June 21st, 10:00 CEST: Yoshiteru Maeno (Kyoto University, Japan)
Superconductivity of Sr2RuO4: beyond “unconventional”?
After more than 25 years since its discovery, studies of the unconventional superconductivity of Sr2RuO4 have been reactivated in the last couple of years. As the main trigger, it was revealed that previous NMR results had a technical problem of overheating the sample. Now the reduction of the spin susceptibility below Tc consistent with the spin-singlet pairing has been confirmed. As another new development, uniaxial pressure can drive its multi-band Fermi surfaces across a Lifshitz transition, and increases its Tc from 1.5 K to 3.5 K. Correspondingly, muon spin resonance indicates a splitting between the enhanced Tc and the onset of the time-reversal-symmetry breaking at TTRSB that remains at about 1.5 K. In combination with other results such as the unusual jump in a shear elastic constant in ultrasound experiments, one promising pairing symmetry is Eg with the chiral d-wave “d + id” order parameter. However, such a state is controversial since it implies a gap structure that does not allow pairing of electrons within the plane despite the quasi-2-dimensional nature. In order to settle this issue, some promising theories propose the inter-orbital pairing. Superconductivity of Sr2RuO4 may help to establish such a pairing state beyond the traditional “unconventional” pairing. We will also mention scanning tunneling microscopy (STM) and junction experiments to investigate the superconducting state of Sr2RuO4.