Electron spin resonance in a strong-rung spin-1/2 Heisenberg ladder

Cu(C₈H₆N₂)Cl₂, a strong-rung spin-1/2 Heisenberg ladder compound, is probed by means of electron spin resonance (ESR) spectroscopy in the field-induced gapless phase above H_c1. The temperature dependence of the ESR linewidth is analyzed in the quantum field theory framework, suggesting that the anisotropy of magnetic interactions plays a crucial role, determining the peculiar low-temperature ESR linewidth behavior. In particular, it is argued that the uniform Dzyaloshinskii-Moriya interaction (which is allowed on the bonds along the ladder legs) can be the source of this behavior in Cu(C₈H₆N₂)Cl₂.