Magnetization plateau in the S=1/2 spin ladder with alternating rung exchange

TL;DR

This paper investigates the ground state phase diagram of an S=1/2 spin ladder with alternating rung exchange, revealing a magnetization plateau at half saturation and analyzing critical fields and phase transitions.

Contribution

It introduces a mapping of the spin ladder with alternating rung exchange to an XXZ chain, and characterizes the magnetization plateau and critical behavior with novel scaling laws.

Findings

Magnetization plateau at half saturation observed.

Plateau width scales as δ^ν with ν=4/5 or 2.

Transitions are of the commensurate-incommensurate type.

Abstract

We have studied the ground state phase diagram of a spin ladder with alternating rung exchange $J^{n}_{\perp} = J_{\perp}[1 + (-1)^{n} \delta ]$ in a magnetic filed, in the limit where the rung coupling is dominant. In this limit the model is mapped onto an $XXZ$ Heisenberg chain in a uniform and staggered longitudinal magnetic fields, where the amplitude of the staggered field is $\sim \delta$. We have shown that the magnetization curve of the system exhibits a plateau at magnetization equal to the half of the saturation value. The width of a plateau scales as $\delta^{\nu}$, where $\nu =4/5$ in the case of ladder with isotropic antiferromagnetic legs and $\nu =2$ in the case of ladder with isotropic ferromagnetic legs. We have calculated four critical fields ($H^{\pm}_{c1}$ and $H^{\pm}_{c2}$) corresponding to transitions between different magnetic phases of the system. We have shown that these transitions belong to the universality class of the commensurate-incommensurate transition.