Fumitoshi Iga

Single crystal growth and physical properties of Kondo insulator YbB12

Abstract YbB12 is the only Yb-based Kondo insulator. We have succeeded for the first time in growing a large single crystal (6mmO × 50 mm) of YbB12 by floating zone method using a newly developed image-furnace with four xenon lamps. The results of the electrical resistivity, Hall effect and magnetic susceptibility measurements indicate that the quality of this crystal is better than that of the best polycrystal ever reported.

Ce- and Yb-based Kondo semiconductors

Abstract Kondo semiconductors are a class of strongly correlated f-electron materials whose low-energy excitations exhibit a (pseudo)gap at low temperatures. The significant difference between the pseudogap in orthorhombic CeNiSn and CeRhSb and the real gap in cubic Ce 3 Bi 4 Pt 3 and YbB 12 is highlighted by a comparison of transport properties. Low-temparature measurements of the magnetoresistance, Hall coefficient and specific heat of CeNiSn have revealed a field-induced excitation of the coherent low-carrier state. A systematic study of CeNi 1− x T x Sn ( T = Co, Cu and Pt ) has shown that the residual carriers in CeNiSn are immobilized by 1% substitution irrespective of the substitutes. Further substitution with 5% Cu induces a magnetic instability at T = 0.

Structural, transport, and thermal properties of the single-crystalline type-VIII clathrate Ba8Ga16Sn30

The analysis of temperature dependence of mH suggests a crossover of a dominant scattering mechanism from ionized impurity to acoustic phonon scattering with increasing temperature. The existence of local vibration modes of Ba atoms in cages composed of Ga and Sn atoms is evidenced by analysis of experimental data of structural refinement and specific heat, which give an Einstein temperature of 50 K and a Debye temperature of 200 K. This local vibration of Ba atoms should be responsible for the low thermal conductivity s1.1 W / m K at 150 Kd. The potential of type-VIII clathrate compounds for thermoelectric application is discussed.

Field-induced metallic state in YbB12 under high magnetic field

High field magnetization and magnetoresistance of YbB 12 are investigated up to 550 kOe at low temperatures with special interest in the semiconductive band gap energy under the field. Very large negative magnetoresistance is found and the system is perfectly metallic around 500 kOe with anomalous increase in the magnetization. The experimental results are explained by introducing the model that the band gap produced by the mixing of the f- and conduction electrons is destroyed by applying the field and the released f-electron gives an anomalous magnetization. The g J -value is estimated as 1.9 for the f-level and this means that \(\varGamma_{8}\) may be most favourable for the responsible f-electron at the Fermi energy.

Optical conductivity of the Kondo insulator YbB12 : Gap formation and low-energy excitations

Optical reflectivity experiments have been conducted on single crystals of the Kondo insulator

Temperature dependence of Sm valence in SmB6 studied by X-ray absorption spectroscopy

{\mathrm{YbB}}_{12}

Detection of Neutron Scattering from Phase IV of Ce0.7La0.3B6: A Confirmation of the Octupole Order

in order to obtain its optical conductivity, \ensuremath{\sigma}(\ensuremath{\omega}). Upon cooling below 70 K, a strong suppression of \ensuremath{\sigma}(\ensuremath{\omega}) is seen in the far-infrared region, indicating the opening of an energy gap of

Magnetic excitations observed by means of inelastic neutron scattering in polycrystalline

\ensuremath{\sim}25\mathrm{meV}.

Low-Energy Electronic Structure of the Kondo Insulator YbB12

This gap development is coincident with a rapid decrease in the magnetic susceptibility, which shows that the gap opening has significant influence on magnetic properties. A narrow, asymmetric peak is observed at

Magnetoresistance and de Haas-van Alphen Effect in UB12

\ensuremath{\sim}40\mathrm{meV}