Negative thermal expansion of MgB${}_2$ in the superconducting state and anomalous behavior of the bulk Grüneisen function

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 Grüneisen 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.

arXiv comment5 figures. submitted to Phys. Rev. Lett

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