Unusual Superconducting State of Underdoped Cuprates

There is increasing experimental evidence that the superconducting energy gap Δ0 in the underdoped cuprates is independent of doping concentration x while the superfluid density is linear in x . We show that under these conditions thermal excitation of the quasiparticles is very effective in destroying the superconducting state, so that Tc is proportional to xΔ0 and part of the gap structure remains in the normal state. We then estimate Hc2 and predict it to be proportional to x2 . We also discuss to what extent the assumptions that go into the quasiparticle description can be derived in the U(1) and SU(2) formulations of the t-J model. copyright 1997 The American Physical Society