretical calculations based on the spin nematic Tomonaga Luttinger Liquid (TLL) state.

cond-mat.str-el↗

arXiv2013

Ground state of the spin-1/2 chain of green dioptase at high fields

The gem-stone dioptase Cu6Si6O18.6H2O has a chiral crystal structure of equilateral triangular helices consisting of Cu-3d spins. It shows an antiferromagnetic order with an easy axis along c at TN = 15.5 K under zero field, and a magnetization jump at HC = 13.5 T when the field is applied along c-axis. By 29Si-NMR measurements, we have revealed that the high-field state is essentially the two sub-lattice structure, and that the component within ab-plane is collinear. The result indicates no apparent match with the geometrical pattern of helical spin chain.

cond-mat.str-el↗

arXiv2013

Ground state of bond-disordered quasi-one-dimensional spin system (CH3)2CHNH3Cu(ClxBr1-x)3 with x = 0, 0.25 and 0.3

The ground state of the quasi-one-dimensional system with bond-disorder (CH3)2CHNH3Cu(ClxBr1-x)3 with x = 0, 0.25 and 0.3 has been investigated by muSR. The Fourier spectrum of electron spin fluctuation in x =0.25 and 0.3 obtained by LF-muSR technique shows that there exists a soft-mode toward a possible phase transition to an exotic phase such as Bose-glass. In gapped system with x = 0, the Fourier spectrum is totally different from the other two, indicating the existence of the quantum critical point at a finite x between 0 and 0.25.

cond-mat.stat-mech↗

arXiv2013

Ground state of the competing spin chain Cs2Cu2Mo3O12

The high-field ground state of the competing-spin-chain compound Cs2Cu2Mo3O12 with the ferromagnetic first-nearest-neighbor J1=-93 K and the antiferromagnetic second-nearest-neighbor J2 = +33 K was investigated by 133Cs-NMR. A divergence of T1-1 and a peak-splitting in spectra were observed at TN = 1.85 K, indicating the existence of a field-induced long range magnetic order. In the paramagnetic region above 4 K, T1-1 showed a power-law temperature dependence T2K-1. The exponent K was strongly field-dependent, suggesting the possibility of the spin-nematic Tomonaga Luttinger Liquid state.

cond-mat.stat-mech↗

arXiv2012

Microscopic phase separation in triangular-lattice quantum spin magnet kappa-(BEDT-TTF)2Cu2(CN)3 probed by muon spin relaxation

The ground state of the quantum spin system kappa-(BEDT-TTF)2Cu2(CN)3 in which antiferromagnetically-interacting S=1/2 spins are located on a nearly equilateral triangular lattice attracts considerable interest both from experimental and theoretical aspects, because a simple antiferromagnetic order may be inhibited because of the geometrical frustration and hence an exotic ground state is expected. Furthermore, recent two reports on the ground state of this system have made it further intriguing by showing completely controversial results; one indicates the gapless state and the other gapped. By utilizing microscopic probe of muSR, we have investigated its spin dynamics below 0.1 K, unveiling its microscopically phase separated ground state at zero field.

cond-mat.str-el↗

arXiv2010

Pressure-induced new magnetic phase in Tl(Cu$_{0.985}$Mg$_{0.015}$)Cl$_3$ probed by muon spin rotation

We carried out zero-field muon-spin-rotation (ZF-$μ$SR) measurements in hydrostatic pressures in impurity-doped quantum spin gap system Tl(Cu$_{0.985}$Mg$_{0.015}$)Cl$_3$ to investigate microscopic magnetic properties of the pressure-induced phase. The spontaneous muon spin precession, which indicates the existence of a long-range coherent order, is observed in pressures. With decreasing temperature in 3.1 kbar, the internal static magnetic field at the muon sites {\it H}$_{\rm int}$ tends to saturate to 280 Oe around 4 K, however, decreases to 240 Oe at 2.3 K. These results suggest a rearrangement of ordered spins, and we speculate that the oblique antiferromagnetic phase, which is observed in the pressure of 14 kbar on the pure TlCuCl$_3$, appears in the Mg-doped system in lower pressures.

cond-mat.str-el↗

arXiv2008

μSR-detection of the soft-mode toward the exotic magnetic ground state in bond-disordered quantum spin system IPA-Cu(Cl$_{0.35}$Br$_{0.65}$)$_3$

The spin-gap system with a bond-disorder (CH3)2CHNH3-Cu(ClxBr1-x)3 is reported by experiments on macroscopic quantities of the uniform susceptibility and the specific heat to show a magnetic order at low temperatures when the value of x is within the limited region between 0.44 and 0.87, and otherwise a gapped behavior. We have investigated microscopically the ground state of the sample with x=0.35 by muon spin relaxation (mSR) to observe that the frequency spectrum of the spin fluctuation tends to concentrate around zero energy as lowering temperatures. This behavior was interpreted as the soft-mode toward the possible magnetic phase transition at absolute zero.

cond-mat.str-el↗

arXiv2007

High-field Magnetic Torque Measurements in (CH$_3$)$_2$CHNH$_3$CuCl$_3$

High-field magnetic torque measurements were carried out in the spin gap system (CH$_3$)$_2$CHNH$_3$CuCl$_3$. It was observed that the magnitude of the magnetic torque $τ$ is almost zero until the critical field and then increases rapidly, which indicates the existence of the magnetic quantum phase transition from the spin-gap phase to the field-induced magnetic ordered phase. From the temperature dependence of the magnetic torque $τ$, the cusp-like minimum indicative of the Bose-Einstein condensation (BEC) of magnons was observed for $H \perp$ C-plane, while the kink-anomaly was observed for $H \perp$ A-plane at the field-induced transition temperature. It was found that this different behavior between $H \perp$ A-plane and $H \perp$ C-plane can be interpreted as the breaking of the magnon BEC picture due to the rotational symmetry breaking. The additional anomaly was also observed in the field-induced magnetic ordered phase at the intermediate angle of the applied magnetic field in the A-plane and discussed in terms of the spin-flop transition, the spin-reorientation transition and the spin-lattice correlations. In order to investigate the spin-lattice correlations in (CH$_3$)$_2$CHNH$_3$CuCl$_3$, we performed the magnetostriction measurements and observed that the magnetostriction appears by vanishing of the spin gap.

cond-mat.str-el↗

arXiv2007

The effect of randomness on the quantum spin system Tl$_{1-x}$K$_x$CuCl$_3$ with ${\boldmath $x$}$ = 0.44 studied by the Zero-field Muon-Spin-Relaxation (ZF-$μ$SR) method

Zero-field muon-spin-relaxation (ZF-{\boldmath $μ$}SR) measurements were carried out down to 80 mK on the randomness bond system Tl$_{1-x}$K$_x$CuCl$_3$ with $x$ = 0.44. Time spectra are well fitted by the stretched exponential function $\exp (-λt)^β$. The muon spin relaxation rate $λ$ increases rapidly with decreasing temperature, and $β$ tends to 0.5 at 80 mK. The divergent increase of $λ$ suggests the critical slowing down of the frequency of the Cu-3{\it d} spin fluctuations toward a spin frozen state below 80 mK, and the root-exponential-like behavior of the time spectrum indicates that the origin of the relaxation is possibly the spatially-fixed fluctuating dilute moments.

cond-mat.str-el↗

arXiv2007

1H-NMR Study on the Magnetic Order in the Mixture of Two Spin Gap Systems (CH3)2CHNH3CuCl3 and (CH3)2CHNH3CuBr3

The antiferromagnetic ordering in the solid-solution of the two spin-gap systems (CH3)2CHNH3CuCl3 and (CH3)2CHNH3CuBr3 has been investigated by 1H-NMR. The sample with the Cl-content ratio x=0.85 showed a clear splitting in spectra below TN=13.5 K, where the spin-lattice relaxation rate T1-1 showed a diverging behavior. The critical exponent of the temperature dependence of the hyperfine field is found to be 0.33.

cond-mat.str-el↗