Gauge-Theory Approach to Planar Doped Antiferromagnets and External Magnetic Fields

Within the framework of a relativistic non-Abelian gauge theory approach to the physics of spin-charge separation in doped quantum antiferromagnetic planar systems, proposed recently by the authors, we are examining here the effects of constant external magnetic fields on excitations about the superconducting state in the model. The electrically-charged Dirac fermions (holons), describing excitations about specific points on the fermi surface, e.g. those corresponding to the nodes of a d-wave superconducting gap in high-T_c cuprates, condense, resulting in the opening of a Kosterlitz-Thouless-like gap (KT) at such nodes. This leads, in general, to a second superconducting phase transition, which occurs at low temperatures (T c ∼ { O} (0.1)\ K), in addition to the high-T_c superconductivity (T c ∼ { O} (100)\ K) due to the bulk of the fermi surface for holons in a (d-wave) spin-charge separated superconductor. In the presence of strong external magnetic fields at the surface regions of the planar superconductor, in the direction perpendicular to the superconducting planes, these KT gaps appear to be enhanced. Our preliminary analysis, based on analytic Schwinger-Dyson treatments, seems to indicate that for an even number of Dirac fermion species, required in our model as a result of gauging a particle-hole SU(2) symmetry, Parity or Time Reversal violation does not necessarily occurs. Based on these considerations, we argue that recent experimental findings, concerning thermal conductivity plateaux of quasiparticles in planar high-T_c cuprates in strong external magnetic fields, may indicate the presence of such KT gaps, caused by charged Dirac-fermion excitations in these materials, as suggested in the above model.