# Origin of magnetic anisotropy in the spin ladder compound (C$_5$H$_{12}$N)$_2$CuBr$_4$

## TL;DR

This study uses high-resolution neutron scattering to analyze magnetic excitations in the spin ladder compound BPCB, revealing the origin of magnetic anisotropy primarily due to weakly anisotropic leg interactions.

## Contribution

It provides the first detailed experimental and theoretical analysis linking magnetic anisotropy in BPCB to weakly anisotropic leg interactions in the spin ladder.

## Key findings

- Observation of a triplet excitation band with a 0.8 meV spin gap
- Detection of a 50 μeV splitting at the band minimum
- Identification of weakly anisotropic leg interactions as the main source of anisotropy

## Abstract

The $S=1/2$ spin ladder compound (C$_5$H$_{12}$N)$_2$CuBr$_4$ (BPCB) is studied by means of high-resolution inelastic neutron scattering. In agreement with previous studies we find a band of triplet excitations with a spin gap of $\sim0.8$~meV and a bandwidth of $\sim0.6$~meV. In addition, we observe a distinct splitting of the triplet band of $50(1)$~$\mu$eV or $40(2)$~$\mu$eV at the band minimum or maximum, respectively. By comparison to a strong coupling expansion calculation of the triplet dispersion for a spin ladder with anisotropic exchange, weakly anisotropic leg interactions are identified as the dominant source of magnetic anisotropy in BPCB. Based on these results, we discuss the nature of magnetic exchange anisotropy in BPCB and in related transition-metal insulators.