Specific heat evidence on the normal state pseudogap

The electronic specific heat of underdoped cuprates reveals an absence of s pectral weight below a shoulder at the pseudogap energy E-g. With hole dopi ng additional spectral weight with entropy similar to k(B)/added hole appea rs on the flanks of the gap and E-g decreases as E-g = J(1 - p/p(crit)), wh ere J approximate to 1300 K and p(crit)similar to 0.18/CuO2 plane. This lea ds to an abrupt cross-over to metallic behaviour and strong superconductivi ty when the gap closes at p(ent)similar to J gamma(n)/k(B). We conclude tha t E-g(p) reflects the energy scale of correlated holes and spins, and is no t due to superconducting fluctuations or additional competing interactions. (C) 1998 Elsevier Science Ltd. All rights reserved.