Scientists have discovered two types of low-electron bubbles in superfluid helium

FEB can serve as a useful model for studying how the energy states of electrons and the interactions between them in a material affect its properties.

In a new study, scientists from the Indian Institute of Science (IISc) have experimentally demonstrated the existence of two types of electron bubbles (FEB) in superfluid helium for the first time. FEBs can serve as useful models for studying how the energy states of electrons and the interactions between them in a material affect their properties.

An electron is injected into the superfluid helium form which creates a single electron bubble (SEB) – a cavity free of helium atoms and containing only electrons. The shape of the bubble depends on the energy state of the electron.

For example, the bubble is spherical when the electron is in the ground state (1S). There are also MEBs – multiple electron bubbles containing thousands of electrons, according to the IISc statement.

On the other hand, FEB is a nanometer-sized cavity in liquid helium that contains only a few free electrons. The number, state, and interactions between free electrons determine the physical and chemical properties of materials.

Therefore, studying FEB can help scientists better understand how some of these properties arise when multiple electrons in a material interact with each other. According to the authors, understanding how FEBs are formed could also provide insight into the self-assembly of soft materials, which may be important for the development of next-generation quantum materials.

However, so far scientists have only theoretically predicted the existence of FEB. “We have now observed FEB experimentally for the first time and understand how to create it,” said Neha Yadav. “These are great new things that have big implications if we can build and lock them.”

Yadav and colleagues were studying the stability of MEB at the nanometer size when they observed the coincidence of FEB. At first, they were both cheerful and skeptical. “It took a lot of experimentation before we were sure these things were really FEB. It must have been a very interesting moment,” said Professor Ghosh.

The researchers first applied a pulse of voltage to the tip of the tungsten on the surface of liquid helium. Then they generate pressure waves on the charged surface using ultrasonic transducers. This allowed them to make 8EB and 6EB, two types of FEB having eight and six electrons, respectively.

These FEBs were found to be stable for at least 15 milliseconds (quantitative changes usually occur over much shorter timescales) which would have allowed researchers to limit and study them.

“FEB forms an interesting system that has both electron-electron interactions and electron-surface interactions,” Yadav said.

There are many phenomena that FEB can help scientists decipher, such as turbulent flow in superfluids and viscous fluids, or heat flow in superfluid helium. 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 eddies, can reduce thermal conductivity. Because FEBs are at the heart of these eddies – as the authors in this study discovered – they can help study how the eddies interact with each other as well as heat flow through the superfluid helium.

“In the near future, we want to know if there are other types of FEB, and to understand the mechanisms by which some are more stable than others. In the long run, we want to use FEB as a quantum simulator, for which a new type of measurement scheme needs to be developed, Ghosh announced.

The research team included Neha Yadav, a former doctoral student in the Department of Physics, Prosingit Sen, professor at the Center for Nanoscale Science and Engineering (CeNSE), and Ambarish Ghosh, professor at CeNSE. This study was published in scientific progress.