IISc scientists find 2 species of electrons in helium

Bengaluru: Scientists at the city-based Indian Institute of Science (IISc) for the first time discovered two species of few electron bubbles in superfluid helium gas, said an official on Monday.

"This is the first time our scientists have shown the existence of two species of few electron bubbles (FEBs) in superfluid helium. These can serve as a model to study how the energy states of electrons and interactions between them influence properties," said the official in a statement here.

The team of scientists, led by Pofessor Ambarish Ghosh at its Centre for Nano Science and Engineering (CeNSE), Assistant Professor Prosenjit Sen and former physics research student Neha Yadav, published their study in 'Science Advances' journal.

"An electron injected into a superfluid form of helium creates a single electron bubble - a cavity free of helium atoms and containing only the electron. The shape of the bubble depends on the energy state of the electron," the study said.

FEBs are nano-metre-sized cavities in liquid helium containing a few free electrons. The number, state and interactions between them dictate the physical and chemical properties of materials.

Studying FEBs will help scientists understand how properties emerge when a few electrons in a material interact with each other. Understanding how FEBs are formed can provide insights into the assembly of soft materials, which can be used to develop next-generation quantum materials.

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

The team applied a voltage pulse to a tungsten tip on the surface of liquid helium. They generated a pressure wave on the charged surface, using an ultrasonic transducer. This allowed them to create 8EBs (electron bubbles) and 6EBs, two species of FEBs containing 8 and 6 electrons.

"FEBs form an interesting system that has electron-electron interaction and electron-surface interaction," said Yadav in the study. FEBs can also help scientists decipher turbulent flows in super-fluids and viscous fluids, or the flow of heat in superfluid helium.

"Like how current flows without resistance in superconducting materials at very low temperatures, superfluid helium also conducts heat efficiently at very low temperatures," the study pointed out.

Defects in the system, called vortices, however, can lower its thermal conductivity. As FEBs are present at the core of such vortices, they can help in studying how vortices interact with each other.

"In the near future, we would like to know if there are other species of FEBs and understand their mechanisms by which some are more stable than others," added Ghosh. In the long-term, the team plans to use FEBs as quantum simulators and develop new measurement schemes.

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