Spin-Charge Binding Mechanism for Superconductivity in Cuprates

TL;DR

This paper explores a spin-charge recombination mechanism in cuprates, showing how it leads to d-wave superconductivity and explains key experimental features through the symmetry of spinon pairs and quantum spin fluctuations.

Contribution

It demonstrates that the d-wave symmetry and T_c doping dependence naturally emerge from the spinon pair symmetry in a Schwinger boson t-J model framework.

Findings

Recombination destroys magnetic order and restores Fermi surface.

Spinon pairing induces d-wave superconductivity.

Superconducting symmetry linked to quantum spin fluctuations.

Abstract

A spin-charge recombination route to superconductivity, proposed earlier (1992)], is examined using the Schwinger boson representation of the t-J model. The representation is known to work well at half-filling. It was shown in the earlier paper that, away from half filling, spin-charge recombination into electrons occurs directly through the hopping term as the system tries to recover the kinetic energy lost in forming the "spins" (magnetic moments). Recombination is strong enough in 2-D to (1) destroys long-range magnetic order and (2) restores the large Fermi surface. But the normal state remains incoherent, characterize by a Fermi liquid of holons and bosonic spinons which are paired into singlets. The singlets evolve out of half-filling and are responsible for the spin-gap behavior. They induce a pairing of holons and bind with the latter (to recover additional kinetic energy). In this paper we show that $d_{x^2-y^2}$ symmetry of the order parameter and the shape of the T_c vs doping curve, observed in cuprate superconductors, emerge naturally as a consequence of the symmetry of the spinon pairs. The latter symmetry (p-wave), however, is already determined at half-filling. Thus the nature of the superconducting state is intimately connected with that of quantum spin fluctuations in the insulator!