In 2018, a team of physicists from Purdue University invented a device that experimentally showed quasiparticles interfering for the first time in the fractional quantum Hall effect at the fill factor v=1/3. Further development of these heterostructures has allowed the Manfra group to extend their research to experiments that explore countercurrently charged edge modes in the 2/3 fractional quantum Hall state.
They recently published their findings, “Half-Integer Conductance Plateau at the ν=2/3 Fractional Quantum Hall State in a Quantum Point Contact,” in Physical examination letters on February 17, 2023.
The team is led by Dr. Michael J. Manfra, Bill and Dee O’Brien Emeritus Professor of Physics and Astronomy, Professor of Electrical and Computer Engineering, Professor of Materials Engineering, and Scientific Director of Microsoft Quantum Lab West Lafayette. The main author of the publication is Dr. James Nakamura, Principal Investigator. Dr. Geoffrey Gardner and graduate student Shuang Liang were also co-authors of this publication, making valuable contributions to heterostructure growth.
In the experiment, the team produced a semiconductor material that contains a sheet of two-dimensional electrons. On top of this semiconductor, they built a quantum point contact made of metal grids with a very narrow gap of 300 nanometers. They used quantum point contact to direct conductive edge states through narrow space. In this configuration, demonstrated by the graph above, they were able to measure an electrical conductance equal to half the fundamental value of e2/h. This experimental result is consistent with long-standing theoretical predictions for the edge states of the fractional quantum Hall state ν = 2/3.
“We have a semiconductor structure that contains electrons arranged in a plane, called a two-dimensional electron system. When you cool electrons to low temperatures and place them in a strong magnetic field, they form special states of matter called quantum Hall states. “, explains Nakamura. “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 edge states that flow around the edge of the sample, and they are chiral, meaning that each edge state only flows in one direction (in the direction clockwise or counter-clockwise).The state ν=2/3 is predicted by theoretical physicists to have the special property that there are two edge states that flow through the opposite direction to each other, one clockwise and the other counterclockwise. This is different from most quantum Hall states, where all edge states flow in the same direction. We used a device with metal gates called a quantum dot contact to control the edge states, and our measurements of the edge states in the quantum dot contacts confirm the countercurrent edge states in our device. Quantum dot contact brings edge states closer together on opposite edges of the sample. We measured a value of el electrical conductance 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 the edge structure with two countercurrent edge states.”
This all-Purdue team of physicists is uniquely set up to succeed at Purdue University due to state-of-the-art facilities covering semiconductor growth, nanofabrication, and low-temperature electrical measurements at the university.
“A crucial aspect is the nanotechnology facilities at Purdue,” says Nakamura. “This includes the machine, called the MBE (molecular beam epitaxy) machine, which is used to produce the semiconductor structures. This highly specialized machine, operated by the Manfra Group, requires expertise to build and operate, so this is a key advantage at Purdue. Liang, under Gardner, is responsible for this aspect of our work. Additionally, the Birck Nanotechnology Center clean room is a state-of-the-art facility with a wide range of equipment at our disposal, which we used to manufacture the quantum dot contact gates. Having all of these resources and expertise available at one institution makes our experiences possible.
This research is part of an ongoing quest to understand and manipulate fractionally charged anyons in the fractional quantum Hall regime, a rich testbed for exploring the impact of topology in condensed matter physics that might possibly 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.
