Exotic s-wave superconductivity in alkali-doped fullerides: equilibrium point of view
15:00 - 16:00
École Polytechnique, Paris
CFEL (Bldg. 99)
Seminar Room I, EG.076
The alkali-doped fullerides (A₃C₆₀, A = K, Rb, Cs) show a highest superconducting transition temperature (Tc) among molecular solids. In the phase diagram, s-wave superconductivity (SC) lies next to Mott insulating phase. This adjacency is similar to the cuprates (d-wave SC) but is more surprising because s-wave SC is believed to be severely suppressed by strong correlations.
In the first part of the talk, I will talk about first principles study on the equilibrium phase diagram [1,2]. A fully ab initio calculation shows that the fullerides are unique multi-orbital systems with an effectively negative exchange interaction (negative J) and a strongly repulsive Hubbard U. We show that this unusual form of intramolecular interaction leads to a surprising cooperation between the strong correlations and phonons in the s-wave pairing, in stark contrast to the conventional phonon mechanism.
In the second part of the talk, I will present recent results which study the effect of several perturbations on SC in three-orbital negative-J Hubbard model. The study is motivated by a recent remarkable report on the SC-like nonequilibrium optical property in K₃C₆₀ at a temperature much higher than the equilibrium transition temperature . I will show that an asymmetric form of Coulomb interaction can enhance superconductivity and that this asymmetric interaction can be realized by the pumping of T₁ᵤ phonons .
 Y. Nomura et al., Science Advances 1, e1500568 (2015)
 Y. Nomura et al., J. Phys.: Condens. Matter 28, 153001 (2016)
 M. Mitrano et al., Nature 530, 461-464 (2016)
 M. Kim et al., in preparation