Pairing correlations above T-c and pseudogaps in underdoped cuprates

We begin with a brief review of experiments on underdoped cuprates which st rongly support the view that the pseudogap is a normal stare precursor of t he superconducting gap due to local, dynamic pairing correlations. We then look at the simplest 2D model system (the attractive Hubbard model) which e xhibits a pseudogap due to pairing, and describe results for the unusual be haviour of various quantities like the spin susceptibility, NMR relaxation rate, momentum distribution and compressibility. We argue that, quite gener ally, pseudogap anomalies arise naturally in the normal slate of 2D superco nductors in a low-density, small pair size regime. The extension of these i deas to d-wave systems and some open problems are briefly described. (C) 19 98 Elsevier Science Ltd. All rights reserved.