Observation of direct transition in S = (1)/(2) quantum spin system Cu-2(C5N12N2)(2)Cl-4 by high-field ESR

Cu-2(C5H12N2)(2)Cl-4 (abbreviated as CHpC) is anew spin-gap system with a g ap of about 10 K, which is proposed to be an S = 1/2 ladder system. We perf ormed the high-field ESR measurements down to 0.5 K using the He-3 cryostat and the Vector Network Analyzer System in IMR. The measurements have been performed in V and W bands using the superconducting magnet up to 14 T. and we have observed the direct transition of CHpC below 0.9 K for the first t ime. Our results are discussed in connection with our previous high-field E SR results above 1.8 K using the pulsed magnetic field. (C) 2001 Elsevier S cience B.V. All rights reserved.