arXiv subjects

N. Ogita

Publications and source records attributed to N. Ogita.

4 recordsLinked to original sources

arXiv2015

Low-Energy Optical Phonon Modes in the Caged Compound LaRu2Zn20

The caged compound LaRu2Zn20 exhibits a structural transition at TS =150 K, whose driving mechanism remains elusive. We have investigated atomic dynamics by the measurements of specific heat C and inelastic X-ray scattering (IXS). The lattice part of the specific heat Clat divided by T3, Clat/T3, shows a broad peak at around 15 K, which is reproduced by two Einstein modes with characteristic temperatures of 35 K and 82 K, respectively. IXS measurements along the [111] and [110] directions reveal optical phonon modes at 3 meV (35 K) and 7 meV (80 K), respectively, whose values agree with the values of Einstein temperatures. The first principles calculation has assigned the phonon modes at 3 meV as the optical modes of Zn atoms located at the middle of two La atoms. The low-energy vibration of the Zn atom perpendicular to the there-fold axis is thought to lead the structural instability of LaRu2Zn20.

cond-mat.str-el↗

arXiv2009

Optical conductivity of rattling phonons in type-I clathrate Ba$_8$Ga$_{16}$Ge$_{30}$

A series of infrared-active optical phonons have been detected in type-I clathrate Ba$_8$Ga$_{16}$Ge$_{30}$ by terahertz time-domain spectroscopy. The conductivity spectra with the lowest-lying peaks at 1.15 and 1.80 THz are identified with so-called rattling phonons, i.e., optical modes of the guest ion Ba$^{2+}(2)$ with $T_{1u}$ symmetry in the oversized tetrakaidecahedral cage. The temperature dependence of the spectra from these modes are totally consistent with calculations based on a one-dimensional anharmonic potential model that, with decreasing temperature, the shape becomes asymmetrically sharp associated with a softening for the weight to shift to lower frequency. These temperature dependences are determined, without any interaction effects, by the Bose-factor for optical excitations of anharmonic phonons with the nonequally spaced energy levels.

cond-mat.mtrl-sci↗

arXiv2003

Pressure induced Superconductor-Insulator transition in the spinel compound CuRh2S4

We performed resistivity measurements in CuRh$_{2}$S$_{4}$ under quasi-hydrostatic pressure of up to 8.0 GPa, and found a pressure induced superconductor-insulator (SI) transition. Initially, with increasing pressure, the superconducting transition temperature $T_c$ increases from 4.7 K at ambient pressure to 6.4 K at 4.0 GPa, but decreases at higher pressures. With further compression, superconductivity in CuRh$_{2}$S$_{4}$ disappears abruptly at a critical pressure $P_{\rm SI}$ between 5.0 and 5.6 GPa, when it becomes an insulator.

cond-mat.supr-con↗

arXiv2001

Micro-Raman scattering investigation of MgB2 and RB2 (R=Al, Mn, Nb and Ti)

Phonon spectra have been investigated by micro-Raman scattering from 290 to 20K. New peaks at 500 and 674 cm-1 appear below 250K in MgB2. These peaks are explained by the Fermi resonance, where Raman-active E2g intereacts with the overtone of E1u due to anhamonicity. Raman spectra of the isostructural RB2 (R=Al, Mn, Nb and Ti) have been also measured at 290K. The line width of E2g phonon of the superconductor MgB2 and NbB2 shows the broader than that of the non-superconductors. It is found that the anharmonicity of phonons is important for the superconductivity for MgB2.

cond-mat.supr-con↗