Pulsed Fourier-Transform ultrasonic spectroscopy for ultralow temperature applications

Pulsed Fourier-Transform (FT) ultrasonic spectroscopy has been developed for the purpose of studying super(3)He at ultralow temperatures. Our method is analogous to pulsed FT NMR and possesses several advantages over conventional time-of-flight acoustic techniques. For use at low temperatures, the most significant advantage is the ability to observe the frequency spectrum while the temperature, pressure, and magnetic field may be independently tuned. More specifically, using longitudinal LiNbO sub(3) transducers operating both on- and off-resonance, we are able to study several broadband windows of frequency, namely 16-25 MHz, 60-70 MHz, and 105-111 MHz. The determination of the energy gap of super(3)He-B is presented as an effective application, and other novel uses are discussed.