Search for Supersolidity in 4He in Low-Frequency Sound Experiments

We present results of the search for supersolid ⁴He using low-frequency, low-level mechanical excitation of a solid sample grown and cooled at fixed volume. We have observed low frequency non-linear resonances that constitute anomalous features. These features, which appear below ∼0.8 K, are absent in ³He. The frequency, the amplitude at which the nonlinearity sets in, and the upper temperature limit of existence of these resonances depend markedly on the sample history.