Pseudogap and symmetry of superconducting order parameter in cuprates

The phase diagram, nature of the normal state pseudogap, type of the Fermi surface, and behavior of the superconducting gap in various cuprates are di scussed in terms of a correlated state with valence bonds. The variational correlated state, which is a band analogue of the Anderson (RVB) states, is constructed using local unitary transformations. Formation of valence bond s causes attraction between holes in the d-channel and corresponding superc onductivity compatible with antiferromagnetic spin order. Our calculations indicate that there is a fairly wide range of doping with antiferromagnetic order in isolated CuO2 planes. The shape of the Fermi surface and phase tr ansition curve are sensitive to the value and sign of the hopping interacti on t' between diagonal neighboring sites. In underdoped samples, the dielec trization of various sections of the Fermi boundary, depending on the sign of t', gives rise to a pseudogap detected in photoemission spectra for vari ous quasimomentum directions. In particular, in bismuth- and yttrium-based ceramics (t'>0), the transition from the normal state of overdoped samples to the pseudogap state of underdoped samples corresponds to the onset of di electrization on the Brillouin zone boundary near k = (0, pi) and transitio n from "large'' to "small'' Fermi surfaces. The hypothesis about s-wave sup erconductivity of La- and Nd-based ceramics has been revised: a situation i s predicted when, notwithstanding the d-wave symmetry of the superconductin g order parameter, the excitation energy on the Fermi surface does not vani sh at all points of the phase space owing to the dielectrization of the Fer mi boundary at k(x) = +/- k(y). The model with orthorhombic distortions and two peaks on the curve of T-c versus doping is discussed in connection wit h experimental data for the yttrium-based ceramic. (C) 1999 American Instit ute of Physics. [S1063-7761(99)01902-2].