ESR Study of the Spin-Ladder Compound (C5H12N)2CuBr4

A systematic multi-frequency electron spin resonance (ESR) study of the spin-ladder material (C5H12N)2CuBr4 (known as BPCB) and its deuterated analog have been performed at temperatures down to 1.3 K in magnetic fields up to 16 T. The energy gap in the magnetic excitation spectrum at k = 0, Delta/kB = 16.45 K, was observed directly, confirming the existence of Dzyaloshinskii-Moriya interaction in this compound. A small (» 140 mK) splitting between the levels of the excited spin-triplet confirmed the presence of a weak interladder coupling. The temperature evolution of the ESR excitation spectrum in BPCB revealed its very unusual behavior, whose origin is discussed.