FERMI-SURFACE EVOLUTION, PSEUDOGAP, AND STAGGERED GAUGE FIELD FLUCTUATIONS IN UNDERDOPED CUPRATES

In the context of the t-J model, we show that besides the long wave le ngth gauge field fluctuation, an additional low energy fluctuation, i. e.? the staggered gauge field fluctuation, Flays a crucial role in the underdoped regime. Including the staggered gauge field fluctuation, w e calculate the spectral function. The line shape near ( pi, 0) is ver y broad in the underdoped case and is quite sharp in the overdoped cas e. The quasiparticle peaks near (0.5 pi, 0.5 pi) are always very sharp in both cases. We also calculate the Fermi surface (FS) crossover fro m a small four-segment-like FS to a large continuous FS. The pseudogap extracted from the photoemission data can be also interpreted by our calculation.