The temperature of phonon pulses injected into liquid4He

Abstract

The characteristic temperature of phonons injected into liquid ⁴He at 0.1K and at 24 bar can be measured using the known dispersion of phonons. However, as this temperature is low, the dispersion is small and it is necessary to use a fast detector with a time constant less than the dispersion time. A new bolometer system has been developed which not only has a time constant of about 0.5 mu s but also measures the actual power absorbed. This bolometer uses the superconducting transition of a Zn film in a thermal feedback circuit which maintains the detecting element at constant temperature. The phonon pulses are injected into the liquid ⁴He by a thin metal film heater on a sapphire substrate. The temperature of the heater film during the current pulse is calculated taking into account for the first time the frequency dependence of the phonon transmission coefficient. The calculated temperatures agree very well with those measured and the model accounts for the break in the slope in the graph of log T against log heater power. The characteristic temperature of the ballistic phonon pulses in the liquid ⁴He is found to be very much lower than the heater temperature and lower than the values previously reported.