The elastic properties of a filled skutterudite PrOs 4 Sb 12 with a heavy fermion superconductivity at T C ϭ1.85 K have been investigated. The elastic softening of (C 11 ϪC 12 )/2 and C 44 with lowering temperature down to T C indicates that the quadrupolar fluctuation due to the CEF state plays a role for the Cooper paring in superconducting phase of PrOs 4 Sb 12 . A Debye-type dispersion in the elastic constants around 30 K revealed a thermally activated ⌫ 23 rattling due to the off-center Pr-atom motion obeying ϭ 0 exp(E/k B T) with an attempt time 0 ϭ8.8ϫ10 Ϫ11 s and an activation energy Eϭ168 K. It is remarkable that the charge fluctuation of the off-center motion with ⌫ 23 symmetry may mix with the quadrupolar fluctuation and enhance the elastic softening of (C 11 ϪC 12 )/2 just above T C .The rare-earth cubic compounds based on Pr 3ϩ ions have received much attention because various unusual properties are expected at low temperatures. The system with a non-ϪJ y 2 favors a quadrupole ordering. We refer the ⌫ 3 ground state systems as a metallic compound PrPb 3 showing the antiferro-quadrupole ordering at T Q ϭ0.4 K 1 and a semiconductor PrPtBi undergoing ferroquadrupole ordering at T Q ϭ1.2 K. 2 PrSb is known as a singlet ground state system. 3 The elastic constant (C 11 ϪC 12 )/2 is responsible for the quadrupolar susceptibility of O 2 0 and O 2 2 with ⌫ 3 symmetry, while C 44 is for the susceptibility of O yz ϭJ y J z ϩJ z J y , O zx ϭJ z J x ϩJ x J z , O xy ϭJ x J y ϩJ y J x with ⌫ 5 symmetry. The softening of (C 11 ϪC 12 )/2 and C 44 is useful proof to clarify the quadrupolar effects of Pr-based compounds.Recently, Bauer et al. have found a new-type of the heavy-fermion superconductor in a filled skutterudite PrOs 4 Sb 12 with space group T h 5 (Im3). 4 The heavy fermion state with a large specific heat coefficient ␥ϭ750 mJ/mol K 2 of PrOs 4 Sb 12 exhibits the superconducting transition at T C ϭ1.85 K associated with a large jump ⌬C/T C ϳ500 mJ/mol K 2 . A sign of the double transition in the specific heat has been found. 5 The thermal transport measurement in fields suggests the two distinct superconducting phases in PrOs 4 Sb 12 . 6 The nuclear spin relaxation rate 1/T 1 of Sb indicates unconventional superconductivity possessing neither a coherence peak nor a T 3 -power law. 7 The muon spin relaxation in PrOs 4 Sb 12 yields a penetration depth indicating a new type of energy gap. 8 The odd-parity Cooper pairing mediated by the quadrupole fluctuation is argued as unconventional heavy-fermion superconductivity in PrOs 4 Sb 12 . 9 Because the magnetic susceptibility is rather silent to distinguish non-magnetic ⌫ 23 doublet from ⌫ 1 singlet, it has not been settled whether the CEF ground state of PrOs 4 Sb 12 is ⌫ 23 doublet or ⌫ 1 singlet. 4,10 The measurement of (C 11 ϪC 12 )/2 and C 44 responsible for the quadrupolar sus-ceptibility in PrOs 4 Sb 12 is a central issue to clarify the CEF state and the interplay of the quadrupolar fluctuation to the superconductivity in PrOs 4 Sb 12 .

We have synthesized off-stoichiometric Ti-Ni-Sn half-Heusler thermoelectrics in order to investigate the relation between randomly distributed defects and thermoelectric properties. A small change in the composition of Ti-Ni-Sn causes a remarkable change in the thermal conductivity. An excess content of Ni realizes a low thermal conductivity of 2.93 W/mK at room temperature while keeping a high power factor. The low thermal conductivity originates in the defects generated by an excess content of Ni. To investigate the detailed defect structure, we have performed first-principles calculations and compared with x ray photoemission spectroscopy measurement. Based on these analyses, we conclude that the excess Ni atoms randomly occupy the vacant sites in the half-Heusler structure, which play as phonon scattering centers, resulting in significant improvement of the figure of merit without any substitutions of expensive heavy elements, such as Zr and Hf.

In Cu 2 ZnSnS 4 (CZTS) photovoltaic cells, a low open-circuit voltage (V OC ) principally causes low conversion efficiency. We investigated the deposition of a CZTS layer by a two-layer process to improve the V OC of the CZTS cells. In this process, the first CZTS layers near a Mo electrode have a high Cu content and the second layer near the surface has a low Cu content. The two-layer process improved the V OC of the CZTS cells from 0.66 to 0.78 V. Finally, the best CZTS cell showed a conversion efficiency of 8.8%. R ecently, the production of compound thin-film photovoltaic cells, for example, the Cu(In,Ga)Se 2 and CdTe types, has been increasing annually. However, the materials used to make these cells include rare metals (In and Ga), which significantly limits the volume that can be produced. The thin-film photovoltaic material Cu 2 ZnSnS x Se 4−x (CZTSSe) has recently become a candidate material for new photovoltaic cells. Wang et al. recently reported the production of CZTS 0.2 Se 0.8 thin-film solar cells with a conversion efficiency of 12.6%. 2) However, CZTSSe contains Se, a harmful element, and has a band-gap energy (E g ) of 1.1 eV, which is smaller than the optimum E g of 1.4-1.5 eV for the solar spectrum. On the other hand, Cu 2 ZnSnS 4 (CZTS) has been attracting increasing attention as a candidate material 1) because it contains no rare (In and Ga) or harmful (Se and Te) elements, and has a suitable E g of 1.4 eV. However, the CZTS cells that were previously reported have efficiencies below 10%. [3][4][5][6] Consequently, for the mass production of photovoltaic cells, it is clear that the development of CZTS cells with high efficiency is required.One of the main reasons for the poor photovoltaic properties of CZTS cells is their open-circuit voltage (V OC ), which is lower than that expected from the E g . The low V OC directly results in low conversion efficiency. To understand the V OC of the CZTS cells, the conduction band offset (CBO) at the interface between the buffer and the CZTS layers should be examined. In general, the interface between the buffer layer and the absorber layer is categorized into either a "cliff " or a "spike" type, where a large cliff lowers V OC and a large spike reduces the short-circuit current density (J SC ). 7,8) Researchers previously reported on the evaluation of the CBO value at the interface between the CdS and CZTS layers using methods such as hard X-ray photoelectron spectroscopy, 9) pump=probe methods [ultraviolet photoelectron spectroscopy (UPS)], 10) and UPS=inverse photoelectron spectroscopy (IPES), 11) and showed it to be sufficiently small, in the range from +0.4 to −0.2 eV, to realize a high-performance cell. Therefore, the CBO value is not the main reason for the low V OC in the CZTS cells.Another possible reason for the low V OC is the influence of an inner potential between the CZTS and window layers. In other words, the lower inner potential between the CZTS and window layers can cause a lower V OC when the hole carrier concentration (N c ) of the CZ...

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We directly and non-destructively measured the valence band offset at the interface between CdS and Cu2ZnSnS4 (CZTS) using hard X-ray photoelectron spectroscopy (HAXPES), which can measure the electron state of the buried interface because of its large analysis depth. These measurements were made using the following real devices; CZTS(t = 700 nm), CdS(t = 100 nm)/CZTS(t = 700 nm), and CdS(t = 5 nm)/CZTS(t = 700 nm) films formed on Mo coated glass. The valence band spectra were measured by HAXPES using an X-ray photon energy of 8 keV. The value of the valence band offset at the interface between CdS and CZTS was estimated as 1.0 eV by fitting the spectra. The conduction band offset could be deduced as 0.0 eV from the obtained valence band offset and the band gap energies of CdS and CZTS.