  We study theoretically the spin transport in a nonmagnetic metal connected to
ferromagnetic injector and detector electrodes. We derive a general expression
for the spin accumulation signal which covers from the metallic to the
tunneling regime. This enables us to discuss recent controversy on spin
injection and detection experiments. Extending the result to a superconducting
device, we find that the spin accumulation signal is strongly enhanced by
opening of the superconducting gap since a gapped superconductor is a low
carrier system for spin transport but not for charge. The enhancement is also
expected in semiconductor devices.
