Specific heat evidence on the normal state pseudogap

The electronic specific heat of underdoped cuprates reveals an absence of spectral weight below a shoulder at the pseudogap energy E(g). With hole doping additional spectral weight with entropy similar to k(B)/added hole appears on the flanks of the gap and E(g) decreases as E(g) = J(1 - p/p(crit)), where J approximate to 1300 K and p(crit)similar to 0.18/CuO(2) plane. This leads to an abrupt cross-over to metallic behaviour and strong superconductivity when the gap closes at p(ent)similar to J gamma(n)/k(B). We conclude that E(g)(p) reflects the energy scale of correlated holes and spins, and is not due to superconducting fluctuations or additional competing interactions. (C) 1998 Elsevier Science Ltd. All rights reserved.