Search for fourth sound propagation in supersolid He4

Abstract

A systematic study is carried out to search for fourth sound propagation solid He4 samples below 500mK down to 40mK between 25 and 56bar using the techniques of heat pulse generator and titanium superconducting transition edge bolometer. If solid He4 is endowed with superfluidity below 200mK, as indicated by recent torsional oscillator experiments, theories predict fourth sound propagation in such a supersolid state. If found, fourth sound would provide convincing evidence for superfluidity and a new tool for studying the new phase. The search for a fourth sound-like mode is based on the response of the bolometers to heat pulses traveling through cylindrical samples of solids grown with different crystal qualities. Bolometers with increasing sensitivity are constructed. The heater generator amplitude is reduced to the sensitivity limit to search for any critical velocity effects. The fourth sound velocity is expected to vary as ∝ρs∕ρ. Searches for a signature in the bolometer response with such a characteristic temperature dependence are made. The measured response signal has not so far revealed any signature of a new propagating mode within a temperature excursion of 5μK from the background signal shape. Possible reasons for this negative result are discussed. Prior to the fourth sound search, the temperature dependence of heat pulse propagation was studied as it transformed from “second sound” in the normal solid He4 to transverse ballistic phonon propagation. Our work extends the studies of [V. Narayanamurti and R. C. Dynes, Phys. Rev. B 12, 1731 (1975)] to higher pressures and to lower temperatures. The measured transverse ballistic phonon propagation velocity is found to remain constant (within the 0.3% scatter of the data) below 100mK at all pressures and reveals no indication of an onset of supersolidity. The overall dynamic thermal response of solid to heat input is found to depend strongly on the sample preparation procedure.