Two-dimensional semiconductors were found to be an almost ideal fractional quantum Hall platform

Researchers at Columbia University report that they have observed a quantum fluid known as Fractional Quantum Hall States (FQHS), one of the most delicate phases of matter, for the first time in a monolayer 2-D semiconductor. Their findings demonstrate the excellent intrinsic quality of 2-D semiconductors and establish them as a unique testing platform for future applications in quantum computing. The study was published online today in Nanotechnology nature.

“We were very surprised to see this state in 2-D semiconductors because they are generally supposed to be too dirty and messy to harbor this effect,” says Cory Dean, professor of physics at Columbia University. “Furthermore, the sequence of FQHS in our experiment reveals unexpected and interesting new behavior that we have never seen before, and in fact suggests that 2-D semiconductors are almost ideal platforms for further study of FQHS.”

Fractional quantum Hall state is a collective phenomenon that occurs when researchers confine electrons to move in a thin two-dimensional plane and subject them to large magnetic fields. First discovered in 1982, the Fractional Quantum Hall effect has been studied for over 40 years, but many fundamental questions remain. One of the reasons for this is that the condition is very fragile and appears only in the cleanest materials.

“Therefore, FQHS observation is often considered an important milestone for a 2-D material, one that only the cleanest electronic systems have reached,” says Jim Hone, professor of mechanical engineering Wang Fong-Jen at Columbia. Engineering.

While graphene is the best-known 2-D material, a large group of similar materials have been identified in the past 10 years, all of which can exfoliate to a single layer thickness. One class of these materials is transition metal dicholcogenide (TMD), such as WSe2, the material used in this new study. Like graphene, they can be peeled to be atomically thin, but unlike graphene, their properties under magnetic fields are much simpler. The challenge has been that the quality of the TMD glass was not very good.

“Since TMD took the stage, it was always thought of as a dirty material with many flaws,” says Hone, whose group has made a significant improvement in the quality of TMD, bringing it to a quality close to graphene, often regarded as the El Maximum purity standard among 2D materials.

In addition to sample quality, studies of 2-D semiconductor materials have been hampered by difficulties in making good electrical contact. To address this, the Columbia researchers have also been developing the ability to measure electronic properties by capacitance, rather than conventional methods of flowing a current and measuring resistance. An important advantage of this technique is that the measurement is less sensitive both to poor electrical contact and to impurities in the material. Measurements for this new study were made under very large magnetic fields, which help stabilize the FQHS, at the National High Magnetic Field Laboratory.

“The fractional numbers that characterize the FQHS that we observe, the proportions of the particle to the magnetic flux number, follow a very simple sequence,” says Qianhui Shi, first author of the article and a postdoctoral researcher at the Columbia Nano Initiative. “The simple sequence is consistent with generic theoretical expectations, but all of the above systems show more complex and irregular behavior. This tells us that we finally have an almost ideal platform for the study of FQHS, where experiments can be directly compared to models simple. “

Among the fractional numbers, one of them has an even denominator. “Looking at the fractional quantum Hall effect itself was surprising, seeing the state of the pair denominator in these devices was truly surprising, as this state had only been observed in the best of the best devices before,” says Dean.

Fractional states with even denominators have received special attention since their first discovery in the late 1980s, as they are believed to represent a new type of particle, one with quantum properties unlike any other known particle in the universe. “The unique properties of these exotic particles,” says Zlatko Papic, an associate professor of theoretical physics at the University of Leeds, “could be used to design quantum computers that are protected from many sources of errors.”

Until now, experimental efforts to understand and exploit even-denominator states have been limited by their extreme sensitivity and the extremely small number of materials in which this state can be found. “This makes discovering the state of the peer denominator on a new and different material platform really exciting,” adds Dean.

The two Columbia University laboratories, the Dean Lab and the Hone Group, worked in collaboration with NIMS Japan, which supplied some of the materials, and Papic, whose group made computer models of the experiments. Both Columbia labs are part of the university’s Materials Research Science and Engineering Center. This project also used clean room facilities at both the Columbia Nano Initiative and City College. Measurements in large magnetic fields were performed at the National High Magnetic Field Laboratory, a user facility funded by the National Science Foundation and based at Florida State University in Tallahassee, Fl.

Now that the researchers have very clean 2-D semiconductors, as well as an effective probe, they are exploring other interesting states that emerge from these 2-D platforms.