Anisotropy of Magnetic Interactions in the Spin-Ladder Compound (C$_5$H$_{12}$N)$_2$CuBr$_4$

TL;DR

This study uses high-resolution ESR spectroscopy to reveal a significant biaxial anisotropy in the magnetic interactions of the spin-ladder compound (C$_5$H$_{12}$N)$_2$CuBr$_4$, highlighting the role of spin-orbit coupling.

Contribution

It provides the first direct evidence of anisotropy in BPCB, challenging the previous isotropic model and advancing understanding of magnetic properties in spin-ladder systems.

Findings

Detected a ~5% biaxial anisotropy in magnetic interactions.

Measured the zero-field excitation gap as 16.5 K.

Identified ESR signatures of inter-ladder exchange interactions.

Abstract

Magnetic excitations in the spin-ladder material (C$_5$H$_{12}$N)$_2$CuBr$_4$ [BPCB] are probed by high-resolution multi-frequency electron spin resonance (ESR) spectroscopy. Our experiments provide a direct evidence for a biaxial anisotropy ($\sim 5\%$ of the dominant exchange interaction), that is in contrast to a fully isotropic spin-ladder model employed for this system previously. It is argued that this anisotropy in BPCB is caused by spin-orbit coupling, which appears to be important for describing magnetic properties of this compound. The zero-field zone-center gap in the excitation spectrum of BPCB, $\Delta_0/k_{B}=16.5$ K, is detected directly. Furthermore, an ESR signature of the inter-ladder exchange interactions is obtained. The detailed characterization of the anisotropy in BPCB completes the determination of the full spin hamiltonian of this exceptional spin-ladder material and shows ways to study anisotropy effects in spin ladders.