Capacitive Generation and Detection of Third Sound Resonances in Saturated Superfluid ⁴He Films

We report on the operation of a third sound resonator for use with saturated superfluid helium films. Measurements of superfluid ⁴He third sound velocities as a function of film thickness are described for films ranging in thickness from 26 nm to 85 nm. The film resides on the gold-covered surface of a 38 mm diameter copper disk, which has been machined with a diamond-tipped cutting tool, and has an rms roughness of about 10 nm.