Uwatoko Yoshiya

Thermoelectric and electrical transport properties of Mg2Si multi-doped with Sb, Al and Zn

Enhanced thermoelectric and electrical transport properties of Mg2Si-based thermoelectric materials have been achieved by multi-doping with Sb, Al and Zn. Results on the investigation of the electrical transport and thermoelectric properties of multi-doped samples prepared using the spark plasma sintering technique are reported. Synchrotron radiation powder X-ray diffraction was used to characterize the structures of the doped samples. The electrical transport properties were determined from mid-infrared reflectivities, Hall effect and conventional quasi-four probe conductivity measurements. Using the electron concentrations (N) determined from the Hall coefficients, the effective masses (m*) were calculated from the frequency of the plasma edge (ωP) of the infrared reflectivities. The thermoelectric performance and thermoelectric figure of merits (ZT) in the temperature range of 300 K to 900 K of the doped Mg2Si compounds were calculated from the measured temperature dependent electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (κ). A maximum ZT of 0.964 was found for Sb0.5%Zn0.5% doped Mg2Si at 880 K. This value is comparable to those of PbTe based thermoelectric materials.

Pressure Collapse of Kondo Gap in Kondo Compound CeRhSb

The effect of pressure on the electrical resistivity of CeRhSb has been studied up to 80 kbar in order to elucidate the electronic Properties of CeRhSb at low temperature. It is found that the Kondo temperature increases with pressure and gap is quenched at P =80 kbar, showing a maximum around 30 kbar.

Change in Unusual Magnetic Properties by Rh Substitution in CeRu2Al10

We have studied the Rh substitution (electron doping) effect in the Kondo semiconductor CeRu2Al10, which orders antiferromagnetically below (\(T_{0}=27\) K), by specific heat \(C\), magnetic susceptibility χ, electrical resistivity ρ, and Hall resistivity ρ\(_{\text{H}}\) measurements of Ce(Ru1-xRhx)2Al10 (\(x=0.12\), 0.2, 0.23, 0.34) single crystals. \(T_{0}\) decreases monotonically with an increase in \(x\). In addition, new anomalies occur at \(T_{1}\sim 6.5\) and \(T_{2}\sim 3.0\) K in all the Rh substitution samples. However, \(T_{1}\) and \(T_{2}\) are almost independent of \(x\). The shape of the anomaly at \(T_{0}\) in χ\((T)\) changes with Rh substitution, which implies the reorientation of the ordered moment. These phenomena indicate that the magnetic properties of CeRu2Al10 are strongly associated with the number of \(4d\) electrons.

Pressure-Induced Superconductivity in the Quasi-One-Dimensional Organic Conductor (TMTTF)2AsF6

We have investigated the transport properties of the quasi-one-dimensional (1-D) organic conductor (TMTTF) 2 AsF 6 at high pressures. A superconducting transition was observed, for the first time, at 2.38 K from a resistance measurement at 4.5 GPa. The presence of zero resistance was also confirmed at ∼2.3 K in the pressure range of 4.65≤ P ≤5 GPa. We discuss the electron correlation in (TMTTF) 2 AsF 6 based on its pressure–temperature phase diagram.

Thermal Expansion Coefficients of CeCu2 and YCu2 Single Crystals

Thermal expansions of CeCu 2 and YCu 2 single crystals have been measured in the temperature range from 2.0 K to 300 K. It is found that the linear thermal expansion coefficients α i ( i = a , b and c ) of CeCu 2 are strongly anisotropic among each crystal axis reflecting orthorhombic crystal structure. A well-defined anomaly due to antiferromagnetic ordering is observed near 3.5 K in the α i - T curves of CeCu 2 . A magnetic contribution to α i ( T ) due to the 4f electron is estimated by comparing the α i - T curve of CeCu 2 with that of YCu 2 .

Pressure Effect on Magnetic Short Range Ordering of LuFe2Ge2

Thermal neutron elastic scattering experiments revealed that iron localized moments form in an antiferromagentic short range ordering in LuFe 2 Ge 2 below 9 K. A long range type antiferromagnetic ordering grows rapidly under high pressure above 1.6 GPa.

Magnetic Phase Diagram of Ce1-xGdxRu2Al10 Single Crystals

CeRu 2 Al 10 shows an unknown phase transition at T 0 ∼27 K, and GdRu 2 Al 10 shows a simple RKKY-type antiferromagnetic one at T N ∼16.5 K. In order to clarify the origin of the transition of CeRu 2 Al 10 , we have investigated the relationship between these phase transitions from the electrical resistivity, magnetization and specific heat measurements using pseudoternary Ce 1- x Gd x Ru 2 Al 10 single crystals grown by the Al self-flux method. When x increases to 0.3, T 0 decreases slightly and then disappears abruptly at x =0.4. On the other hand, T N decreases monotonically with x , and coexists with T 0 at x =0.1 and 0.3. These results indicate that the unknown phase transition of CeRu 2 Al 10 is independent of the antiferromagnetic one of GdRu 2 Al 10 , which suggests that the unknown phase transition cannot be described by only RKKY-type interaction.

Magnetic and Electrical Properties in CePtSi3 without Inversion Symmetry in the Crystal Structure

We succeeded in growing a single crystal of CePtSi 3 with the tetragonal BaNiSn 3 -type crystal structure by the Sn-flux method. CePtSi 3 is found to be an antiferromagnet with successive two trans...

Electrical Resistivity of (TMTTF)2PF6 under High Pressure

We investigated the resistivity of quasi 1D organic conductor (TMTTF) 2 PF 6 under high pressure ( P ≦10 GPa) using a cubic anvil pressure system, which can generate a high quality hydrostatic pressure. We observed a superconducting state of (TMTTF) 2 PF 6 under 4 ≦ P ≦5 GPa. Zero resistivity was observed under 4 ≦ P ≦4.5 GPa for the first time. We will discuss the pressure-induced superconductivity with a P – T phase diagram of (TMTTF) 2 PF 6 .

Effect of Pressure on the Electrical Resistivity of YbAl2

We report the electrical resistivity and specific heat of the intermediate-valence intermetallic compound YbAl 2 . A single-crystalline YbAl 2 was synthesized by the Li-flux-growth technique. Electrical resistivity measurements were performed under high pressures up to 2.3 GPa. The value of the electrical resistivity increases with increasing pressure. This behavior is attributed to the gradual increase of the Yb valence in YbAl 2 .