IISc scientists discover two species of few-electron bubbles in superfluid helium

Scientists at the Indian Institute of Science (IISc) have experimentally shown the existence of two species of few electron bubbles (FEBs) in superfluid helium for the first time. There are several phenomena that FEBs can help scientists decipher, such as turbulent flows in superfluids and viscous fluids, or the flow of heat in superfluid helium, a statement by the institute said.

Bengaluru: In a new study conducted by the Indian Institute of Science (IISc), scientists at the institute have experimentally shown the existence of two species of few electron bubbles (FEBs) in superfluid helium for the first time. These FEBs can serve as a useful model to study how the energy states of electrons and interactions between them in a material influence its properties.

The team included Neha Yadav, a former PhD student at the Department of Physics, Prosenjit Sen, Associate Professor at the Centre for Nano Science and Engineering (CeNSE) and Ambarish Ghosh, Professor at CeNSE. The study was published in 'Science Advances'.

An electron injected into a superfluid form of helium creates a single electron bubble (SEB) – a cavity that is free of helium atoms and contains only the electron. The shape of the bubble depends on the energy state of the electron. There are also multiple electron bubbles (MEBs) that contain thousands of electrons.

FEBs, on the other hand, are nanometre-sized cavities in liquid helium containing just a handful of free electrons. The number, state and interactions between free electrons dictate the physical and chemical properties of materials. According to the authors, understanding how FEBs are formed can also provide insights into the self-assembly of soft materials, which can be important for developing next-generation quantum materials. However, scientists have only theoretically predicted the existence of FEBs so far.

“We have now experimentally observed FEBs for the first time and understood how they are created,” Yadav said. “These are nice new objects with great implications if we can create and trap them.”

Yadav and colleagues were studying the stability of MEBs at nanometre sizes when they serendipitously observed FEBs. Initially, they were both elated and sceptical. “It took a large number of experiments before we became sure that these objects were indeed FEBs. Then it was certainly a tremendously exciting moment,” said Ghosh.

There are several phenomena that FEBs can help scientists decipher, such as turbulent flows in superfluids and viscous fluids, or the flow of heat in superfluid helium, a statement by the institute said.

Just like how current flows without resistance in superconducting materials at very low temperatures, superfluid helium also conducts heat efficiently at very low temperatures. But defects in the system, called vortices, can lower its thermal conductivity. Since FEBs are present at the core of such vortices – as the authors have found in this study – they can help in studying how the vortices interact with each other as well as heat flowing through the superfluid helium, the statement added.

“In the immediate future, we would like to know if there are any other species of FEBs, and understand the mechanisms by which some are more stable than the others,” Ghosh said.

“In the long term, we would like to use these FEBs as quantum simulators, for which one needs to develop new types of measurement schemes.”