Piezoelectric generation of longitudinal waves in strontium titanate at cryogenic temperatures

Thin single-crystal plates of strontium titanate (STO) have been used as non-resonant surface excited piezoelectric transducers to launch longitudinal ultrasonic waves into rutile crystals at low-temperatures for frequencies as high as 500 MHz. In contrast to the recently discovered giant static piezoelectric effect which shows a strong increase on the piezoelectric strain constant d₃₁₁ in the quantum paraelectric regime of STO (T⩽10 K), the ultrasonic electromechanical conversion efficiency at 4 K for longitudinal waves is comparable to that of quartz and it is temperature independent from 1.3 to 30 K. High frequency ultrasonic echoes remain detectable up to 90 K. Preliminary studies at 30 MHz by means of quartz resonant transducers of the low-temperature propagation of ultrasonic longitudinal and shear waves in bulk STO samples show temperature anomalies in the effective elastic constants below 30 K