Degenerate states of narrow semiconductor rings in the presence of spin-orbit coupling: Role of time-reversal and large gauge transformations

The electron Hamiltonian of narrow semiconductor rings with the Rashba and Dresselhaus spin-orbit terms is invariant under a time-reversal operation followed by a large gauge transformation. We find that all the eigenstates are doubly degenerate when integer or half-integer quantum fluxes thread the quantum ring. The wave functions of a degenerate pair are related to each other by the symmetry operation. These results are valid even in the presence of a disorder potential. When the Zeeman term is present only some of these degenerate levels anticross.