Pinned Balseiro-Falicov model of tunneling and photoemission in the cuprates

sThe smooth evolution of the tunneling gap of Bi2Sr2CaCu2O8 with doping fro m a pseudogap state in the underdoped cuprates to a superconducting state a t optimal and overdoping, has been interpreted as evidence that the pseudog ap must be due to precursor pairing. We suggest an alternative explanation, that the smoothness reflects a hidden SO(N) instability group near the ( p i,0) points of the Brillouin zone (with N= 3, 4, 5, or 6). Because of this group structure, the pseudogap could actually be due to any of a number of nesting instabilities, including charge or spin density waves or more exoti c phases. We present a detailed analysis of this competition for one partic ular model: the pinned Balseiro-Falicov model of competing charge density w ave and (s-wave) superconductivity. We show that most of the anomalous feat ures of both tunneling and photoemission follow naturally from the model, i ncluding the smooth crossover, the general shape of the pseudogap phase dia gram, the shrinking Fermi surface of the pseudogap phase, and the asymmetry of the tunneling gap away from optimal doping. Below T-c, the sharp peak a t Delta(1) and the dip seen in the tunneling and photoemission near 2 Delta (1) cannot be described in detail by this model, but we suggest a simple ge neralization to account for inhomogeneity, which does provide an adequate d escription. We show that it should be possible, with a combination of photo emission sand tunneling, to demonstrate the extent of pinning of the Fermi level to the Van Hove singularity. A preliminary analysis of the data sugge sts pinning in the underdoped, but not in the overdoped regime. [S0163-1829 (99)03325-1].