  A simple interlayer pair tunneling is solved exactly. We find that in the
normal state spin-1/2 particle and hole excitations are gapped. But the state
is an unusual metal, characterized by novel fermionic spin zero and charge +2e
and -2e excitations that exist about their own Fermi surfaces. The model is
consistent with a number of properties of the underdoped cuprates. At finite
temperature the Fermi surface appears partially gapped, and at high
temperatures the spin gap disappears completely. Superconductivity is induced
by an additional intralyer interaction which opens a gap in the charge-fermion
spectrum. The symmetry of this charge gap (and hence that of the order
parameter) is igenerally different from the symmetry of one-electron or - hole
gap. The former can be a d wave, while the latter is more complex.

Condensed Matter