Search for an ac Josephson effect in superfluidHe4using a low-frequency acoustic resonator

A search for a Josephson effect in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ has been carried out with use of a low-frequency acoustic resonator in analogy to an rf-biased superconducting quantum interference device. Orifices from 100 to 400 nm in diameter in free-standing foils ∼100 nm thick were used as weak links. Observations were made at temperatures from 0.4 to 1.1 K and at pressures from 13 to 40 kPa. Aside from the suggestive behavior seen in one early run, no sign of a Josephson effect has been observed. Values of the order-parameter phase difference through the weak link at the critical rate of flow ranged from 9×2π to 33×2π rad at T=0.95 K.