Abstract
We describe a cw method for simultaneous ultrasonic phase velocity and attenuation measurements. This method uses a phase locked loop technique to lock the carrier frequency of a frequency modulated drive to the center of the mechanical resonance of the composite oscillator consisting of the sample and the transducers. At the same time, by measuring the rf amplitude modulation index at twice the modulation frequency, the change in the resonator Q factor, and therefore the attenuation, can also be measured. We have applied this method to measure the sound velocity and attenuation of a heavy fermion superconductor UPt3 down to 5 mK. With only 5 nW of rf excitation, we could detect changes in the phase velocity as small as Δv/v∼5×10−9.
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