ESR Measurements at Ultra-Low Temperatures Using a Dilution Refrigerator

Abstract

An ESR measurement system with a high sensitivity vector network analyzer has been developed for use at ultralow temperatures below 1 K. The microwave frequency can be varied between 45 GHz and 110 GHz. The resonant cavity was installed in a bottom of a 3 He- 4 He dilution refrigerator, which realizes the temperatures down to 160 mK. In order to avoid a heating of the cavity and sample by microwaves or heat leaks, attenuators and thermal anchors were placed at suitable positions. 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. The evaluation of a new ESR mode, 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 of the thermal excitation.