Non-BCS superconductivity for underdoped cuprates by spin-vortex attraction

Abstract

Within a gauge approach to the t-J model, we propose a new, non-BCS mechanism of superconductivity for underdoped cuprates. The gluing force of the superconducting mechanism is an attraction between spin vortices on two different N\'eel sublattices, centered around the empty sites described in terms of fermionic holons. The spin fluctuations are described by bosonic spinons with a gap generated by the spin vortices. Due to the no-double occupation constraint, there is a gauge attraction between holon and spinon binding them into a physical hole. Through gauge interaction the spin vortex attraction induces the formation of spin-singlet (RVB) spin pairs with a owering of the spinon gap. Lowering the temperature the approach exhibits two crossover temperatures: at the higher crossover a finite density of incoherent holon pairs are formed leading to a reduction of the hole spectral weight, at the lower crossover also a finite density of incoherent spinon RVB pairs are formed, giving rise to a gas of incoherent preformed hole pairs, and magnetic vortices appear in the plasma phase. Finally, at a even lower temperature the hole pairs become coherent, the magnetic vortices become dilute and superconductivity appears. The superconducting mechanism is not of BCS-type since it involves a gain in kinetic energy (for spinons) coming from the spin interactions

Gauge approach to the ‘pseudogap’ phenomenology of the spectral weight in highTccuprates

“…For 2D Coulomb gases with the above parameters a pairing develops for a temperature T ph ≈ J/2π , which turns out to appear in the SM. As it occurs for non-weakly coupled attractive Fermi systems, below T ph there is a temperature at which the pairs condense, and it will turn out to be the superconducting transition temperature T c , see [2]; in-between there is a crossover, T ps , due to spinons, but it will not be discussed here. This omission affects only the lower temperature region for the hole Green function discussed in this paper; in principle the approach is able to derive this correction, but it would highly complicate the calculations without affecting most of the main features.…”

“…In this section we discuss in detail the pairing among holons in the SM region of the t-t -J model in the BCS approximation, adapting the framework developed in [2], in turn inspired by [18]. A brief comment on the modifications needed for the PG region are added at the end.…”

“…However, since T ph > T * , when we enter in the PG region from the SM the pairing is already active and this implies that the spectral weight is already strongly peaked near the node in the MBZ. (2) The interval of energy where the attraction is effective between spinon and holon mediated by the slave-particle gauge field is much shorter in the PG than in the SM. This region is in fact triggered by the Reizer momentum, which in the PG is approximately δ 2/3 times smaller than the one in the SM.…”

“…In this paper we develop an 'explanation' of the Fermi arcs within a generalization of a gauge approach to superconductivity in cuprates recently proposed [2], comparing the results with ARPES and tunnelling data. To understand our proposal it is useful first to sketch the pairing mechanism, eventually leading to superconductivity, for underdoped cuprates presented in [2] within a gauge approach to the t-J model: as we dope, a vortex-like quantum distortion of the AF background is generated around the empty sites (described in terms of fermionic spinless holons) with opposite chirality for cores on the two Néel sublattices.…”

“…As a consequence of this attraction at a crossover temperature, denoted T ph , a finite density of incoherent holon pairs are formed. In [2] it was claimed that this phenomenon produces a reduction of the hole spectral weight on the Fermi surface (FS) and it was proposed to identify this temperature with the experimentally observed 'upper pseudogap', where the in-plane resistivity deviates from the linear behaviour. At a lower crossover temperature, denoted T ps , also a finite density of incoherent spinon pairs appears, giving rise to a gas of incoherent preformed hole pairs through holon-spinon gauge attraction.…”