Electron spin resonance study of Cu benzoate using a He-3-He-4 dilution refrigerator

An electron spin resonance (ESR) measurement system for use at ultralow tem peratures using a He-3-He-4 dilution refrigerator has been developed. As th e first experiment. the ESR was measured on a quantum spin chain Cu benzoat e clown to 160 ml. in which a field-induced gap was recently found.((4)) Th e evaluation of a new ESR mode. the so-called breather mode, was found at t he lowest temperatures. In the previous ESR down to 0.5 K, this mode was cl early found for H // c. in which the magnitude of the gap is largest. In th e present work. the achievement of low temperatures enabled us to observe a smaller gap in H // b by suppressing the thermal excitation. The magnitude of the field-induced gap evaluated from the nonlinear field dependence of the mode shows good agreement with that determined by previous specific hea t measurements.