日 時： 2014年7月30日（水）16:00～17:30
場 所： 工学部6号館3階325号室（物工会議室）
講 師： P. Wzietek 氏（Laboratoire de Physique des Solides, Université Paris-Sud, Orsay, France）
題 目： NMR investigation of the Pressure induced transition from Mott insulator to Superconductor in the Cs3C60 isomeric compounds
概要： The A3C60 metallic compounds has for long been considered as metals close to regular Fermi liquids displaying superconducting BCS behavior. Among various evidences many come from NMR studies : the Pauli-like normal state electronic susceptibility as measured by NMR Knight shift and T1T data, the decrease of Knight shift in the superconducting state as expected for singlet pairing, and finally the observation of the Hebel-Slichter coherence peak pointing towards a conventional s-wave superconductivity.
On the other hand, the marked increase of Tc with the inter-ball distance and some other experimental observations led to conjectures on the role of electron correlations enhanced by a Jahn-Teller distortion in these materials. The debate on this issue has been reopened by the recent discovery of two fulleride Cs3C60 isomeric phases which present the most expanded lattice in this family and which also happen to be Mott insulators at ambient pressure. In fact, although superconductivity involves an electron-phonon coupling, the electronic correlations are of primary importance for enhancing the Tc values, as had been anticipated from DMFT calculations.
I will present NMR experiments preformed in the A15 and fcc-Cs3C60 phases for increasing hydrostatic pressure through the transition from a Mott insulator to a superconductor. These measurements clearly indicate the existence of a 3D Mott transition to a superconducting state in this family. Our findings are consistent with theoretical models which emphasize the role of the coulomb correlations and the Jahn-Teller distortion that lifts the multi-orbital molecular state degeneracy in these materials.