doi:10.1038/nindia.2014.112 Published online 19 August 2014
Electron-trapping bubbles in liquid helium
Researchers have gained new insights into how spherical bubbles form, trap electrons and then disappear on the surface of liquid helium1. These insights into electron-trapping bubbles (also known as multielectron bubbles) will be useful for better understanding various physical phenomena, such as quantum melting and sonoluminescence.
Multielectron bubbles are short lived and hence difficult to study. The researchers generated such bubbles on the surface of liquid helium by applying a few-kilovolt voltage pulse to a sharp tungsten tip placed in a plastic tube above the helium surface. They observed the formation of large bubbles with diameters of a few hundred micrometres at temperatures below 2.17 kelvin.
They found that these large bubbles shrank to sizes below a few micrometres within a few milliseconds and then disappeared, whereas smaller bubbles with diameters between 10 and 100 micrometres survived for longer at temperatures near 2.5 kelvin.
The researchers used a fast camera to image the bubbles and measured their charge and radii. They detected the formation of three types of bubbles — uncharged bubbles, unstably trapped multielectron bubbles and stably trapped multielectron bubbles. They observed that stably trapped bubbles lasted for hundreds of milliseconds and hence could be observed for longer. They found that a stable multielectron bubble with a radius of 0.5 micrometre contained 2,750 electrons.
The researchers say that probing multielectron bubbles can open up new avenues in the study of two-dimensional electron systems that form inside the bubbles. They add that the two-dimensional electron system inside a single multielectron bubble can exhibit large surface density changes under electric fields that are unattainable in any other system.
1. Vadakkumbatt, V. et al
. Studying electrons on curved surfaces by trapping and manipulating multielectron bubbles in liquid helium. Nat. Comm.
5, 4571 (2014) doi: 10.1038/ncomms5571