Possible charge-density wave, superconductivity, andf-electron valence instability inEuBiS2F

Abstract

Superconductivity (SC) and charge-density wave (CDW) are two contrasting yet relevant collective electronic states which have received sustained interest for decades. Here we report that, in a layered europium bismuth sulfofluoride, EuBiS2F, a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K, without any extrinsic doping. The Eu ions were found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane Bi-6p states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by ∼ 50 times, owing to the significant hybridizations between Eu-4f and Bi-6p electrons, as verified by band-structure calculations. Thus, EuBiS2F manifests itself as an unprecedented material that simultaneously accommodates SC, CDW and f -electron valence instability.

Electrical and Thermal Transport of Layered Bismuth-Sulfide EuBiS2F at Temperatures between 300 and 623 K

“…The bond valence sum (BVS) of Eu was calculated to be 2.16(1), 13,14) which was consistent with the previously reported value, 9) indicating electron doping into the BiS2 conduction layer.…”

“…9) Given the results shown in the previous study, it was controversial from the viewpoint of thermoelectricity, whether Eu 4f and Bi 6p hybridized orbitals are effective in modulating the Fermi surface and enhancing S, 10) or reducing S owing to the coexistence of electrons and holes, i.e., mixed conduction. In this study, we demonstrate the electrical and thermal transport of EuBiS2F at temperatures between 300 and 623 K.Polycrystalline EuBiS2F was prepared by a solid-state reaction using a sealed silica tube, following the method reported by Zhai et al 9) The relative density of the sample was calculated to be 90%. The sample purity was examined by X-ray diffraction (XRD) collected using CuK radiation (Rigaku RINT 2500).…”

“…2(a). On the other hand, Zhai et al reported  of ~2.2 mcm at 300 K with a positive temperature coefficient (d/dT) between 300 and 350 K. 9) This different T plot is probably due to self-doping owing to the mixed valence of Eu and/or inevitable Bi off-stoichiometry, which is previously examined using X-ray diffraction 15) and scanning tunneling microscopy. 16) Figure 2(b) shows S as a function of temperature.…”

“…Polycrystalline EuBiS2F was prepared by a solid-state reaction using a sealed silica tube, following the method reported by Zhai et al 9) The relative density of the sample was calculated to be 90%. The sample purity was examined by X-ray diffraction (XRD) collected using CuK radiation (Rigaku RINT 2500).…”

“…9) Specific heat measurement showed significant hybridization between Eu 4f and Bi 6p electrons, while density functional theory (DFT) calculation indicates that the valence band maximum consists of Eu 4f orbitals. 9) Given the results shown in the previous study, it was controversial from the viewpoint of thermoelectricity, whether Eu 4f and Bi 6p hybridized orbitals are effective in modulating the Fermi surface and enhancing S, 10) or reducing S owing to the coexistence of electrons and holes, i.e., mixed conduction. In this study, we demonstrate the electrical and thermal transport of EuBiS2F at temperatures between 300 and 623 K.…”