Negative thermal expansion of MgB2 in the superconducting state and anomalous behavior of the bulk Gruneisen function -: art. no. 220505

The thermal expansion coefficient alpha of MgB2 is revealed to change from positive to negative on cooling through the superconducting transition temperature T-c. The Gruneisen 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.