arXiv subjects

Masayuki Hagiwara

Publications and source records attributed to Masayuki Hagiwara.

50 records · Page 3Linked to original sources

arXiv2016

Magnetoelectric Behavior from $S=1/2$ Asymmetric Square Cupolas

Magnetoelectric properties are studied by a combined experimental and theoretical study of a quasi-two-dimensional material composed of square cupolas, Ba(TiO)Cu$_4$(PO$_4$)$_4$. The magnetization is measured up to above the saturation field, and several anomalies are observed depending on the field directions. We propose a $S$=1/2 spin model with Dzyaloshinskii-Moriya interactions, which well reproduces the full magnetization curves. Elaborating the phase diagram of the model, we show that the anomalies are explained by magnetoelectric phase transitions. Our theory also accounts for the scaling of the dielectric anomaly observed in experiments. The results elucidate the crucial role of the in-plane component of Dzyaloshinskii-Moriya interactions, which is induced by the noncoplanar buckling of square cupola. We also predict a `hidden' phase and another magnetoelectric response both of which appear in nonzero magnetic field.

cond-mat.str-el↗

arXiv2016

Long-range magnetic order and interchain interactions in the $S=2$ chain system MnCl$_3$(bpy)

A compound with very weakly interacting chains, MnCl$_3$(bpy), has attracted a great deal of attention as a possible $S=2$ Haldane chain. However, long-range magnetic order of the chains prevents the Haldane gap from developing below 11.5~K. Based on a four-sublattice model, a description of the antiferromagnetic resonance (AFMR) spectrum up to frequencies of 1.5~THz and magnetic fields up to 50~T indicates that the interchain coupling is indeed quite small but that the Dzaloshinskii-Moriya interaction produced by broken inversion symmetry is substantial (0.12~meV). In addition, the antiferromagnetic, nearest-neighbor interaction within each chain (3.3~meV) is significantly stronger than previously reported. The excitation spectrum of this $S=2$ compound is well-described by a $1/S$ expansion about the classical limit.

cond-mat.mtrl-sci↗

arXiv2015

Observation of the orbital quantum dynamics in the spin-1/2 hexagonal antiferromagnet Ba3CuSb2O9

We explore orbital dynamics in the spin liquid candidate Ba3CuSb2O9 using multi-frequency electron spin resonance. We prepared two high quality single crystals. The crystal with a slight copper deficiency shows a structural phase transition at around 200 K due to the cooperative Jahn-Teller effect, accompanied with orbital ordering. In contrast, the crystal with almost perfect stoichiometry shows no orbital ordering down to the lowest temperature of 1.5 K. Dramatic change in the g-factor anisotropy as a function of frequency and temperature demonstrates orbital quantum fluctuations at a nearly constant time scale of ~ 100 ps below 20 K, evidencing the emergence of an orbital liquid state in this quantum spin liquid compound.

cond-mat.str-el↗

arXiv2014

Unconventional magnetism in the layered oxide LaSrRhO$_4$

We have prepared polycrystalline samples of LaSrRh$_{1-x}$Ga$_x$O$_4$ and LaSr$_{1-x}$Ca$_x$RhO$_4$,and have measured the x-ray diffraction, resistivity, Seebeck coefficient, magnetization and electron spin resonance in order to evaluate their electronic states. The energy gap evaluated from the resistivity and the Seebeck coefficient systematically changes with the Ga concentration, and suggests that the system changes from a small polaron insulator to a band insulator. We find that all the samples show Curie-Weiss-like susceptibility with a small Weiss temperature of the order of 1 K, which is seriously incompatible with the collective wisdom that a trivalent rhodium ion is nonmagnetic. We have determined the $g$ factor to be $g$=2.3 from the electron spin resonance, and the spin number to be $S$=1 from the magnetization-field curves by fitting with a modified Brillouin function. The fraction of the $S$=1 spins is 2--5%, which depends on the degree of disorder in the La/Sr/Ca-site, which implies that disorder near the apical oxygen is related to the magnetism of this system. A possible origin for the magnetic Rh$^{3+}$ ions is discussed.

cond-mat.str-el↗

arXiv2012

JASMIN: Japanese-American study of muon interactions and neutron detection

Experimental studies of shielding and radiation effects at Fermi National Accelerator Laboratory (FNAL) have been carried out under collaboration between FNAL and Japan, aiming at benchmarking of simulation codes and study of irradiation effects for upgrade and design of new high-energy accelerator facilities. The purposes of this collaboration are (1) acquisition of shielding data in a proton beam energy domain above 100 GeV; (2) further evaluation of predictive accuracy of the PHITS and MARS codes; (3) modification of physics models and data in these codes if needed; (4) establishment of irradiation field for radiation effect tests; and (5) development of a code module for improved description of radiation effects. A series of experiments has been performed at the Pbar target station and NuMI facility, using irradiation of targets with 120 GeV protons for antiproton and neutrino production, as well as the M-test beam line (M-test) for measuring nuclear data and detector responses. Various nuclear and shielding data have been measured by activation methods with chemical separation techniques as well as by other detectors such as a Bonner ball counter. Analyses with the experimental data are in progress for benchmarking the PHITS and MARS15 codes. In this presentation recent activities and results are reviewed.

physics.acc-ph↗

arXiv2012

Neutron energy spectrum from 120 GeV protons on a thick copper target

Neutron energy spectrum from 120 GeV protons on a thick copper target was measured at the Meson Test Beam Facility (MTBF) at Fermi National Accelerator Laboratory. The data allows for evaluation of neutron production process implemented in theoretical simulation codes. It also helps exploring the reasons for some disagreement between calculation results and shielding benchmark data taken at high energy accelerator facilities, since it is evaluated separately from neutron transport. The experiment was carried out using a 120 GeV proton beam of 3E5 protons/spill. Since the spill duration was 4 seconds, proton-induced events were counted pulse by pulse. The intensity was maintained using diffusers and collimators installed in the beam line to MTBF. The protons hit a copper block target the size of which is 5cm x 5cm x 60 cm long. The neutrons produced in the target were measured using NE213 liquid scintillator detectors, placed about 5.5 m away from the target at 30^{\circ} and 5 m 90^{\circ} with respect to the proton beam axis. The neutron energy was determined by time-of-flight technique using timing difference between the NE213 and a plastic scintillator located just before the target. Neutron detection efficiency of NE213 was determined on basis of experimental data from the high energy neutron beam line at Los Alamos National Laboratory. The neutron spectrum was compared with the results of multi-particle transport codes to validate the implemented theoretical models. The apparatus would be applied to future measurements to obtain a systematic data set for secondary particle production on various target materials.

physics.acc-ph↗

arXiv2011

Magnetic Field and Pressure Phase Diagrams of Uranium Heavy-Fermion Compound U$_2$Zn$_{17}$

We have performed magnetization measurements at high magnetic fields of up to 53 T on single crystals of a uranium heavy-fermion compound U$_2$Zn$_{17}$ grown by the Bridgman method. In the antiferromagnetic state below the Néel temperature $T_{\rm N}$ = 9.7 K, a metamagnetic transition is found at $H_c$ $\simeq$ 32 T for the field along the [11$\bar{2}$0] direction ($a$-axis). The magnetic phase diagram for the field along the [11$\bar{2}$0] direction is given. The magnetization curve shows a nonlinear increase at $H_m$ $\simeq$ 35 T in the paramagnetic state above $T_{\rm N}$ up to a characteristic temperature $T_{χ{\rm max}}$ where the magnetic susceptibility or electrical resistivity shows a maximum value. This metamagnetic behavior of the magnetization at $H_m$ is discussed in comparison with the metamagnetic magnetism of the heavy-fermion superconductors UPt$_3$, URu$_2$Si$_2$, and UPd$_2$Al$_3$. We have also carried out high-pressure resistivity measurement on U$_2$Zn$_{17}$ using a diamond anvil cell up to 8.7 GPa. Noble gas argon was used as a pressure-transmitting medium to ensure a good hydrostatic environment. The Néel temperature $T_{\rm N}$ is almost pressure-independent up to 4.7 GPa and starts to increase in the higher-pressure region. The pressure dependences of the coefficient of the $T^2$ term in the electrical resistivity $A$, the antiferromagnetic gap $Δ$, and the characteristic temperature $T_{ρ{\rm max}}$ are discussed. It is found that the effect of pressure on the electronic states in U$_2$Zn$_{17}$ is weak compared with those in the other heavy fermion compounds.

cond-mat.str-el↗

arXiv2010

High Field ESR and Magnetization of the Triangular Lattice Antiferromagnet NiGa2S4

We report the experimental and the analytical results of electron spin resonance (ESR) and magnetization in high magnetic fields up to about 68 T of the quasi two-dimensional triangular lattice antiferromagnet NiGa$_2$S$_4$. From the temperature evolution of the ESR absorption linewidth, we find a distinct disturbing of the development of the spin correlation by $Z_2$-vortices between 23 K and 8.5 K. Below $T_{\rm{v}}=8.5$ K, spin-wave calculations based on a 57$^{\circ}$ spiral spin order well explains the frequency dependence of the ESR resonance fields and high field magnetization processes for $H$$\parallel$$c$ and $H$$\perp$$c$, although the magnetization for $H$$\perp$$c$ at high fields is different from the calculated one. Furthermore, we explain the field independent specific heat with $T^2$-dependence by the same spin-wave calculation, but the magnitude of the specific heat is much less than the observed one. Accordingly, these results suggest the occurrence of a $Z_2$ vortex-induced topological transition at $T_{\rm{v}}$ and may indicate quantum effects beyond the descriptions based on the above classical spin models.

cond-mat.str-el↗

arXiv2009

Upper critical fields of the 11-system iron-chalcogenide superconductor FeSe$_{0.25}$Te$_{0.75}$

We have performed electrical resistivity measurements of a polycrystalline sample of FeSe$_{0.25}$Te$_{0.75}$, which exhibits superconductivity at $T_{\rm c} \sim 14$ K, in magnetic fields up to 55 T to determine the upper critical field $μ_{0}H_{\rm c2}$. In this compound, very large slopes of $μ_{0}H_{\rm c2}$ at the onset, the mid-point, the zero-resistivity temperatures on superconductivity are determined to be -13.7, -10.1, and -6.9 T/K, respectively. The observed $μ_{0}H_{\rm c2}(T)$s of this compound are considerably smaller than those expected from the Werthamer-Helfand-Hohenberg model, manifesting the Pauli limiting behavior. These results suggest that this compound has a large Maki parameter, but it is smaller than that calculated for a weak-coupling superconductor, indicating a large superconducting gap of this compound as a strong-coupling superconductor.

cond-mat.supr-con↗

arXiv2008

Exact solution of the geometrically frustrated spin-1/2 Ising-Heisenberg model on the triangulated Kagome (triangles-in-triangles) lattice

The geometric frustration of the spin-1/2 Ising-Heisenberg model on the triangulated Kagome (triangles-in-triangles) lattice is investigated within the framework of an exact analytical method based on the generalized star-triangle mapping transformation. Ground-state and finite-temperature phase diagrams are obtained along with other exact results for the partition function, Helmholtz free energy, internal energy, entropy, and specific heat, by establishing a precise mapping relationship to the corresponding spin-1/2 Ising model on the Kagome lattice. It is shown that the residual entropy of the disordered spin liquid phase is for the quantum Ising-Heisenberg model significantly lower than for its semi-classical Ising limit (S_0/N_T k_B = 0.2806 and 0.4752, respectively), which implies that quantum fluctuations partially lift a macroscopic degeneracy of the ground-state manifold in the frustrated regime. The investigated model system has an obvious relevance to a series of polymeric coordination compounds Cu_9X_2(cpa)_6 (X=F, Cl, Br and cpa=carboxypentonic acid) for which we made a theoretical prediction about the temperature dependence of zero-field specific heat.

cond-mat.stat-mech↗

arXiv2005

Thermodynamic properties of a tetramer ferro-ferro-antiferro-antiferromagnetic Ising-Heisenberg bond alternating chain as a model system for Cu(3-Clpy)$_2$(N$_3$)$_2$

Thermodynamic properties of a tetramer ferro-ferro-antiferro-antiferromagnetic Ising-Heisenberg bond alternating chain are investigated by the use of an exact mapping transformation technique. Exact results for the magnetization, susceptibility and specific heat in the zero as well as nonzero magnetic field are presented and discussed in detail. The results obtained from the mapping are compared with the relevant experimental data of Cu(3-Clpy)$_2$(N$_3$)$_2$ (3-Clpy=3-Chloropyridine).

cond-mat.mtrl-sci↗

arXiv2000

Evidence for the singlet-dimer ground state in an S = 1 antiferromag netic bond alternating chain

Susceptibility, ESR and magnetization measurements have been performed on si ngle crystals of an S=1 bond alternating chain compound: [Ni(333-tet)(μ-NO _2)](ClO_4) (333-tet = N,N'-bis(3-aminopropyl)propane-1,3-diamine) and the c ompound doped with a small amount of Zn. We observed an anomalous angular de pendence in the Zn-doped sample. These behaviors are well explained by the m odel based on the VBS picture for the singlet-dimer phase. The picture impli es that the free spins of S=1 with a positive single-ion anisotropy are indu ced at the edges of the chains without forming the singlet-dimer.

cond-mat.mtrl-sci↗

arXiv1998

Magnetic Properties of a Quantum Ferrimagnet: NiCu(pba)(D_2O)_3 . 2D_2O

We report the results of magnetic measurements on a powder sample of NiCu(pba)(D_2O)_3 \cdot 2D_2O$ (pba=1,3-propylenebis(oxamato)) which is one of the prototypical examples of an $S$=1/2 and 1 ferrimagnetic chain. Susceptibility($χ$) shows a monotonous increase with decreasing temperature (T) and reaches a maximum at about 7 K. In the plot of $χT$ versus $T$, the experimental data exhibit a broad minimum and are fit to the $χT$ curve calculated for the ferrimagnetic Heisenberg chain composed of S=1/2 and 1. From this fit, we have evaluated the nearest-neighbor exchange constant $J/k_B=121 K$, the g-values of Ni$^{2+}$ and Cu$^{2+}$, $g_{Ni}$=2.22 and $g_{Cu}$=2.09, respectively. Applied external field dependence of $χT$ at low temperatures is reproduced fairly well by the calculation for the same ferrimagnetic model.

cond-mat.mtrl-sci↗

arXiv1997

Low-temperature properties of the spin-1 antiferromagnetic Heisenberg chain with bond-alternation

We investigate the low-temperature properties of the spin-1 antiferromagnetic Heisenberg chain with bond-alternation by the quantum Monte Carlo method (loop algorithm). The strength of bond-alternation at the gapless point is estimated as $δ_{c}=0.2595\pm0.0005$. We confirm numerically that the low-temperature properties at the gapless point are consistent with field theoretical predictions. The numerical results are compared with those of the spin-1/2 antiferromagnetic Heisenberg chain and recent experimental results for [\{Ni(333-tet)($μ$-N$_3$)\}$_n$](ClO$_4$)$_n$ (333-tet=tetraamine $N,N^{\prime}$-bis(3-aminopropyl)-1,3-propanediamine).

cond-mat.stat-mech↗