Symmetry-protected Z2-quantization and quaternionic Berry connection with Kramers degeneracy

As for a generic parameter-dependent Hamiltonian with time reversal (TR) invariance, a non-Abelian Berry connection with Kramers (KR) degeneracy is introduced by using a quaternionic Berry connection. This quaternionic structure naturally extends to the many-body system with KR degeneracy. Its topological structure is explicitly discussed in comparison with the one without KR degeneracy. Natural dimensions to have nontrivial topological structures are discussed by presenting explicit gauge fixing. Minimum models to have accidental degeneracies are given with/without KR degeneracy, which describe the monopoles of Dirac and Yang. We have shown that the Yang monopole is literally a quaternionic Dirac monopole. The generic Berry phases with/without KR degeneracy are introduced by the complex/quaternionic Berry connections. As for the symmetry-protected Z(2)-quantization of these general Berry phases, a sufficient condition of the Z(2)-quantization is given as the inversion/reflection equivalence. Topological charges of the SO(3) and SO(5) nonlinear alpha-models are discussed in relation to the Chern numbers of the CP1 and HP1 models as well.