Yuta Saiga

Superconductivity and Structural Phase Transitions in Caged Compounds RT2Zn20 (R = La, Pr, T = Ru, Ir)

Electrical resistivity ρ, specific heat C , magnetization M measurements are reported on four compounds, LaRu 2 Zn 20 , PrRu 2 Zn 20 , LaIr 2 Zn 20 , and PrIr 2 Zn 20 , which crystallize in a cubic CeCr 2 Al 20 -type structure. LaRu 2 Zn 20 , LaIr 2 Zn 20 , and PrIr 2 Zn 20 show superconducting transitions at T C = 0.2, 0.6, and ∼0.05 K, respectively, whereas PrRu 2 Zn 20 remains a normal state down to 0.04 K. This is the first observation of superconductivity in the family of RT 2 X 20 (T = transition metal; X = Al and Zn). Furthermore, structural phase transitions manifest themselves at T s = 150, 138, and 200 K for LaRu 2 Zn 20 , PrRu 2 Zn 20 , and LaIr 2 Zn 20 , respectively. No magnetic transition is found in PrRu 2 Zn 20 and PrIr 2 Zn 20 down to 1.8 K. On cooling PrIr 2 Zn 20 below 2 K, the specific heat divided by temperature, C / T , continuously increases and reaches 5 J/(K 2 ·mol) at 0.4 K, suggesting that Pr 4 f 2 electrons are involved in the heavy-fermion state, as observed in a related compo...

Formation of a Hybridization Gap in a Cage-Like Compound CeFe2Al10

We report on the magnetic, transport, and thermal properties of a cage-like compound CeFe 2 Al 10 that crystallizes in the orthorhombic YbFe 2 Al 10 -type structure. A broad peak in the magnetic susceptibility at 70 K indicates that CeFe 2 Al 10 is a valence fluctuation compound. The electrical resistivity and the Hall coefficient exhibit sharp upturns below 20 K, where the thermopower shows a rapid decrease. These low-temperature anomalies in the transport properties resemble those of a typical Kondo semiconductor CeRhSb. These features indicate the formation of a hybridization gap in CeFe 2 Al 10 on cooling below 20 K. The energy gap is estimated as 15 K from the thermal activation energy of the resistivity. The magnetic contribution of the specific heat shows a Schottky-type maximum at 30 K that provides another evidence for the gap formation in CeFe 2 Al 10 .

Pressure-induced Magnetic Transition in a Single Crystal of YbCo2Zn20

We have measured the electrical resistivity of a single crystal of YbCo 2 Zn 20 at pressures up to 2.37 GPa and at temperatures from 50 mK to 300 K. Above a critical pressure P c (∼1 GPa), we have found a resistivity anomaly at T M (∼0.15 K at 1 GPa) that increases with the pressure. At the ambient pressure, the system shows a nonmagnetic ground state described by the Fermi-liquid model. The T 2 coefficient of the electrical resistivity A strongly increases with the pressure upon approaching P c . However, in the vicinity of P c , the temperature dependence of the resistivity deviates from the Fermi-liquid description. These observations suggest that the application of hydrostatic pressure induces a magnetically ordered state for P ≥ P c and T ≤ T M .

High thermoelectric performance of Cu substituted type-VIII clathrate Ba8Ga16-xCuxSn30 single crystals

Single crystalline samples of type-VIII clathrate Ba8Ga16–xCuxSn30 (0 ≤ x ≤ 0.033) were prepared by the Sn-flux method. Upon substituting Cu for Ga, the carrier mobility at 300 K increases twice while the carrier density stays in the range 3.1−4.2×1019/cm3. Consequently, the electrical resistivity is decreased from 5.3 mΩcm for x = 0 to 3.2 mΩcm for x = 0.033. Irrespective of x, the Seebeck coefficient is largely negative and linearly changes with temperature in the range 300 < T < 600 K. The thermal conductivity is in the range 0.68−0.74 W/Km at 300 K for all samples. The dimensionless figure of merit ZT for x = 0.033 reaches the maximum of 1.35 at 540 K.

Enhancement of thermoelectric efficiency in type-VIII clathrate Ba8Ga16Sn30 by Al substitution for Ga

Single-crystalline samples of type-VIII clathrate Ba8Ga16−xAlxSn30 (0≤x≤12) were grown from Sn flux to characterize the structural and thermoelectric properties from 300 to 600 K. The lattice parameter increases by 0.5% as x is increased to 10.5 whose value is the solubility limit of Al. The Seebeck coefficients of all samples are largely negative and the absolute values increase to approximately 300 μV/K on heating to 600 K. This large thermopower coexists with the metallic behavior in the electrical resistivity. The values of resistivity for 1≤x≤6 at 300 K are in the range 3.3–3.8 mΩ cm which is 70% of that for x=0. As a result, the power factor for x=4 and 6 has a rather large maximum of 1.83×10−3 W/m K2 at 480 K. The thermal conductivity stays at a low level of 0.72 W/mK up to 480 K, and the sample with x=6 reaches a ZT value of 1.2 at 500 K.

Magnetic properties of gapless semiconductors: PbPdO2 and PbPd0.9Co0.1O2

PbPdO2 is a new class of gapless semiconductors, which is extremely sensitive to external influences such as temperature, magnetic field, and carrier doping, because of their peculiar band structure. With varying temperature, a broad transition from a high-temperature metallic behavior to a low-temperature insulating behavior was observed at TMI=100 K in the electrical resistivity, which is related to the thermally assisted excitation near the Fermi level due to its gapless band structure. By doping 10% Co for Pd in PbPdO2, the number of hole charge carriers was increased by ten times, and the transition temperature was increased to TMI=150 K. When applying a magnetic field, a ferromagnetic component was found at low temperatures in the magnetization curves of both materials, in addition to diamagnetic background signals for PbPdO2 and paramagnetic background signals for PbPd0.9Co0.1O2. In the low temperature regime, the slope of magnetoresistance is negative, while it is changed into positive with a quad...

Crystal growth and thermoelectric properties of type-VIII clathrate Ba8Ga15.9Sn30.1−xGex with p-type charge carriers

Type-VIII clathrate Ba8Ga16Sn30 is a promising thermoelectric material in the 400?600?K range. We report on the effects of Ge substitution for Sn on both crystal growth and thermoelectric properties. In a series of samples prepared from the initial composition of Ba8 Ga40 Sn30?X GeX (0???X???6), the size of single crystals reaches the maximum of 8?mm at X?=?0.5. The atomic composition of single crystals is found to be described as Ba8Ga15.9Sn30.1?xGex (0???x???4.73). Ge substitution results in the increase in the positive Seebeck coefficient and electrical resistivity by 1.3 and 2 times at the maximum, respectively, whereas the hole carrier density does not change substantially. Taken together with the thermal conductivity data, the maximum of the dimensionless figure of merit ZT reaches 0.87 at 540?K for x?=?0.07.

Magnetic properties of Mn and Co doped PbPdO2

We report the dramatic change of gapless semiconductor properties by different chemical doping elements of Co and Mn into PbPdO2. The metal-insulatorlike transition temperature TMI = 100 K for PbPdO2 shifts to a higher temperature of 150 K by the Co doping and to a lower temperature of 70 K by the Mn doping. Because of the anisotropic band structure with the majority of heavy holes and the minority of light electrons, the transport and magnetic properties are significantly changed by the chemical doping elements. At low temperatures, the Co doping enhances ferromagnetic interactions, whereas the Mn doping favors antiferromagnetic interactions. These results are of great interests because you can control the magnetic ordering as well as manipulate the carrier density by changing the doping elements. These materials could be a good candidate for spintronics applications.

Thermoelectric performance of Zn-substituted type-VIII clathrate Ba8Ga16Sn30 single crystals

We have grown single-crystalline samples of Zn-substituted type-VIII clathrate Ba8Ga16Sn30 with n-type carriers by Sn-flux method. The actual compositions of the single crystals were found to be described as Ba8Ga15.8−2yZnySn30.2+y (y = 0 ∼ 0.54), where the charge balance is well maintained. As y goes from 0 to 0.42, the resistivity at 300 K decreases from 5.3 to 3.0 mΩ cm gradually, but the effective mass is essentially constant at 1.2 ∼ 1.5m0, indicating intact band structure near the conduction band minima upon Zn substitution for Ga. At elevated temperatures, the ambipolar effect on the thermal conductivity becomes less pronounced upon Zn doping, and the dimensionless figure of merit ZT for y = 0.07 and 0.42 remains at rather high values compared with the nondoped sample.

Study of α ↔ β transformation in the dimorphic clathrate Ba8Ga16Sn30

The intermetallic clathrate Ba8Ga16Sn30 (BGS) is dimorphic, possessing α-phase (type-VIII) and β-phase (type-I). To determine the stability ranges of the two phases, we performed a study of the phase diagram by combining differential thermal analysis (DTA) and power X-ray diffraction (XRD) on both samples of bulk single crystals (≅1 mm) and powdered crystals (<50 µm). It was found that bulk samples of both α- and β-phases melt congruently at 500°C, but a powder specimen of β-phase has a broad endothermic DTA peak at 466°C, just below the melting point. Intensive studies on this event revealed a partially reversible and strongly kinetically controlled β → α structural phase transition. This transition has been confirmed by in situ synchrotron XRD at SPring-8. After heating the powder specimens to 530°C and cooling to room temperature, the main phase remained intact in both modifications. When the soaking temperature was increased from 530 to 910°C, a weak exothermic peak for nucleation of Ba(Ga/Sn)4 appeared at about 570°C in the cooling process. The main phase of the resulting materials was found to be β-phase, irrespective of the starting phases and grain sizes. Based on the results of DTA and XRD, we constructed a tentative Sn–Ba(Ga/Sn)4 pseudo-binary phase diagram. It suggests that large single crystals of β-BGS can be grown from the Sn flux without being disturbed by the α ↔ β structural phase transition.