Zero modes of velocity field and topological invariant in quantum torus

Abstract: We propose the velocity field approach to characterize topological invariants of quantum states. We introduce the indexes of the velocity field flow based on the zero modes of the velocity field and find that these zero modes play the role of effective topological charges or defects linking to Euler characteristic by the Poincar\'{e}-Hopf theorem. The global property of the indexes is topological invariants against the parameter deformation. We demonstrate this approach by the quantum torus model and compare the topological invariant with that obtained from the Chern number. We find that the physical mechanism of the topological invariant based on the zero modes of the velocity field is different from that of the topological invariant by the Chern number. The topological invariant characterized by the velocity field describes a homeomorphic topological invariant associated with the zero modes on the submanifold of the base manifold of the SU(2)-fibre bundle for quantum torus, whereas the Chern number characterizes a homotopy invariant associated with the exceptional points in the Brillouin zone. We also propose the generalized winding number in terms of the velocity field for both Hermitian and non-Hermitian systems. This gives a connection between the zero mode and winding number in the velocity space. These results enrich the topological invariants of quantum states and promises us a novel insight to understanding topological invariants of quantum states as well as expected to be further applied in more generic models.