arXiv subjects

H. Kikuchi

Publications and source records attributed to H. Kikuchi.

8 recordsLinked to original sources

arXiv2026

Magnetic phase diagram and spin Hamiltonian of antiferromagnet Cs$_2$CoI$_4$

We report comprehensive thermodynamic and neutron scattering measurements on the $S$ = 3/2 antiferromagnet Cs$_2$CoI$_4$, a member of the thoroughly studied family of frustrated magnets Cs$_2MX_4$ ($M$ = Cu, Co, Ru, $X$ = Br, Cl, I, O). Unlike previously studied members, Cs$_2$CoI$_4$ undergoes a structural phase transition, for which we determine the low-temperature crystallographic structure. The resulting symmetry reduction strongly affects both the magnetic exchange interactions and single-ion anisotropy. Despite the large parameter space, we propose a minimal magnetic Hamiltonian that reasonably captures the observed excitation spectrum, analyzed using extended SU(4) linear spin-wave theory.

cond-mat.str-el↗

arXiv2024

Spinons in a new Shastry-Sutherland lattice magnet Pr$_2$Ga$_2$BeO$_7$

Identifying the elusive spinon excitations in quantum spin liquid (QSL) materials is what scientists have long sought for. Recently, thermal conductivity ($\kappa$) has emerged to be a decisive probe because the fermionic nature of spinons leads to a characteristic nonzero linear $\kappa_0/T$ term while approaching zero Kelvin. So far, only a few systems have been reported to exhibit such term. Here, we report a $\kappa_0/T \approx$ 0.01 WK$^{-2}$m$^{-1}$, the largest $\kappa_0/T$ value ever observed in magnetic oxide QSL candidates, in a new quantum magnet Pr$_2$Ga$_2$BeO$_7$ with a Shastry-Sutherland lattice (SSL). Its QSL nature is further supported by the power-law temperature dependence of the specific heat, a plateau of muon spin relaxation rate, and gapless inelastic neutron spectra. Our theoretical analysis reveals that the introduction of XY spin anisotropy is the key for Pr$_2$Ga$_2$BeO$_7$ to be the first QSL realized on the SSL, after more than four decades of extensive studies on this celebrated magnetically frustrated lattice.

cond-mat.str-el↗

arXiv2019

Thermal Conductivity and Magnetic Phase Diagram of CuB2O4

We have measured temperature and magnetic field dependences of the thermal conductivity along the c-axis, kc, and that along the [110] direction, k110, of CuB2O4 single crystals in zero field and magnetic fields along the c-axis and along the [110] direction. It has been found that the thermal conductivity is nearly isotropic and very large in zero field and that the thermal conductivity due to phonons is dominant in CuB2O4. The temperature and field dependences of kc and k110 have markedly changed at phase boundaries in the magnetic phase diagram, which has been understood to be due to the change of the mean free path of phonons caused by the change of the phonon-spin scattering rate at the phase boundaries. It has been concluded that thermal conductivity measurements are very effective for detecting magnetic phase boundaries.

cond-mat.str-el↗

arXiv2013

Nonmagnetic Impurity Effect of the S=1/2 Spin Ladder System (pipdH)_2Cu_{1-x}Zn_xBr_4

We report the synthesis and the magnetic susceptibility of (pipdH)2Cu1-xZnxBr4, which is a nonmagnetic impurity-doped S=1/2 spin ladder system. The samples were synthesized from a solution by using a slow evaporation method. Samples were confirmed to be in a single phase and to have the same crystal structure as the pure system (pipdH)2CuBr4 by using X-ray diffraction measurements. To check the magnetic properties, we performed magnetic susceptibility and magnetization measurements with a SQUID magnetometer. A plot of the inverse magnetic susceptibility indicates the presence of dominant antiferromagnetic coupling. The magnetic susceptibility shows a broad maximum due to low dimensionality and a spin gap behavior related to the two-leg spin ladder at low temperature. The spin gap and the Curie constants of Zn-doped samples, as estimated from an analysis of the magnetic susceptibility, monotonically decrease as the Zn concentration decreases. A nonmagnetic impurity of S=1/2 spin ladder system affects the spin gap.

cond-mat.mtrl-sci↗

arXiv2009

Spin Configuration in the 1/3 Magnetization Plateau of Azurite Determined by NMR

High magnetic field $^{63,65}$Cu NMR spectra were used to determine the local spin polarization in the 1/3 magnetization plateau of azurite, Cu$_3$(CO$_3$)$_2$(OH)$_2$, which is a model system for the distorted diamond antiferromagnetic spin-1/2 chain. The spin part of the hyperfine field of the Cu2 (dimer) sites is found to be field independent, negative and strongly anisotropic, corresponding to $\approx$10 % of fully polarized spin in a $d$-orbital. This is close to the expected configuration of the "quantum" plateau, where a singlet state is stabilized on the dimer. However, the observed non-zero spin polarization points to some triplet admixture, induced by strong asymmetry of the diamond bonds $J_1$ and $J_3$.

cond-mat.str-el↗

arXiv2007

Quantum spin liquid states in the two dimensional kagome antiferromagnets, ZnxCu4-x(OD)6Cl2

A three-dimensional system of interacting spins typically develops static long-range order when it is cooled. If the spins are quantum (S = 1/2), however, novel quantum paramagnetic states may appear. The most highly sought state among them is the resonating valence bond (RVB) state in which every pair of neighboring quantum spins form entangled spin singlets (valence bonds) and the singlets are quantum mechanically resonating amongst all the possible highly degenerate pairing states. Here we provide experimental evidence for such quantum paramagnetic states existing in frustrated antiferromagnets, ZnxCu4-x(OD)6Cl2, where the S = 1/2 magnetic Cu2+ moments form layers of a two-dimensional kagome lattice. We find that in Cu4(OD)6Cl2, where distorted kagome planes are weakly coupled to each other, a dispersionless excitation mode appears in the magnetic excitation spectrum below ~ 20 K, whose characteristics resemble those of quantum spin singlets in a solid state, known as a valence bond solid (VBS), that breaks translational symmetry. Doping nonmagnetic Zn2+ ions reduces the distortion of the kagome lattice, and weakens the interplane coupling but also dilutes the magnetic occupancy of the kagome lattice. The VBS state is suppressed and for ZnCu3(OD)6Cl2 where the kagome planes are undistorted and 90% occupied by the Cu2+ ions, the low energy spin fluctuations in the spin liquid phase become featureless.

cond-mat.str-el↗

arXiv2002

The Fermion Determinant, its Modulus and Phase

We consider path integration of a fermionic oscillator with a one-parameter family of boundary conditions with respect to the time coordinate. The dependence of the fermion determinant on these boundary conditions is derived in a closed form with the help of the self-adjoint extension of differential operators. The result reveals its crucial dependence on them, contrary to the conventional understanding that this dependence becomes negligible over sufficiently long time evolution. An example in which such dependence plays a significant role is discussed in a model of supersymmetric quantum mechanics.

hep-th↗

arXiv2002

Fermion Determinant Calculus

The path-integral of the fermionic oscillator with a time-dependent frequency is analyzed. We give the exact relation between the boundary condition to define the domain in which the path-integral is performed and the transition amplitude that the path-integral calculates. According to this relation, the amplitude suppressed by a zero mode does not indicate any special dynamics, unlike the analogous situation in field theories. It simply says the path-integral picks up a combination of the amplitudes that vanishes. The zero mode that is often neglected in the reason of not being normalizable is necessary to obtain the correct answer for the propagator and to avoid an anomaly on the fermion number. We give a method to obtain the fermionic determinant by the determinant of a simple (2\times 2) matrix, which enables us to calculate it for a variety of boundary conditions.

hep-th↗