This image shows a monolayer WSe2 housing "composite fermions," a quasi-particle created from the interaction between electrons. It's also responsible for the sequence of fractional quantum Hall states. (Image Credit: Cory Dean/Columbia University)

Researchers from Columbia University successfully examined a quantum fluid called the fractional quantum Hall states (FQHS) in a monolayer 2D semiconductor. The team's discovery shows the high intrinsic quality of 2D semiconductors and establishes them as a unique test platform for future applications in quantum computer systems. The team presented their findings in the Nature Nanotechnology journal.


"We were very surprised to observe this state in 2D semiconductors because it has generally been assumed that they are too dirty and disordered to host this effect," says Cory Dean, professor of physics at Columbia University. "Moreover, the FQHS sequence in our experiment reveals unexpected and interesting new behavior that we've never seen before, and in fact, suggests that 2D semiconductors are close-to-ideal platforms to study FQHS further."


Graphene is the best 2D material. However, a large set of identical materials have been discovered over the past decade, which can be exfoliated down to a single-layer thickness. Transition metal dichalcogenides (TMDs), such as WSe2, are a class of these materials the research team used in this study. Similar to graphene, their layers can be removed until they're atomically thin. One downside is that their properties under magnetic fields are simpler, unlike graphene. The problem is that TMDs do not posses great crystal quality.


Along with sample quality, observations of semiconductor 2D materials have been hampered since it has been difficult to make good electrical contact. To overcome this issue, the team developed the ability to measure electronic properties by capacitance instead of using traditional methods. Those methods include causing a current to flow and measuring the resistance. One advantage of this new approach is that the measurement isn't very sensitive to poor electrical contact and impurities in the material. Measurements were taken at the National High Magnetic Field Lab under large magnetic fields.


"The fractional numbers that characterize the FQHS we observed—the ratios of the particle to magnetic flux number—follow a very simple sequence," says Qianhui Shi, a postdoctoral researcher at the Columbia Nano Initiative. "The simple sequence is consistent with generic theoretical expectations, but all previous systems show more complex and irregular behavior. This tells us that we finally have a nearly ideal platform for the study of FQHS, where experiments can be directly compared to simple models."


One of the fractional numbers has an even denominator. "Observing the fractional quantum Hall effect was itself surprising, seeing the even-denominator state in these devices was truly astonishing since previously this state has only been observed in the very best of the best devices," says Dean.


Since their discovery in the late 1980s, fractional states with even denominators received special attention. This is because they represent a new type of particle with quantum properties that are different from other known particles.


To date, experiments designed to understand and manipulate the even denominator states have been restricted. This is due to their high sensitivity and small amount of materials in which this state is detected. "This makes the discovery of the even denominator state in a new—and different—material platform, really very exciting," Dean adds.


With extremely clean 2D semiconductors and an effective probe, the scientists are now investigating other unique states that arise from these 2D platforms.


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