The layered pnictide oxides, ATi₂Pn₂O (A = Ba, (SrF)₂, Na₂; Pn = As, Sb, Bi), consist of alternating stacked (Ti₂Pn₂O)^2– and A²⁺ layers, the former of which contains a Ti₂O suare lattice. Among them, BaTi₂Sb₂O and BaTi₂Bi₂O exhibit superconductivity at 1.2 and 4.6 K, respectively. In contrast, the isostructural nonsuperconducting compound BaTi₂As₂O shows an electronic nematic-like transition at T_a = 200 K. The nematic phase is suppressed by isovalent pnictide substitution (As^3–→Sb^3–), resulting in the emergence of superconductivity. Further isovalent substitution (Sb^3–→Bi^3–) results in the disappearance of the neamatic phase and a two-dome structure in T_c is observed, which is similar as that of the iron-based superconductors¹. The role of the electronic nematic phase in the superconductivity and the origin of the two-dome structure in BaTi₂Pn₂O have not been clarified yet, but may provide a helpful clue to the high-T_c superconductivity.

To tackle this issue, we have studied the superconductivity and electronic nematic phases of BaTi₂Pn₂O. In the solid solutions of BaTi₂(As_1‒xSb_x)₂O, with decreasing temperature, the superconducting samples (x = 0.9 and 1) showed a sign change in the Seebeck coefficient from negative to positive at T_a whereas As-rich side of x ≤ 0.8 did not. Thus, this change in the electronic state must be important to the emergence of superconductivity. In BaTi₂Bi₂O, another nematic phase was found by ²⁰⁹Bi-NMR/NQR measurements and X-ray diffraction². Though the T_a is similar to that of BaTi₂Sb₂O, the orthorhombicity of BaTi₂Bi₂O was much larger than that of BaTi₂Sb₂O, implying a significant difference for the nematic phases of both compounds. Hence, the two different nematic phases may be responsible for the emergence of the two-dome structure in BaTi₂Pn₂O.

1) T. Yajima, Condens. Matter, 2, 4 (2017).
2) S. Kitagawa, K. Ishida, W. Ishii, T. Yajima, and Z. Hiroi, Phys. Rev. B, 98, 220507(R) (2018).

Keywords: Titanium pnictide oxides, Superconductivity, Electronic nematic phase