An 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 which respect a SU(2) symmetry -- a symmetry previously known to be important at half filling. The mean field phase diagram is found to be consistent with the phases observed in the cuprate superconductors, which contains d-wave superconductor, spin gap, strange metal, and Fermi liquid phases. The spin gap phase is best understood as the staggered flux phase, which is nevertheless translationally invariant for physical quantities. The physical electron spectral function shows small Fermi segments at low doping which continuously evolve into the large Fermi surface at high doping concentrations. The close relation between the SU(2) and the U(1) slave-boson theory is discussed. The low energy effective theory for the low lying fluctuations is derived, and new lying modes (which were over looked in the U(1) theory) are identified.

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