In 2018, a team of physicists from Purdue University invented a device that experimentally demonstrated that quasiparticles overlap for the first time in a fractional quantum Hall effect at a fill factor of v = 1/3. Further development of these heterostructures allowed Manfra’s group to broaden their research to include experiments that explore the patterns of charged edges present in the quantum Hall 2/3 state.
They recently published their findings, “Integer semiconduction plateau at ν = 2/3 fractional quantum junction Hall state”, in Physical examination letters On February 17, 2023.
The team is led by Dr. Michael J. Manfra, Bill and D’O’Brien Professor Emeritus of Physics and Astronomy, Professor of Electrical and Computer Engineering, Professor of Materials Engineering, and Scientific Director of Microsoft Quantum Laboratory West Lafayette. The lead author of the publication is Dr. James Nakamura, principal investigator. Dr. Jeffrey Gardner and graduate student Shuang Liang were also co-authors of this publication, and made valuable contributions to the growth of heterostructure.
In the experiment, the team produced a semiconductor material that contained a two-dimensional sheet of electrons. On top of this semiconductor, they built a quantum contact point made of metallic grids with a very narrow 300 nanometer gap. They used a quantum contact point to guide conductive edge states through a narrow space. In this configuration, shown in the diagram above, they were able to measure an electrical conductivity equal to half of the base value of e2/ h. This experimental result is consistent with old theoretical predictions for the terminal states of the fractional Hall state ν = 2/3.
“We have a semiconductor structure that contains electrons arranged in a plane, called a two-dimensional electron system. When electrons are cooled to low temperatures and placed in a strong magnetic field, they form special states of matter called quantum Hall states,” Nakamura explains. “At a certain value of the magnetic field , the quantum Hall state is called the fractional quantum Hall state ν = 2/3. In all quantum Hall states, electric current is carried by the edge states that flow around the edge of the sample, which are chiral, meaning that each edge state only flows in one direction (clockwise or counterclockwise). ν = 2/3 theoretical physicists expect to have the special property of having two terminal states that flow in the opposite direction of each other, one clockwise and the other counterclockwise. This differs from most quantum Hall states, where all edge states flow in the same direction. We used a metal-gated device called a quantum dot contact to control the edge states, and our measurements of the edge states in quantum dot contacts confirm the countercurrent edge states in our device. Quantum dot communication brings the edge states closer together on opposite edges of the sample. We measured the value of electrical conductivity at the terminals of the device equal to half the value e2/h, where e is the charge of the electron and h is Planck’s constant. This conductance value is strong experimental evidence that our system has an edge structure with two oppositely current edge states.”
This team of all Purdue physicists is uniquely created to succeed at Purdue because of the state-of-the-art facilities covering semiconductor growth, nanofabrication, and low-temperature electrical measurements at the university.
“One critical aspect is the nanotechnology facilities at Purdue,” Nakamura says. This includes the machine, called the MBE (molecular beam epitaxy) machine, which is used to produce semiconductor structures. Operated by the Manfra Group, this highly specialized machine requires construction and operation experience, so this is an essential asset at Purdue. Liang, under Gardner’s leadership, is responsible for this aspect of our work. In addition, the clean room at the Birck Nanotechnology Center is a state-of-the-art facility with a wide range of equipment at our disposal, which we used to fabricate quantum dot contact gates. Having all these resources and expertise in one organization makes our experiences possible.
This research is part of the ongoing quest to understand and manipulate partially charged ions in a fractional quantum Hall regime, a rich test base for exploring the influence of topology in condensed matter physics that can be used to create qubits.
This research is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under award number DE-SC0020138.
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