Increasing superconducting Tc’s by a factor of 1000 with large hopping anisotropies in two-dimensional t−J model systems

Abstract

We have studied the enhancement of the superconducting transition temperature Tc in a t−J−U model of electrons moving on a square lattice in which anisotropic electronic hopping is introduced. For this model we have calculated Tc for singlet pairing using the non-self-consistent Thouless criterion, and find a dramatic enhancement of Tc induced by large hopping anisotropies. Further, the maximum increase in Tc is obtained when the system is pushed towards the extreme anisotropy limit, that is, when the hopping of electrons is confined to occur in only one dimension, while superexchange couples the electrons both parallel and perpendicular to the direction in which hopping occurs. We demonstrate that in this limit the increase in Tc, with respect to the isotropic system, can be of the order of 1000. We have also determined that in the extreme anisotropy limit the superconducting gap is an equal mixture of s and d pairing symmetries (two choices of such a combination being s+d and s+id) owing to the reduced (square to rectangular) symmetry of the system in the presence of hopping anisotropies. Thus the presence of d-wave superconducting features in materials whose symmetry is very different from that of a two-dimensional square lattice is not unexpected.

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