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Electron bubbles in liquid helium

Electrons near the surface of liquid helium are attracted towards the liquid-vapor interface due to the image charge induced in the liquid, but face a repulsive potential of approximately 1 eV at atomic distances. Accordingly, it is possible to produce a two-dimensional layer of electrons trapped above the surface of liquid helium, which has been the subject of much theoretical and experimental investigation in the last few decades. When the liquid surface is charged beyond a critical value, the surface can become mechanically unstable resulting in the formation of micron sized (1–1000 μm) bubbles containing 106 to 109 electrons.These objects, known as multielectron bubbles (MEBs) form a rich 2D Coulomb system on curved surfaces, and offer exciting possibilities towards the study of quantum phase transitions. Unfortunately, there have been limited experimental efforts towards the study of MEBs, and as of now there are unresolved questions regarding their stability and hydrodynamic properties. Our current experiments are aimed towards understanding the hydrodynamic behaviour of MEBs, and to investigate the various factors that affect their stability.

Apart from multielectron bubbles, it is also possible to have a nanometre sized bubble in liquid helium containing a single electron, which is one of the most fundamental representations of a “particle in a box”, and is an excellent probe to study novel phenomena in superfluid helium, including the nature of quantized vortices and quasiparticles. In recent years, the usage of a novel acousto-optic technique has facilitated the detection of single electron bubbles, resulting in the visualization of single vortex lines in superfluid helium. Our current experiments are aimed towards understating the optical properties of this novel quantum system.