Symmetry Protected Topological Order and Spin Susceptibility in Superfluid 3He-B

We here demonstrate that the superfluid 3He-B under a magnetic field in a particular direction stays topological due to a discrete symmetry, that is, in a symmetry protected topological order. Due to the symmetry protected topological order, helical surface Majorana fermions in the B phase remain gapless and their Ising spin character persists. We unveil that the competition between the Zeeman magnetic field and dipole interaction involves anomalous quantum phase transition where topological phase transition takes place together with spontaneous breaking of symmetry. Based on the quasiclassical theory, we illustrate that the phase transition is accompanied by anisotropic quantum criticality of spin susceptibilities on the surface, which is detectable in NMR experiments.