Anisotropy of magnetic interactions in the spin-ladder compound<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mtext>C</mml:mtext><mml:mn>5</mml:mn></mml:msub><mml:msub><mml:mtext>H</mml:mtext><mml:mrow><mml:mn>12</mml:mn></mml:mrow></mml:msub><mml:mtext>N</mml:mtext></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mtext>CuBr</mml:mtext></mml:mrow><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>

Magnetic excitations in the spin-ladder material ${({\text{C}}_{5}{\text{H}}_{12}\text{N})}_{2}{\text{CuBr}}_{4}$ (BPCB) are probed by high-resolution multifrequency electron spin resonance (ESR) spectroscopy. Our experiments provide a direct evidence for a pronounced anisotropy ($\ensuremath{\sim}5\mathrm{%}$ 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, ${\ensuremath{\Delta}}_{0}/{k}_{B}=16.5\text{ }\text{K}$, is detected directly. Furthermore, an ESR signature of the interladder 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.

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