Negative thermal expansion of MgB$_{2}$ in the superconducting state and anomalous behavior of the bulk Gr\"uneisen function

TL;DR

This study reveals that MgB$_{2}$ exhibits negative thermal expansion below its superconducting transition temperature, with anomalous behavior in the Gr"uneisen function indicating unusual electron-phonon interactions.

Contribution

It uncovers the temperature-dependent sign change in thermal expansion and Gr"uneisen function in MgB$_{2}$, highlighting novel electron-phonon coupling phenomena in the superconducting state.

Findings

Thermal expansion coefficient becomes negative below T_c

Gr"uneisen function turns negative at T_c and then increases dramatically

Evidence of anomalous coupling between electrons and phonons in MgB$_{2}$

Abstract

The thermal expansion coefficient $\alpha$ of MgB$_2$ is revealed to change from positive to negative on cooling through the superconducting transition temperature $T_c$. The Gr\"uneisen function also becomes negative at $T_c$ followed by a dramatic increase to large positive values at low temperature. The results suggest anomalous coupling between superconducting electrons and low-energy phonons.