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

¹ Laboratoire de Physique des Solides, Université Paris-Sud 11, CNRS UMR 8502 - 91405 Orsay, France, EU
² Dipartimento di Fisica, Università di Parma - Via G. P. Usberti 7/a, 43100 Parma, Italy, EU

^a yihara@phys.sci.hokudai.ac.jp

Received: 18 January 2011
Accepted: 22 March 2011

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 p_c from a Mott insulator to a superconductor. We evidence that for p ≫ p_c the (T₁T)^{− 1} data above T_c display metallic-like Korringa constant values which match quantitatively previous data taken on other A₃C₆₀ compounds. However below the pressure for which T_c 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 ≳ p_c singular T-dependences of (T₁T)^{− 1} are detected for T ≫ T_c. It will be shown that they can be ascribed to a large variation with temperature of the Mott transition pressure p_c towards a liquid-gas-like critical point, as found at high T for usual Mott transitions.