  We study theoretically spin current generation from a band insulator with PT
symmetry, which is associated with Zener tunneling in strong dc electric
fields. Each band in this system is doubly degenerate with opposite spins, but
spin rotational symmetry is not preserved in general. We consider the condition
for spin current generation in connection with the nature of the wave function,
which ultimately depends on a geometric quantity known as the shift vector.
From an analysis of a two-band model, we find that the shift vector is
necessary for spin current generation in a PT symmetric system. We also present
zigzag chain models that have shift vectors, and confirm from numerical
calculations that a nonzero tunneling spin current occurs in spin-degenerate
systems.
