Observation of anisotropic exchange in a spin ladder by ESR

We report on high resolution X-band electron spin resonance (ESR) spectroscopy studies of the spinladder material (C₅H₁₂N)₂CuBr₄. Our experiments provide a direct evidence for the presence of anisotropy in (C₅H₁₂N)₂CuBr₄ in contrast to a fully isotropic spin-ladder model employed for this system previously. Low-temperature angular dependence of ESR transitions is analyzed employing a simple spin-1/2 dimer model with the symmetric anisotropic exchange interaction.