Enhancement of the thermal expansion of organic charge transfer salts by strong electronic correlations

TL;DR

This paper investigates how strong electronic correlations enhance the thermal expansion anomalies in organic charge transfer salts, linking these effects to electronic entropy and bond properties using computational models.

Contribution

It demonstrates that electronic correlations significantly amplify thermal expansion anomalies in organic salts, providing a theoretical framework relating entropy and bond parameters.

Findings

Electronic correlations enhance thermal expansion below 100 K.

Thermal expansion relates to entropy dependence on Hamiltonian parameters.

Calculated expansion magnitude is smaller than experimental observations.

Abstract

Organic charge transfer salts exhibit thermal expansion anomalies similar to those found in other strongly correlated electron systems. The thermal expansion can be anisotropic and have a non-monotonic temperature dependence. We show how these anomalies can arise from electronic effects and be significantly enhanced, particularly at temperatures below 100 K, by strong electronic correlations. For the relevant Hubbard model the thermal expansion is related to the dependence of the entropy on the parameters ($t$, $t'$, and $U$) in the Hamiltonian or the temperature dependence of bond orders and double occupancy. The latter are calculated on finite lattices with the Finite Temperature Lanczos Method. Although many features seen in experimental data, in both the metallic and Mott insulating phase, are described qualitatively, the calculated magnitude of the thermal expansion is smaller than that observed experimentally.