Pseudogaps in underdoped cuprates

Abstract

It has become clear in the past several years that the cuprates show many unusual properties, both in the normal and superconducting (SC) states, especially in the underdoped region. In particular, gap-like behavior is observed in magnetic properties, c-axis conductivity and photoemission, whereas in-plane transport properties are only slightly affected by the pseudogap. I shall argue that these experimental evidences must be viewed in the context of the physics of a doped Mott insulator and that they support the notion of spin charge separation. I shall review recent theoretical developments, concentrating on studies based on the t– J model. I shall describe a model based on quasiparticle (qp) excitations, which predicts the doping dependence of T c and anomalous energy-gap-to- T c ratios. Finally, I shall outline how the model may be derived from a microscopic formulation of the t– J model. After a brief review of the U(1) formulation, I shall explain some of the difficulties encountered there, and how a new SU(2) formulation can resolve some of the difficulties.