Competition between Charge Ordering and Superconductivity in Layered Organic Conductors $\alpha$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ (M = K, NH$_4$)

TL;DR

This study investigates the competition between charge ordering and superconductivity in layered organic conductors, revealing how proximity to a quantum phase transition influences their electronic ground states.

Contribution

It provides a theoretical explanation for the coexistence and competition of charge order and superconductivity based on extended Hubbard model calculations.

Findings

Superconducting salt remains metallic down to 2 K.

Non-superconducting K-analog shows a pseudogap below 200 K.

Charge fluctuations near a quantum phase transition drive different ground states.

Abstract

While the optical properties of the superconducting salt $\alpha$-(BEDT-TTF)$_2$NH$_4$Hg(SCN)$_4$ remain metallic down to 2 K, in the non-superconducting K-analog a pseudogap develops at frequencies of about 200 cm$^{-1}$ for temperatures T < 200 K. Based on exact diagonalisation calculations on an extended Hubbard model at quarter-filling we argue that fluctuations associated with short range charge ordering are responsible for the observed low-frequency feature. The different ground states, including superconductivity, are a consequence of the proximity of these compounds to a quantum phase charge-ordering transition driven by the intermolecular Coulomb repulsion.