Dark Matter Detection Using Helium Evaporation and Field Ionization.

We describe a method for dark matter detection based on the evaporation of helium atoms from a cold surface and their subsequent detection using field ionization. When a dark matter particle scatters off a nucleus of the target material, elementary excitations (phonons or rotons) are produced. Excitations which have an energy greater than the binding energy of helium to the surface can result in the evaporation of helium atoms.

Anomalous softening of ⁴He crystals.

We have measured the elasticity of high quality ultrapure ⁴He single crystals in the low temperature region where supersolidity is supposed to occur. At 20 mK, our results are consistent with elastic coefficients previously measured at 1.2 K.

Observation of a new type of electron bubble in superfluid helium.

We report on the observation of a new type of electron bubble in superfluid helium-4. This object appears to be larger than the normal electron bubble and is associated with the presence of quantized vortices in the liquid.

Analysis of the early stage of coalescence of helium drops.

We analyze the growth of the neck that forms between two liquid drops that have come into contact. The analysis is for a fluid in which the velocity of each point on the surface is proportional to the local curvature and directed normal to the interface. For this system, we show that the radius of the neck is proportional to t(1/3), where the time t is measured from the moment at which coalescence commences.

Detection of excited-state electron bubbles in superfluid helium.

We report on experiments in which the pressure oscillation associated with a sound wave is used to explode electron bubbles in liquid helium. Using this technique, we are able to detect the presence of electron bubbles in excited states.