A systematic study is carried out to search for fourth sound propagation solid 4He samples below500 mK down to 40 mK between 25 and 56 bar using the techniques of heat pulse generator andtitanium superconducting transition edge bolometer. If solid 4He is endowed with superfluiditybelow 200 mK, as indicated by recent torsional oscillator experiments, theories predict fourthsound propagation in such a supersolid state. If found, fourth sound would provide convincingevidence for superfluidity and a new tool for studying the new phase. The search for a fourthsound-like mode is based on the response of the bolometers to heat pulses traveling through cylindricalsamples of solids grown with different crystal qualities. Bolometers with increasing sensitivityare constructed. The heater generator amplitude is reduced to the sensitivity limit tosearch for any critical velocity effects. The fourth sound velocity is expected to vary as s /.Searches for a signature in the bolometer response with such a characteristic temperature dependenceare made. The measured response signal has not so far revealed any signature of a newpropagating mode within a temperature excursion of 5 K from the background signal shape.Possible reasons for this negative result are discussed. Prior to the fourth sound search, the temperaturedependence of heat pulse propagation was studied as it transformed from “secondsound” in the normal solid 4He to transverse ballistic phonon propagation. Our work extends thestudies of V. Narayanamurti and R. C. Dynes, Phys. Rev. B 12, 1731 1975 to higher pressuresand to lower temperatures. The measured transverse ballistic phonon propagation velocityis found to remain constant within the 0.3% scatter of the data below 100 mK at all pressuresand reveals no indication of an onset of supersolidity. The overall dynamic thermal response ofsolid to heat input is found to depend strongly on the sample preparation procedure.
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