High pressure and superconductivity: Intercalated graphite CaC6 as a model system

Abstract

Superconducting CaC6 is found to exhibit two important pressure effects: (i) a large P-induced Tc enhancement up to 15.1 K at 7.5 GPa, the highest Tc value hitherto reported for graphite intercalated compounds; and (ii) a dramatic Tc drop down to ∼3 K at a critical pressure of ∼9 GPa suggestive of a structural instability. We show that a combined electrical resistivity and x-ray diffraction study under high pressures provides a comprehensive account of both phenomena within the frame of the BCS theory in terms of a P-induced softening of the in-plane Ca mode relevant to the electron-phonon coupling. Our data analysis indicates that, below ∼8 GPa, the softening contributes to the Tc enhancement whilst, at higher pressures, it drives the system to a disordered phase presumably characterized by a disordering of the Ca sublattice. Thus, pressure induces a simultaneous order-disorder and lattice-softening phase transition from a good metal phase with high Tc to a bad metal phase with low Tc. © 2010 Springer Science+Business Media B.V.