  We study resonant transport through a superconducting double barrier
structure. At each barrier, due to the proximity effect, an incident electron
can either reflect as an electron or a hole (Andreev reflection). Similarly,
transport across the barrier can occur via direct tunneling as electrons as
well as via the crossed Andreev channel, where a hole is transmitted. In the
subgap regime, for a symmetric double barrier system (with low transparency for
each barrier), we find a new T=1/4 resonance (T is the transmission probability
for electrons incident on the double barrier structure) due to interference
between electron and hole wave-functions between the two barriers, in contrast
to a normal double barrier system which has the standard transmission resonance
at T=1. We also point out as an application that the resonant value of T=1/4
can produce pure spin current through the superconducting double barrier
structure.
