arXiv subjects

Kunihiko Oka

Publications and source records attributed to Kunihiko Oka.

3 recordsLinked to original sources

arXiv2019

Superconductivity in a Scandium Borocarbide with a Layered Crystal Structure

The discovery of nearly room-temperature superconductivity in superhydrides has motivated further materials research for conventional superconductors. To realize the moderately high critical temperature $(T_\mathrm{c})$ in materials containing light elements, we explored new superconducting phases in a scandium borocarbide system. Here, we report the observation of superconductivity in a new ternary Sc-B-C compound. The crystal structure, which was determined through a Rietveld analysis, belongs to tetragonal space group $P4/ncc$. By complementarily using the density functional theory calculations, a chemical formula of the compound was found to be expressed as Sc$_{20}$C$_{8-x}$B$_x$C$_{20}$($x=1\:\mathrm{or}\:2$). Interestingly, a small amount of B is essential to stabilize the present structure. Our experiments revealed the typical type-II superconductivity at $T_\mathrm{c}=7.7\:\mathrm{K}$. Additionally, we calculated the density of states within a first-principles approach and found that the contribution of the Sc-3d orbital was mainly responsible for the superconductivity.

cond-mat.supr-con↗

arXiv2015

Magnetic State of the Geometrically Frustrated Quasi-One-Dimensional Spin System Cu$_3$Mo$_2$O$_9$ Studied by Thermal Conductivity

We have measured the thermal conductivity of the geometrically frustrated quasi-onedimensional spin system Cu$_3$Mo$_2$O$_9$ in magnetic fields. A contribution of the thermal conductivity due to spins has been observed in the thermal conductivity along the spin chains. The thermal conductivity due to phonons, $κ_{\rm phonon}$, has been found to decrease by the application of a magnetic field, which has been explained as being due to the reduction in the spin gap originating from the spin-singlet dimers. Moreover, it has been found that $κ_{\rm phonon}$ increases with increasing field in high fields above ~7 T at low temperatures. This suggests the existence of a novel field-induced spin state and is discussed in terms of the possible spin-chirality ordering in a frustrated Mott insulator.

cond-mat.str-el↗

arXiv2011

Electric polarization induced by Neel order without magnetic superlattice: experimental study of Cu3Mo2O9 and numerical study of a small spin cluster

We clarify that the antiferromagnetic order in the distorted tetrahedral quasi-one dimensional spin system induces electric polarizations. In this system, the effects of the low dimensionality and the magnetic frustration are expected to appear simultaneously. We obtain the magnetic-field-temperature phase diagram in Cu3Mo2O9 by studying the dielectric constant and the spontaneous electric polarization. Around the tricritical point at 10 T and 8 K, the change of the direction in the electric polarization causes a colossal magnetocapacitance. We calculate the charge redistribution in the small spin cluster consisting of two magnetic tetrahedra to demonstrate the electric polarization induced by the antiferromagnetism.

cond-mat.str-el↗