Topological Properties of \tau-Type Organic Conductors with a Checkerboard Lattice

TL;DR

This paper demonstrates that au-type organic layered conductors with a checkerboard lattice can host topological phases, featuring Dirac cones, SOC-induced gaps, and helical edge states, suggesting they are organic topological insulators with potential spin Hall effects.

Contribution

It reveals the emergence of topological phases in au-type organic conductors with a checkerboard lattice, highlighting the role of SOC and strain in creating topologically nontrivial states.

Findings

Presence of quadratic band touching in the conduction and valence bands.

Splitting into Dirac cones under uniaxial strain.

Existence of helical edge states indicating topological insulator behavior.

Abstract

Although the topological phases are difficult to be realized in organic molecular crystals, we demonstrate here that they can emerge in the \tau-type organic layered conductors, \tau-(EDO-S,S-DMEDT-TTF)_2X_{1+y} and \tau-(P-S,S-DMEDT-TTF)_2X_{1+y} (X=AuBr_2, I_3, IBr_2), where EDO-S,S-DMEDT-TTF and P-S,S-DMEDT-TTF denote the planar donor molecules ethylenedioxy-S,S-dimethyl(ethylenedithio)tetrathiafulvalene and pyrazino-S,S-dimethyl(ethylenedithio)tetrathiafulvalene, respectively. The conducting layers of these conductors have a highly symmetric checkerboard structure, which can be regarded as a modified Mielke lattice. Because their electronic structure inherits that of the Mielke lattice, their conduction and valence bands exhibits the quadratic band touching. The contact point splits into a pair of Dirac cones under uniaxial strain which breaks C_4-symmetry. In \tau-type conductors, we can expect rather large spin-orbit coupling (SOC) as organic conductors. We show that the SOC in this case opens a topologically nontrivial gap at the band contact point, and the helical edge states exist in the gap. The actual \tau-type conductors could be regarded as heavily-doped topological insulators, which could exhibit finite spin Hall effect.