Bipolaronic charge excitations in t-J two-leg ladders

TL;DR

This paper develops a low-energy effective model for charge excitations in two-leg t-J ladders, revealing bipolaronic behavior and predicting correlation exponents and superconductivity features at low doping.

Contribution

It introduces a simplified effective theory based on mean-field and Luther-Emery liquid concepts, focusing on charge dynamics and excluding spin degrees of freedom.

Findings

Charge excitations behave as bipolaronic entities.

The effective model predicts specific correlation exponents.

Superconductivity order parameter is analyzed within the model.

Abstract

We present a low-energy effective model for the charge degrees of freedom in two-leg t-J ladders. Starting from SU(2) mean-field theory, we exclude the spin degrees of freedom which have an energy gap. At low temperatures, the mean-field solution is the staggered-flux phase. For gapless charge excitations the effective theory is the Luther-Emery liquid. Our analysis is applicable at low doping and in the ``physical'' range of parameters $t/J\sim 3$ where there is only one massless mode in the charge sector and no massless modes in the spin sector. Within our model we make predictions about correlation exponents and the superconductivity order parameter, and discuss the comparison with the existing numerical results.