Surface electric structure of possible high-Tc surface superconductor WO3 based on the first-principles calculation
*Takuya Sekikawa1, Hiroyuki Kawai2, Yoshiaki Ono1

Tungsten oxide WO3 is a band insulator with a band gap of 3eV. When carrier (electron) is introduced in WO3 by alkali metal doping, AxWO3 (A=Na, K, Rb, Cs) becomes metallic and shows the superconductivity for 0.1 < x < 0.35, where the superconducting transition temperature Tc is found to increase with decreasing x up to 7 K [1,2]. At further low doping x~0.05, remarkable two dimensional superconductivities on the surface of AxWO3 are reported at Tc=91 K for A=Na [3] and Tc=120 K for A=H [4]. Theoretically, x-dependence of Tc in the bulk AxWO3 is well accounted for by the first-principles calculation with including the plasmon effect [5,6]. The possible surface superconductivity, however, was not discussed there. In this paper, we investigate the electronic structure on the surface of WO3 by using the first-principles calculation as a first step to discuss the surface superconductivity in WO3. We use the OpenMX code and apply the slab approach to reproduce the surface state.

Figures 1 (a) and (b) show the band dispersions of the tetragonal WO3 in the cases of the bulk and the fourteen-layer slab, respectively. In the fourteen-layer slab case, a remarkable narrow in-gap band appears at ~1eV below the conduction band. Detailed analysis of the wave functions reveal that the in-gap band mainly consists of orbitals in the first and second layers on the surface. Within the rigid band shift, the in-gap band is occupied by a small amount of doped electrons on the surface and then is expected to play a crucial role for the surface superconductivity in WO3.

Numerical calculations were performed using Oakforest-PACS system provided by Multidisciplinary Cooperative Research Program in CCS, University of Tsukuba, and MASAMUNE-IMR system provided in CCMS, IMR, Tohoku University.

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[5] T. Sekikawa, R. Watabe, J. Ishizuka, Y. Nitta, K. Sano and Y. Ōno, JPS. Conf. Proc. 30, 011043 (2020).
[6] K. Sano, Y. Nitta and Y. Ōno, J. Phys. Soc. Jpn. 89, 113704 (2020).

Keywords: High-Tc superconductivity, surface physics, first-principles calculation, WO3