Magnetic ordering induced by interladder coupling in the spin-1/2 Heisenberg two-leg ladder antiferromagnet C$_9$H$_{18}$N$_2$CuBr$_4$

TL;DR

This study investigates a quantum antiferromagnet with coupled spin ladders, revealing that interladder interactions are close to inducing magnetic order without external fields, supported by specific heat and neutron scattering data.

Contribution

The paper provides detailed experimental characterization of a spin-1/2 ladder antiferromagnet, highlighting the critical interladder coupling near the threshold for magnetic ordering, which is rare among similar materials.

Findings

Magnetic ordering occurs at T_N=1.99 K.

Interladder coupling is close to the critical value for Ne9el order.

Energy gap of 0.30 meV due to spin anisotropy.

Abstract

We present specific-heat and neutron-scattering results for the \emph{S}=1/2 quantum antiferromagnet (dimethylammonium)(3,5-dimethylpyridinium)CuBr$_4$. The material orders magnetically at \emph{T}$_N$=1.99(2)\,K, and magnetic excitations are accompanied by an energy gap of 0.30(2) meV due to spin anisotropy. The system is best described as coupled two-leg spin-1/2 ladders with the leg exchange $J_{\rm leg}$=0.60(2)~meV, rung exchange $J_{\rm rung}$=0.64(9)~meV, interladder exchange $J_{\rm int}$=0.19(2)~meV, and an interaction-anisotropy parameter $\lambda$=0.93(2), according to inelastic neutron-scattering measurements. In contrast to most spin ladders reported to date, the material is a rare example in which the interladder coupling is very near the critical value required to drive the system to a N\'eel-ordered phase without an assistance of a magnetic field.