Pressure gradients in solid 4He: Thermal quenching and annealing

Torsional oscillator measurements on solid He-4 show a frequency increase at low temperatures that suggests mass decoupling or nonclassical rotational inertia (NCRI). The magnitude of the NCRI appears to be larger when the helium is frozen and cooled rapidly. Annealing at high temperatures usually reduces the NCRI, with an accompanying drop in pressure, suggesting that defects are involved. Measurements in quenched or deformed crystals show a T-2 term in the temperature dependence of the pressure which has also been attributed to defects. We have built a cell with two capacitive gauges to measure the temperature dependence of the pressure and the magnitude of pressure gradients in solid helium. The helium can be melted in a few seconds using a heater embedded in the crystal and can be refrozen and quenched to low temperature very rapidly. From the maximum pressure differences in the cell, we infer a yield stress of order 4 kPa for solid He-4 near melting. The pressure gradients relax when the temperature is raised above about 0.5 K via a thermally activated annealing process with an activation barrier of about 5 K. The existence of significant pressure gradients makes it difficult to measure the thermodynamic temperature dependence of the pressure, for example to extract information about glassy regions or other defects.