Optical properties of non-centrosymmetric Th-based superconductors: a density functional theory study

Non-centrosymmetric superconductors are the subject of intensive study because the lack of an inversion center can lead to mixed spin-singlet and spin-triplet pairing states[1]. Here, we investigate the optical properties with the help of electronic transitions for non-centrosymmetric superconductors based on Th with the stoichiometry Th₇T₃ (T=Fe, Co, Ni) [3-5], which exhibit weak electron correlations. The full potential linear augmented plane wave (FP-LAPW) method was employed, as well as scalar and fully relativistic treatments. The Th₇T₃ ​ compounds crystallize in the hexagonal Th₇Fe₃-type structure (space group P6_3​mc) and undergo a superconducting transition at Tc≈2TK. The crystal unit cell contains three inequivalent thorium positions (Th1, Th2 at 6c, Th3 at 2b) and one transition metal position (6c). By replacing the Fe with Co or Co with Ni, one additional electron is added to the system. This substitution of one transition metal atom for another, along with the change in atomic masses and the role of the relativistic effect of the thorium atoms, leads to interesting properties in the electronic and optical structures of the Th₇T₃ compounds. We emphasize the role of the ASOC strength since anisotropic spin splitting, both in the band structure and optical properties, emerges when one takes into account the spin–orbit coupling in the calculations. By providing the frequency-independent dielectric function calculations at 2, 10, and 50 K temperatures, we assume that transitions between Th-6d and T-d valence electrons are mainly responsible for the superconducting properties.

[1]Bauer, E. & Sigrist, M. Non-Centrosymmetric Superconductors: Springer-Verlag Berlin Heidelberg 847 (2012).

[2-3] Sahakyan, M. & Tran, V. H. Journal of Physics: Cond Mat 28, 205701 (2016), Phil. Mag. 42, 957-966 (2017).

[4] Tran, V. H. & Sahakyan, M. Scientific Reports 7, 15769 (2017).

[5] R Idczak, M Sahakyan, VH Tran Journal of Physics: Condensed Matter 30 (47), 475802 (2018).