Quantum phase transition by cyclic four-spin exchange interaction for S=1/2 two-leg spin ladder

TL;DR

This study uses the density matrix renormalization group method to identify a quantum phase transition in an S=1/2 two-leg spin ladder induced by cyclic four-spin exchange interactions, revealing a gapless phase at a critical interaction ratio.

Contribution

It demonstrates the existence of a quantum phase transition driven by cyclic four-spin exchange interactions in a two-leg spin ladder, which was not previously characterized.

Findings

Spin gap vanishes at J4/Jleg ≈ 0.3 for J_rung=J_leg.

Correlation length becomes extremely large at the transition point.

String correlation function decays algebraically near the critical point.

Abstract

We investigate an $S=1/2$ two-leg spin ladder with a cyclic four-spin exchange interaction whose interaction constant is denoted by $J_4$, by using the density matrix renormalization group method. The interchain and the intrachain interaction constant are denoted by $J_{rung}$ and $J_{\rm leg}$, respectively and assumed to be antiferromagnetic. It turns out that a spin gap between the singlet ($S_{tot}^z=0$) and the triplet ($S_{tot}^z=1$) states vanishes at $J_4/J_{leg} \simeq 0.3$ for $J_{rung}=J_{leg}$. This result is in contrast with the fact that the $S=1/2$ antiferromagnetic Heisenberg ladder, that is the case of $J_{rung} \neq J_{leg}, J_4=0$, has a spin gap for all nonzero value of interchain interaction $J_{\rm rung}>0$. We find a larger value of the correlation length for the spin-pair correlation function than a linear size $L$ of the system at $J_4/J_{\rm leg}=0.3$ and $J_{rung}=J_{leg}$: the correlation length $\xi$ is about 204 times of the lattice constant for L=84 for these values of interactions. We also find that the string correlation function decays rather algebraically than exponentially at $J_4/J_{leg}=0.3$ and $J_{rung}=J_{leg}$. These results suggest that there is a quantum phase transition at $J_4/J_{leg} \simeq 0.3$ for $J_{rung}=J_{leg}$. We estimate a phase boundary where the spin gap vanishes in a $J_4/J_{leg}-J_{rung}/J_{leg}$ plane and obtain a consistent result with that by a perturbation theory for $J_{\rm rung}/J_{leg}>1$.