Dr. Takeshi Mizushima (Department of Physics, Okayama University)
"Symmetry protected topological order and spin susceptibility in superfluid 3He-B"

The superfluid 3He-B has been recognized as one of the most concrete examples of topological superconductors and superfluids, where time- reversal invariant superfluid 3He-B has a nontrivial bulk topological number and the bulk-edge correspondence ensures the existence of helical Majorana fermions. It is however stated that any time-reversal breaking such as a magnetic field wipe out the topological nature. In this seminar, we clarify that the B-phase under a magnetic field in a particular direction stays topological due to a discrete symmetry, that is, in a symmetry protected topological phase. It is shown that the Ising spin character of the helical Majorana fermion is a direct consequence of the symmetry protected topological phase. Based on the quasiclassical theory, we demonstrate that the competition between the dipole interaction and the magnetic field involves the topological phase transition and simultaneously the spontaneous symmetry breaking of the discrete symmetry, which is accompanied by novel behaviors of spin susceptibilities on the surface. These work was in collaboration with Masatoshi Sato and Kazushige Machida.