Model construction and superconductivity analysis of organic conductors \beta-(BDA-TTP)_2MF_6 (M=P, As, Sb, Ta) based on first principles band calculation

TL;DR

This study uses first principles calculations to model the electronic structure and superconductivity of organic conductors eta-(BDA-TTP)_2MF_6, revealing new insights into their Fermi surface, band structure, and pairing symmetry.

Contribution

The paper constructs an accurate tight-binding model from first principles and applies RPA to analyze superconductivity, highlighting differences from previous extended Huckel models.

Findings

Density of states correlates with SOMO band width.

Superconducting gap exhibits four sign changes (d-wave-like).

Anion dependence of Tc aligns with experiments.

Abstract

We perform a first principles band calculation for a group of quasi-two-dimensional organic conductors \beta-(BDA-TTP)2MF6 (M=P, As, Sb, Ta). The ab-initio calculation shows that the density of states (DOS) is correlated with the band width of singly occupied (highest) molecular orbital (SOMO), while it is not necessarily correlated with the unit cell volume. The direction of the major axis of the cross section of the Fermi surface lies in the \Gamma-B direction, which differs from that obtained by the extended Huckel calculation. Then, we construct a tight-binding model which accurately reproduces the ab-initio band structure. The obtained transfer energies give smaller dimerization than in the extended Huckel band. As for the difference of the anisotropy of the Fermi surface, the transfer energies along the inter-stacking direction are smaller than those obtained in the extended Huckel calculation. Assuming spin-fluctuation-mediated superconductivity, we apply random phase approximation (RPA) to a two-band Hubbard model. This two-band Hubbard model is composed of the tight-binding model derived from the first principles band structure and an on-site (intra-molecule) repulsive interaction taken as a variable parameter. The obtained superconducting gap changes sign four times along the Fermi surface like in a d-wave gap, and the nodal direction is different from that obtained in the extended Huckel model. Anion dependence of Tc is qualitatively consistent with the experimental observation.