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We estimated Grüneisen parameter (γG) to investigate lattice anharmonicity in a layered BiS2-based superconductor system REO1-xFxBiS2 (RE = La, Ce, Pr, Nd), where in-plane chemical pressure was tuned by the ionic radii of the RE elements, and the carrier concentration was tuned by F concentration. With increasing in-plane chemical pressure, bulk modulus remarkably increases, and a larger γG is observed for RE = Nd. The dependence of γG on in-plane chemical pressure exhibits a good correlation with Tc, suggesting that a larger γG results in a higher Tc in the system. In addition, γG shows a slight decrease by a decrease of F concentration (x) in REO1-xFxBiS2. Our results suggest that the enhancement of anharmonicity in REO1-xFxBiS2 is achieved by the suppression of in-plane disorder by in-plane chemical pressure and carrier doping. From analogy to thermoelectric LaOBi(S,Se)2, in which anharmonic vibration of Bi along the c-axis was enhanced by in-plane chemical pressure, we propose that the anharmonic vibration of Bi along the c-axis is present and in REO1-xFxBiS2, and the enhancement of the anharmonicity is positively linked to superconducting Tc and pairing mechanisms. [ABSTRACT FROM AUTHOR]

Electron-doped TiNCl is a superconductor, for which exotic mechanisms of the superconductivity have been discussed. However, difficulty in preparing large single-phase samples has prevented the direct observation of its electronic structure and how that changes with electron doping. In this study, micro-photoemission spectroscopy (μ-PES) was used to reveal the electronic structures of TiNCl and Na-intercalated TiNCl (Na-TiNCl). Comparison of Ti 2p core-level spectra shows the enhancement of a spectral feature in Na-TiNCl that suggests an increase in its metallic character. This indicates the introduction of electron carriers to the TiNCl layers. Although the overall valence band electronic structure of parent TiNCl could be reproduced by first-principle calculations, that of Na-TiNCl showed a marked deviation from the rigid band model near the Fermi level (EF). The spectral shape observed near EF of Na-TiNCl was found to be similar to a result in early transition metal oxides that has been attributed to the effect of strong electron correlation. The present study also demonstrated the usefulness of μ-PES to obtain reliable electronic structure data for samples that are difficult to make at large sizes. [ABSTRACT FROM AUTHOR]

One of the ultimate goals of the study of iron-based superconductors is to identify the common feature that produces the high critical temperature (Tc). In the early days, based on a weak-coupling viewpoint, the nesting between hole- and electron-like Fermi surfaces (FSs) leading to the so-called s± state was considered to be one such key feature. However, this theory has faced a serious challenge ever since the discovery of alkali-metal-doped FeSe (AFS) superconductors, in which only electron-like FSs with a nodeless superconducting gap are observed. Several theories have been proposed, but a consistent understanding is yet to be achieved. Here we show experimentally that a hole-like band exists in KxFe2-ySe2, which presumably forms a hole-like Fermi surface. The present study suggests that AFS can be categorized in the same group as iron arsenides with both hole- and electron-like FSs present. This result provides a foundation for a comprehensive understanding of the superconductivity in iron-based superconductors. [ABSTRACT FROM AUTHOR]

We show that the strongly correlated 4 f-orbital symmetry of the ground state is revealed by linear dichroism in corelevel photoemission spectra, as we have discovered for YbRh2Si2 and YbCu2Si2. Theoretical analysis shows us that the linear dichroism reflects the anisotropic charge distributions resulting from a crystalline electric field. We have successfully determined the ground-state 4 f symmetry for both compounds from the polarization-dependent angleresolved core-level spectra at a temperature well below the first excitation energy. The excited-state symmetry is also probed by temperature dependence of the linear dichroism where the high measurement temperatures are on the order of the crystal-field-splitting energies. [ABSTRACT FROM AUTHOR]