arXiv subjects

Takatsugu Masuda

Publications and source records attributed to Takatsugu Masuda.

35 records · Page 2Linked to original sources

arXiv2021

Nontrivial temperature dependence of magnetic anisotropy in multiferroics Ba$_2$MnGe$_2$O$_7$

We measured the temperature dependences of the static magnetization and the spin excitation in a square-lattice multiferroics Ba$_2$MnGe$_2$O$_7$. An anisotropy gap of the observed low energy mode is scaled by electric polarization rather than a power of sublattice moment. Spin nematic interaction in effective spin Hamiltonian, which is equivalent to interaction of electric polarization, is responsible for the easy-axis anisotropy. The nontrivial behavior of the anisotropy gap can be rationalized as change of the hybridized $d$-$p$ orbital with temperature, leading to the temperature dependence of the spin nematic interaction.

cond-mat.str-el↗

arXiv2021

Spin-gap formation due to spin-Peierls instability in $π$-orbital-ordered NaO$_2$

We have investigated the low-temperature magnetism of sodium superoxide (NaO$_{2}$), in which spin, orbital, and lattice degrees of freedom are closely entangled. The magnetic susceptibility shows anomalies at $T_{1}=220$ K and $T_{2}=190$ K, which correspond well to the structural phase transition temperatures, and a sudden decrease below $T_{3}=34$ K. At 4.2 K, the magnetization shows a clear stepwise anomaly around 30 T with a large hysteresis. In addition, the muon spin relaxation experiments indicate no magnetic phase transition down to $T=0.3$ K. The inelastic neutron scattering spectrum exhibits magnetic excitation with a finite energy gap. These results confirm that the ground state of NaO$_{2}$ is a spin-singlet state. To understand this ground state in NaO$_{2}$, we performed Raman scattering experiments. All the Raman-active libration modes expected for the marcasite phase below $T_{2}$ are observed. Furthermore, we find that several new peaks appear below $T_{3}$. This directly evidences the low crystal symmetry, namely, the presence of the phase transition at $T_{3}$. We conclude the singlet-ground state of NaO$_{2}$ due to the spin-Peierls instability.

cond-mat.str-el↗

arXiv2020

Magnetic order in the chemically-substituted frustrated antiferromagnet CsCrF$_{4}$

The effect of chemical substitution on the ground state of the geometrically frustrated antiferromagnet CsCrF$_4$ has been investigated through a neutron powder diffraction experiment. Magnetic Fe-substituted CsCr$_{0.94}$Fe$_{0.06}$F$_{4}$ and nonmagnetic Al-substituted CsCr$_{0.98}$Al$_{0.02}$F$_{4}$ samples are measured, and magnetic Bragg peaks are clearly observed in both samples. Magnetic structure analysis revealed a 120$^{\circ}$ structure having a magnetic propagation vector $\mathbf{k}_{\rm mag}=(0,0,1/2)$ in CsCr$_{0.94}$Fe$_{0.06}$F$_{4}$. For CsCr$_{0.98}$Al$_{0.02}$F$_{4}$, a quasi-120$^{\circ}$ structure having $\mathbf{k}_{\rm mag}=(1/2,0,1/2)$ is formed. It is notable that the identified magnetic structure in CsCr$_{0.94}$Fe$_{0.06}$F$_{4}$ belongs to a different phase of ground states from those in CsCr$_{0.98}$Al$_{0.02}$F$_{4}$ and the parent CsCrF$_{4}$. These results suggest that the Fe-substitution strongly influences the ground state of CsCrF$_{4}$.

cond-mat.str-el↗

arXiv2020

Spin-waves in 2D honeycomb lattice $XXZ$-type van der Waals antiferromagnet CoPS$_3$

The magnetic excitations in CoPS$_3$, a two-dimensional van der Waals (vdW) antiferromagnet with spin $S=3/2$ on a honeycomb lattice, has been measured using powder inelastic neutron scattering. Clear dispersive spin waves are observed with a large spin gap of ~13 meV. The magnon spectra were fitted using an $XXZ$-type $J_1-J_2-J_3$ Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb layers. The best-fit parameters show ferromagnetic exchange $J_1=-2.08$ meV and $J_2=-0.26$ meV for the nearest and second-nearest neighbors and a sizeable antiferromagnetic exchange $J_3=4.21$ meV for the third-nearest neighbor with the strong easy-axis anisotropy $K=-2.06$ meV. The suitable fitting could only be achieved by the anisotropic $XXZ$-type Hamiltonian, in which the exchange interaction for the out-of-plane component is smaller than that for the in-plane one by a ratio $α=J_z/J_x=0.6$. Moreover, the absence of spin-orbit exciton around 30 meV indicates that Co$^{2+}$ ions in CoPS$_3$ have a $S=3/2$ state rather than a spin-orbital entangled $J_\rm{eff}=1/2$ ground state. Our result directly shows that CoPS$_3$ is an experimental realization of the $XXZ$ model with a honeycomb lattice in 2D vdW magnets.

cond-mat.mtrl-sci↗

arXiv2020

Establishing the carrier scattering phase diagram for ZrNiSn-based half-Heusler thermoelectric materials

Chemical doping is one of the most important strategies for tuning electrical properties of semiconductors, particularly thermoelectric materials. Generally, the main role of chemical doping lies in optimizing the carrier concentration, but there can potentially be other important effects. Here, we show that chemical doping plays multiple roles for both electron and phonon transport properties in half-Heusler thermoelectric materials. With ZrNiSn-based half-Heusler materials as an example, we use high-quality single and polycrystalline crystals, various probes, including electrical transport measurements, inelastic neutron scattering measurement, and first-principles calculations, to investigate the underlying electron-phonon interaction. We find that chemical doping brings strong screening effects to ionized impurities, grain boundary, and polar optical phonon scattering, but has negligible influence on lattice thermal conductivity. Furthermore, it is possible to establish a carrier scattering phase diagram, which can be used to select reasonable strategies for optimization of the thermoelectric performance.

cond-mat.mtrl-sci↗

arXiv2020

Zero-energy excitation in the classical kagome antiferromagnet NaBa$_{2}$Mn$_{3}$F$_{11}$

We performed inelastic neutron scattering measurements on a polycrystalline sample of a classical kagome antiferromagnet NaBa$_{2}$Mn$_{3}$F$_{11}$ to investigate the possibility of a dispersionless zero-energy excitation associated with rotation of spins along the chains. The observed spectra indeed exhibit such an excitation with strong intensity at low energy, as well as dispersive excitations with weak intensity at high energy. Combining the measurements with calculations from linear spin-wave theory reveals that NaBa$_{2}$Mn$_{3}$F$_{11}$ is a good realization of the classical kagome antiferromagnet which exhibits a dispersionless mode lifted by the magnetic dipole-dipole interaction.

cond-mat.str-el↗

arXiv2020

Emergent spin-$1$ Haldane gap and ferroelectricity in a frustrated spin-$1/2$ ladder

We report experimental and theoretical evidence that Rb$_2$Cu$_2$Mo$_3$O$_{12}$ has a nonmagnetic tetramer ground state of a two-leg ladder comprising antiferromagnetically coupled frustrated spin-$1/2$ chains and exhibits a Haldane spin gap of emergent spin-1 pairs. Three spin excitations split from the spin-1 triplet by a Dzyaloshinskii-Moriya interaction are identified in inelastic neutron-scattering and electron spin resonance spectra. A tiny magnetic field generates ferroelectricity without closing the spin gap, indicating a novel class of ferroelectricity induced by a vector spin chirality order.

cond-mat.str-el↗

arXiv2019

Spin gap and L modulated intensity at the low-energy incommensurate magnetic fluctuations in the superconducting state of Sr2RuO4

Low-energy incommensurate (IC) magnetic fluctuations in the multiband superconductor Sr$_2$RuO$_4$ is investigated by high-resolution inelastic neutron scattering measurements and random phase approximation (RPA) calculations. Below $T_\text{c}$, the substantial spin gap is observed at $\mathbf{Q}_\text{IC}=(0.3, 0.3, L)$ where the quasi-one-dimensional $α$ and $β$ sheets consisting of the Fermi surfaces are in good nesting conditions. $L$ modulated intensity of the low-energy IC magnetic fluctuations and our RPA calculations indicate that the superconducting gaps regarding the $α$ and $β$ sheets have the horizontal line nodes.

cond-mat.supr-con↗

arXiv2019

Magnon Pairs and Spin-Nematic Correlation in the Spin-Seebeck Effect

Investigating exotic magnetic materials with spintronic techniques is effective at advancing magnetism as well as spintronics. In this work, we report unusual field-induced suppression of the spin-Seebeck effect (SSE) in a quasi one-dimensional frustrated spin-$\frac{1}{2}$ magnet LiCuVO$_4$, known to exhibit spin-nematic correlation in a wide range of external magnetic field $B$. The suppression takes place above $|B| > 2$ T in spite of the $B$-linear isothermal magnetization curves in the same $B$ range. The result can be attributed to the growth of the spin-nematic correlation while increasing $B$. The correlation stabilizes magnon pairs carrying spin-2, thereby suppressing the interfacial spin injection of SSE by preventing the spin-1 exchange between single magnons and conduction electrons at the interface. This interpretation is supported by integrating thermodynamic measurements and theoretical analysis on the SSE.

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↗

arXiv2018

Neutron spin resonance in the 112-type iron-based superconductor

We use inelastic neutron scattering to study the low-energy spin excitations of 112-type iron pnictide Ca$_{0.82}$La$_{0.18}$Fe$_{0.96}$Ni$_{0.04}$As$_{2}$ with bulk superconductivity below $T_c=22$ K. A two-dimensional spin resonance mode is found around $E=$ 11 meV, where the resonance energy is almost temperature independent and linearly scales with $T_c$ along with other iron-based superconductors. Polarized neutron analysis reveals the resonance is nearly isotropic in spin space without any $L$ modulations. Due to the unique monoclinic structure with additional zigzag arsenic chains, the As $4p$ orbitals contribute to a three-dimensional hole pocket around $Γ$ point and an extra electron pocket at $X$ point. Our results suggest that the energy and momentum distribution of spin resonance does not directly response to the $k_z$ dependence of fermiology, and the spin resonance intrinsically is a spin-1 mode from singlet-triplet excitations of the Cooper pairs in the case of weak spin-orbital coupling.

cond-mat.supr-con↗

arXiv2018

Magnetic State Selected by Magnetic Dipole Interaction in Kagome Antiferromagnet NaBa$_{2}$Mn$_{3}$F$_{11}$

We have studied the ground state of the classical Kagome antiferromagnet NaBa$_{2}$Mn$_{3}$F$_{11}$. Strong magnetic Bragg peaks observed in the $d$-spacing shorter than 6.0 Å were indexed by the propagation vectors of $\boldsymbol{k}_{0} = (0,0,0)$. Additional peaks with weak intensities in the range of the $d$-spacing longer than 8.0 Å were indexed by the incommensurate vectors of $\boldsymbol{k}_{1}=(0.3209(2),0.3209(2),0)$ and $\boldsymbol{k}_{2}=(0.3338(4),0.3338(4),0)$. Magnetic structure analysis exhibits that the 120$^{\circ}$ structure with the {\it tail-chase} geometry having $\boldsymbol{k}_0$ is modulated by the incommensurate vectors. The classical calculation of the Kagome Heisenberg antiferromagnet having the antiferromagnetic 2nd-neighbor interaction, the ground state of which is degenerated 120$^{\circ}$ structures with $\boldsymbol{k}_0$, reveals that the magnetic dipole-dipole (MDD) interaction including up to the 4th neighbor terms selects the tail-chase structure. The observed modulation of the tail-chase structure is indicated to be due to a small perturbation such as the long-range MDD interaction or the interlayer interaction.

cond-mat.str-el↗

arXiv2017

YUI and HANA: Control and Visualization Programs for HRC in J-PARC

We developed control and visualization programs, YUI and HANA, for High- Resolution Chopper spectrometer (HRC) installed at BL12 in MLF, J-PARC. YUI is a comprehensive program to control DAQ-middleware, the accessories, and sample environment devices. HANA is a program for the data transformation and visualization of inelastic neutron scattering spectra. In this paper, we describe the basic system structures and unique functions of these programs from the viewpoint of users.

physics.ins-det↗

arXiv2017

Spin Dynamics in a Stripe-ordered Buckled Honeycomb Lattice Antiferromagnet Ba$_{2}$NiTeO$_{6}$

We carried out inelastic neutron scattering experiments on a buckled honeycomb lattice antiferromagnet Ba$_{2}$NiTeO$_{6}$ exhibiting a stripe structure at a low temperature. Magnetic excitations are observed in the energy range of $\hbar ω\lesssim 10$ meV having an anisotropy gap of 2 meV at 2 K. We perform spin-wave calculations to identify the spin model. The obtained microscopic parameters are consistent with the location of the stripe structure in the classical phase diagram. Furthermore, the Weiss temperature independently estimated from a bulk magnetic susceptibility is consistent with the microscopic parameters. The results reveal that a competition between the NN and NNN interactions that together with a relatively large single ion magnetic anisotropy stabilize the stripe magnetic structure.

cond-mat.str-el↗

arXiv2017

Magnetic Structure and Dielectric State in the Multiferroic Ca$_{2}$CoSi$_{2}$O$_{7}$

Magnetic structure of the multiferroic Ca$_{2}$CoSi$_{2}$O$_{7}$ was determined by neutron diffraction techniques. Combination of the polycrystalline and single-crystal samples experiments revealed a collinear antiferromagnetic structure with the easy axis along $<$100$>$ directions. The dielectric state was discussed in the framework of the spin-dependent $d$-$p$ hybridization mechanism, leading to the realization of the antiferroelectric structure. The origin of the magnetic anisotropy was discussed in comparison with the isostructural Ba$_{2}$CoGe$_{2}$O$_{7}$.

cond-mat.str-el↗

arXiv2013

63,65Cu Nuclear Resonance Study of the Coupled Spin Dimers and Chains Compound Cu2Fe2Ge4O13

Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) of Cu have been measured in a coupled spin dimers and chains compound Cu2Fe2Ge4O13. Cu NQR has also been measured in an isostructural material Cu2Sc2Ge4O13 including only spin dimers. Comparison of the temperature dependence of the 63Cu nuclear spin-lattice relaxation rate between the two compounds reveals that the Fe chains in Cu2Fe2Ge4O13 do not change a spin gap energy of the Cu dimers from that in Cu2Sc2Ge4O13, contributing additionally to the relaxation rate at the Cu site. A modestly large internal field of 3.39 T was observed at the Cu site in the antiferromagnetic state of Cu2Fe2Ge4O13 at 4.2 K, which is partly because of quantum reduction of the ordered moment of a Cu atom. The internal field and the ordered moment of Cu are noncollinear due to large anisotropy of the hyperfine interaction at the Cu site. A model analysis of the internal field based on the fourfold planar coordination of Cu suggests that a 3d hole of the Cu2+ ion is mainly in the d(x2-y2) orbital state.

cond-mat.str-el↗

arXiv2011

Spin Density Wave in Insulating Ferromagnetic Frustrated Chain LiCuVO$_4$

We study field induced quantum phase in weakly-coupled ferromagnetic frustrated chain LiCuVO$_4$ by neutron diffraction technique. A new incommensurate magnetic peak is observed at $H \ge 8.5$ T. The field dependent propagation vector is identified with the spin density wave correlation in the theoretically predicted magnetic quadrupole order. Quantum fluctuation, geometrical frustration, and interchain interaction induce the exotic spin density wave long-range order in the insulating magnet.

cond-mat.str-el↗