SU(2) FORMULATION OF THE T-J MODEL - APPLICATION TO UNDERDOPED CUPRATES

We develop a slave-boson theory for the t-J model at finite doping tha t respects an SU(2) symmetry: a symmetry previously known to be import ant at half filling. The mean-field phase diagram is found to be consi stent with the phases observed in the cuprate superconductors, which c ontain d-wave superconductor, spin-gap, strange metal, and Fermi-liqui d phases. The spin-gap phase is best understood as the staggered flux phase, which is nevertheless translationally invariant for physical qu antities. The physical electron spectral function shows small Fermi se gments at low doping that continuously evolve into the large Fermi sur face at high-doping concentrations. The close relation between the SU( 2) and the U(1) slave-boson theory is discussed. The low-energy effect ive theory for the low-lying fluctuations is derived and additional ly ing modes [which were overlooked in the U(1) theory] are identified.