Soft x-ray spectroscopy of Ba24Ge100: Electronic phase transition and Ba-atom rattling

Determination of the local structure of CsBi4-xPbxTe6 (x = 0, 0.5) by X-ray absorption spectroscopy

We have studied the local structure and valence electronic unoccupied states of thermoelectric CsBi₄Te₆ and superconducting CsBi_3.5Pb_0.5Te₆ (T_c ∼ 3 K) by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) measurements. The Bi-L₃ edge EXAFS reveals wide Bi-Te distance distribution for both compounds indicating complex atomic arrangements in the studied system. The mean square relative displacements (MSRDs) of the Bi-Te bond distances appear largely increased in Pb substituted system due to larger overall local disorder, however, one of the Bi-Te bonds shows a reduced disorder. On the other hand, the Bi-L₃ edge XANES is hardly affected by Pb substitution while the Te-L₁ edge XANES reveals increased density of unoccupied Te 5p states. This suggests that the carriers introduced by the Pb substitution in CsBi_4-xPb_xTe₆ preferentially goes on Te sites. Similarly, the Cs-L₃ edge XANES also shows small changes due to Pb-substitution and reduced local disorder indicated by the reduced width of the Cs-L₃ edge white line. We have also shown that the X-ray photoemission spectroscopy (XPS) measurements on various electronic core levels are in a qualitative agreement with the XANES results. These findings are consistent with carrier doping and a reduced disorder in one direction to be likely factors to drive the thermoelectric CsBi₄Te₆ into a bulk superconductor by Pb-substitution in CsBi_4-xPb_xTe₆.

Electron-phonon interactions of Si100 and Ge100 superconductors with Ba atoms inside

The absolute density of states of isostructural Ba24Si100 and Ba24Ge100 near the Fermi level, which is very relevant for discussion on phonon-mediated superconductivity, is quantitatively evaluated to be 0.28  states  eV⁻¹/Ge  atom and 0.18  states  eV⁻¹/Si   atom by combining soft x-ray photoelectron spectroscopy and magnetic susceptibility measurements. The energetics of the anharmonic motions of the endohedral Ba atoms are also discussed on the basis of the 4d-core-level spectra. The discussion based on these important physical parameters concludes that the unique electron-phonon interactions do not occur due to the local Jahn-Teller distortion of the fragile open cage structure in the phonon-mediated BCS formalism. Compounds with a rigid cage would be essential to give rise to anticipated unique interactions as a key ingredient favored for superconducting paring.

Avila M, Suekuni K, Takabatake T, Guo F, Kobayashi K, Akai K, Tanigaki K. Soft x-ray photoelectron spectroscopy study of type-I clathrates

Extensive soft x-ray photoelectron spectroscopy studies are performed on Ba8 Ga16 Ge30 (BGG) and Sr8Ga16Ge30 (SGG) single crystals ranging from Fermi to core levels, at a high-energy facility. Valence band x-ray photoelectron spectroscopy (XPS) experiments with theoretical calculations revealed that the valence band is mainly constructed by the Ge/Ga 4s and 4p wave functions with little contribution of the Ba/Sr atomic orbitals. Surprisingly, unexpected features evidencing the different shift for the 2a- and 6d- sites between Ba 4d and Sr 3d are observed. The detailed analyses including theoretical support by first-principles band-structure calculations lead to the conclusion that the component distributions of the larger tetrakaidecahedral cage are different depending on the endohedral atoms, which contrasts with the past consensus that BGG and SGG have the same framework structure. This may give thorough reconsiderations on earlier interpretations of experimental data.