An electron entering liquid helium experiences a repulsive potential of 1 eV. This originates in the interaction between the injected electron and electrons of the closed shells of helium atoms through Pauli exclusion principle. If the energy of the electron exceeds 1 eV, it can penetrate the liquid, form a cavity free from helium atoms and subsequently localize itself within the cavity. This is known as a single electron bubble (SEB). In another configuration, a system of electrons of energy less than 1 eV can form a floating charged layer above the surface of liquid helium: a two-dimensional system that has been studied in great detail over last few decades. If the number of electrons in this layer exceeds a critical value of 2×〖10〗^13 electrons〖/m〗^2, an electro hydrodynamical instability sets in, giving rise to multielectron bubbles (MEBs), referring to micron to mm sized cavities containing a large number of electrons.
In the experiments to be discussed in my talk, the primary technique is based on cavitation of liquid helium using pulsed ultrasound. After a brief introduction of the experimental technique, I will present the main results obtained during my Ph.D., as follows:
First, we have observed a new species of electron bubble which cavitates at a negative pressure approximately 80 percent lower magnitude than SEBs. We conclude that these are multielectron bubbles with small (<20) number of electrons; I’ll be presenting various evidences supporting this claim and compare our results with related experiments previously reported. The second result is related to cavitation in superfluid helium in a general manner. We have observed that after cavitation, the bubble is pushed out of the acoustic focus and can grows up to a size as large as a millimetre. The growth and collapse of these bubbles can be understood through Rayleigh-Plesset equation at low temperatures, but this description fails near the lambda transition. We suspect this is related to the large density of vortices nucleated near the bubble surface during the growth and collapse of the cavitating bubble. Lastly, we have shown how a charged helium surface can be rendered unstable using ultrasound, such as to create MEBs with high charge density.
Host: Jack Harris (jack.harris@yale.edu)