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Scientists Observe Fractional Quantum Hall States in Monolayer 2D Semiconductors

For the first time, scientists from Columbia University in the City of New York (Columbia University) have successfully visualized a quantum fluid, called the fractional quantum Hall states (FQHS), in a monolayer two-dimensional (2D) semiconductor. FQHS is one of the most fragile phases of matter.

A monolayer semiconductor is found to be a close-to-ideal platform for fractional quantum Hall state—a quantum liquid that emerges under large perpendicular magnetic fields. The image illustrates monolayer WSe2 hosting “composite fermions,” a quasi-particle that forms due to the strong interactions between electrons and is responsible for the sequence of fractional quantum Hall states. Image Credit: Cory Dean/Columbia University in the City of New York.

The latest findings not only reveal the exceptional inherent quality of 2D semiconductors but also establish them as a special test platform for upcoming applications in the field of quantum computing. The study was recently published online 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. 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.

Cory Dean, Professor, Department of Physics, Columbia University in the City of New York

The term fractional quantum Hall state can be described as a collective occurrence that materializes when scientists limit electrons to travel in a thin 2D plane and expose them to bulk magnetic fields.

The fractional quantum Hall effect was initially identified in 1982, and since then, it has been explored for over 40 years. But despite these vast analyses, several fundamental questions continue to persist. One reason for this is that the state is highly delicate and occurs exclusively in the cleanest materials.

Observation of the FQHS is therefore often viewed as a significant milestone for a 2D material—one that only the very cleanest electronic systems have reached.

Jim Hone, Wang Fong-Jen Professor of Mechanical Engineering, Columbia Engineering, Columbia University in the City of New York

Graphene is the most familiar 2D material. However, a huge set of analogous materials have been discovered in the past decade, which can be collectively exfoliated down to a one-layer thickness. Transition metal dichalcogenides (TMDs), like WSe2, are a class of these materials. The researchers have used WSe2 material in their latest study.

TMDs are similar to graphene, and their layers can be removed to make them atomically thin. However, their characteristics under magnetic fields are relatively simpler, unlike graphene. The persistent challenge was that TMDs do not have excellent crystal quality.

Ever since TMD came on the stage, it was always thought of as a dirty material with many defects,” added Hone, whose team has improved the quality of TMDs considerably, pushing them to an almost graphene-like quality—usually regarded the de facto standard of purity among the 2D materials.

Apart from sample quality, the difficulties in making excellent electrical contact have hampered the analyses of semiconductor 2D materials. To deal with this problem, the Columbia University scientists have also been advancing the potential to quantify electronic characteristics by capacitance, instead of using the traditional techniques of making current flow and quantifying the resistance.

This method provides one major advantage—the measurement is not highly sensitive to poor electrical contact and also to material impurities. For this latest study, the measurements were carried out under extremely large magnetic fields—which made it possible to stabilize the FQHS—at the National High Magnetic Field Lab.

The fractional numbers that characterize the FQHS we observed—the ratios of the particle to magnetic flux number—follow a very simple sequence. The simple sequence is consistent with generic theoretical expectations, but all previous systems show more complex and irregular behavior.

Qianhui Shi, Study First Author and Postdoctoral Researcher, Columbia Nano Initiative, Columbia University in the City of New York

Shi continued, “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,” Dean added.

Ever since their discovery in the late 1980s, fractional states with even denominators have attracted exclusive attention. This is because they are believed to denote a new type of particle—one that has quantum properties unlike any other familiar particle in the universe.

The unique properties of these exotic particles,” observed Zlatko Papic, associate professor in theoretical physics at the University of Leeds, “could be used to design quantum computers that are protected from many sources of errors.”

To date, experimental attempts made to interpret and manipulate the even denominator states have been restricted by their high sensitivity and also by the very small number of materials, where this state might be detected.

This makes the discovery of the even denominator state in a new—and different—material platform, really very exciting,” added Dean.

The two laboratories at Columbia University—the Dean Lab and the Hone Group—worked in association with the NIMS Japan, which delivered a few of the materials, and Papic, whose research team conducted computational modeling of the experiments.

Both laboratories at Columbia University are part of the university’s Material Research Science and Engineering Center. The study also made use of cleanroom facilities available at both the Columbia Nano Initiative and City College.

In addition, measurements at huge magnetic fields were made at the National High Magnetic Field Laboratory, a user facility financially supported by the National Science Foundation and headquartered at Florida State University in Tallahassee, Florida.

Since the scientists have extremely clean 2D semiconductors and also an effective probe, they are analyzing other fascinating states that arise from such 2D platforms.

Journal Reference:

Shi, Q., et al. (2020) Odd- and even-denominator fractional quantum Hall states in monolayer WSe2. Nature Nanotechnology.

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