ESR study on spin ladder compound (C5H9NH3)2CuBr4

ESR measurements have been performed on a S=1/2 Heisenberg antiferromagnetic ladder-like system (C5H9NH3)2CuBr4 (abbreviate to (CPA)2CuBr4). Poly crystal samples have been investigated by Millimeter wave Vector Network Analyzer (MVNA, ABmm) in a magnetic field up to 15 T and in the temperature down to 1.5k. At high temperatures, a broad ESR spectrum has been observed. The estimated g-value and linewidth are 2.20 and 0.49 t, respectively in the measurements with 35GHz at 188K. With decreasing temperature below 10K, which corresponds to the broad maximum of the susceptibility, we have observed the damping of the EPR intensity and the development of new absorptions. The resonance frequencies are proportional to B2/3 at low temperatures. The temperature dependence of the linewidth behaves as the one in the breather and spinon excitation mode of Cu-Benzoate