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

An electron spin resonance (ESR) measurement system for use at ultralow temperatures using a ³He-⁴He dilution refrigerator has been developed. As the first experiment, the ESR was measured on a quantum spin chain Cu benzoate down to 160 mK, in which a field-induced gap was recently found.⁴⁾ The evaluation of a new ESR mode, the so-called breather mode, was found at the lowest temperatures. In the previous ESR down to 0.5 K, this mode was clearly found for H ∥ c, in which the magnitude of the gap is largest. In the 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 heat measurements.