The two-leg t-J ladder: A mean-field description

TL;DR

This paper develops a mean-field theory for two-leg t-J ladders in the strong rung interaction regime, unifying insulating and doped phases, and revealing superconductivity driven by RVB correlations with detailed spectral analysis.

Contribution

It introduces a mean-field approach combining phase string and bond-operator methods to describe the phase diagram and properties of two-leg t-J ladders.

Findings

Ground state is a superconductor with d-wave symmetry driven by RVB correlations.

Phase separation occurs beyond a critical J/t ratio at fixed doping.

The spin gap in the doped phase is linked to quasi-particle excitations.

Abstract

Two-leg t-J ladders are investigated in the framework of a combination of the phase string formulation and bond-operator representation. We develope a mean-field theory in the strong rung interaction regime, i.e. $J_{\perp}\gg J, t$, which provides a unified description of the undoped insulating phase and the low doping phase --- the so-called C1S0 phase. Both of them are characterized by the resonating-valence-bond (RVB) order parameter, with gap opened up in all spin excitations. The ground state of the doped phase is intrinsically a superconductor with a d-wave symmetry, which is driven by the RVB correlations. The ground-state energy is in good agreement with numerical results. Phase separation is shown to occur beyond some critical value of J/t for given doping concentration. We also show that the spin gap in the doped phase is determined by quasi-particle-like excitations. The local structure of hole pairs as well as the spectra of various spin and charge modes are analyzed in comparison with other approaches.