Spin dynamics at the Mott transition and in the metallic state of the Cs3C60 superconducting phases

Abstract

We present here 13C and 133Cs NMR spin lattice relaxation T1 data in the A15 and fcc-Cs3C60 phases for increasing hydrostatic pressure through the transition at pc from a Mott insulator to a superconductor. We evidence that for p≫pc the (T1T)− 1 data above Tc display metallic-like Korringa constant values which match quantitatively previous data taken on other A3C60 compounds. However below the pressure for which Tc goes through a maximum, (T1T)− 1 is markedly increased with respect to the Korringa values expected in a simple BCS scenario. This points out the importance of electronic correlations near the Mott transition. For p≳pc singular T-dependences of (T1T)− 1 are detected for T≫Tc. It will be shown that they can be ascribed to a large variation with temperature of the Mott transition pressure pc towards a liquid-gas-like critical point, as found at high T for usual Mott transitions.