GCSU lecturer co-publishes new insight on physics phenomena

Dr. Kushan Wijewardena, lecturer of physics and astronomy at Georgia College & State University, is part of a team of researchers who recently co-authored a paper on a popular phenomenon well-known in physics called the “fractional quantum Hall effect” or FQHE.

His research, done in collaboration with Professor Ramesh Mani at Georgia State University, was published in the journal Communications Physics. It adds new insight into the behavior of electrons.

“The results were fascinating,” Wijewardena said. “It took several years for us to have a feasible explanation for the experimental observations that we made.”

“The study demonstrates the importance of the high-quality crystals developed at the Swiss Federal Institute of Technology Zurich by Professor Werner Wegscheider and Dr. Christian Reichl,” he added. 

Wijewardena joined Georgia College faculty last year after completing his doctoral studies in Mani’s physics lab at GSU. That’s where Wijewardena began working on measurements to explore “excited levels of fractional quantum Hall effect” by applying a direct current bias. 

This is the first time these findings have been reported. 

At Science.org, FQHE is explained as “a fascinating quantum liquid made up solely of electrons confined to a plane surface. Found only at temperatures near absolute zero and in extremely strong magnetic fields, this liquid can flow without friction. The excited states of this liquid consist of peculiar particle-like objects that carry an exact fraction of an electron charge. Called quasiparticles, these excitations can themselves condense into new liquid states.”

Wijewardena explained it this way: “We are exploring properties such as electrical resistance and conductance in a two-dimensional electron system, where the electrons can behave as gas or liquid, depending on their environment. Here, two-dimensionality for electrons is realized by trapping the electrons at the planar interface of two different materials. Our sample of electrons is similar in shape to a very small sheet of paper with no thickness (about 1 mm x 0.5 mm). When we place the sample in a perpendicular magnetic field and at an extremely low temperature close to absolute zero, the electrons exhibit quantized circular orbits, analogous to concentric circles drawn on the sheet of paper. The radius of these orbits depends on the energy of the electrons, which is influenced by the charge of the electron, and the magnetic field strength. The energies of these orbits are quantized into ‘Landau levels,’ meaning their energy changes in discrete steps rather than continuously. One might think of the lowest energy state corresponding to the smallest circle as the ground state, and the higher lying levels as excited states.”

Wijewardena continued: “Typically, in FQHE, researchers study the ground state, which is the lowest possible energy state. However, there could also be higher lying, or excited states, that the correlated electron system can occupy, which we have examined in the publication.”

GSU’s Mani said “fractional quantum Hall effects are thought to reflect the correlated ground states of electronic systems in a magnetic field. Loosely speaking, one might say it’s like investigating the ground floor of a building.” 

He added, “Then, one might ask whether the building structure has floors at higher levels and whether they can be investigated and what these upper floors look like. Our study reports an investigation of these upper floors or excited states of fractional quantum Hall effect. Surprisingly, with a simple technique, we were able to access these upper floors, find complex signatures and, most importantly, find the excited states of fractional quantum Hall effects.”

In its announcement of Mani’s and Wijewardena’s research, Georgia State University noted everyday modern conveniences “like cellphones, computers, GPS, LED lighting, solar cells and even self-driving cars” were made possible by advances in past research in “condensed matter physics,” a subfield of physics.

This research led to “flatland science and flatland materials (that) are now being studied in condensed matter physics with the aim to realize more energy-efficient, flexible, faster and lighter-weight future electronics, including novel sensors, higher efficiency solar cells, quantum computers and topological quantum computers.”

According to GSU’s statement, the team “observed all the FQHE states splitting unexpectedly, followed by crossings of split branches, which allowed them to explore the new non-equilibrium states of these quantum systems and reveal entirely new states of matter.”

Several Nobel Prizes have been given for FQHE research over the years.

The original quantum Hall effect—which involved a complex physics formula with two-dimensional electron systems subjected to low temperatures and strong magnetic fields—was discovered by German scientist Klaus von Klitzing in 1980. He was awarded a Nobel Prize for his work in 1985. Mani worked with Klitzing in Stuttgart, Germany, for many years.

Shortly after quantum Hall effect was established, scientists at Bell Laboratories reported their discovery of FQHE. It provided evidence of fractional or one-third of an electron’s charge, winning Robert Laughlin, Horst Störmer, and Daniel Tsui a Nobel Prize in 1998.

In 2010, another Nobel Prize was awarded for the discovery of graphene. This provided quantum Hall effect evidence for electrons without mass, which behaved differently from regular electrons. Then, theories for “topological phase transitions and topological matter” arising from QHE received the Nobel Prize in 2016. 

“From this progression,” Wijewardena said, “one can see the quantum Hall effect related to science has been a very active field in condensed matter physics.”

“Since the discovery of the fractional quantum Hall effect, there has been quite a lot of research work on this exciting topic,” he said. “However, this is the first report of excited state fractionally quantized Hall effects induced by applying a direct current bias.”

In the photo: Dr. Kushan Wijewardena gets ready to load a sample into the dilution refrigerator.