Orbital currents and cheap vortices in underdoped cuprates

In the past several years, we have developed a theory for the underdoped cuprates in collaboration with Wen, based on an SU(2) formulation of the t-J model. In this formulation, the staggered flux state plays a central role as the progenitor of the Neel state at half-filling and a close competitor to the d-wave superconductor with small doping where it is characterized by staggered orbital currents. We found support for this point of view when we discovered fluctuating orbital currents in the Gutzwiller projected BCS wave function. We shall argue that low-energy vortices, where the staggered flux state is stabilized in the core, are needed to explain many of the unusual properties of underdoped cuprates. Proposed experiments to look for these orbital currents will be discussed. (C) 2003 Elsevier Science B.V. All rights reserved.