Semiconductor demonstrates elusive quantum physics model

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Cornell researchers have shown that a single material system can switch between two of the craziest states in Condensed Matter physics.

Scientists have never been able to demonstrate a model that was proposed more than a decade ago because a suitable material didn't exist. The researchers' breakthrough could lead to advances in quantum devices.

The paper was published in Nature. The co-lead authors are a former researcher and a student.

Kin Fai Mak, associate professor of physics in the College of Arts and Sciences, and Jie Shan, professor of applied and engineering physics in the College of Engineering, are the co-senior authors of the paper. The researchers came to Cornell through the provost's NextNano initiative and are members of the Kavli Institute at Cornell.

The lab specializes in exploring the electronic properties of 2D quantum materials, often by stacking ultrathin monolayers of semiconductors so their slightly mismatches creates a lattice pattern. There are electrons that can be deposited and interact with each other.

The researchers twisted MoTe2 with WSE2 at a 180-degree angle for the new project.

They observed a Hall effect after applying a voltage. The Hall effect, first observed in the late 19th century, is a phenomenon in which electrical current flows through a sample and is bent by a magnetic field that is applied at a certain angle.

The supersized version of the quantum Hall effect, discovered in 1980, is a phenomenon in which a far greater magnetic field is applied, triggering even stranger phenomena.

The same effect is achieved by the Hall insulator, but without the intervention of a magnetic field, the electrons speed along the edge as if on a highway.

"For a long time people thought that a magnetic field was needed for the quantum Hall effect, but they actually don't need one," Mak said. What replaces the role of a magnetic field? It turns out that it's magnetism. The material has to be magnetic.

The MoTe2/WSe2 stack is one of a few materials that are known to be Hall insulators. Half of its appeal is still there.

The researchers found that by tweaking the voltage, they could turn their stack into a 2D topological insulator, which is a cousin of the quantum Hall insulator, except that it exists in duplicate. The electron highway flows clockwise around the edge in one "copy" andcounterclockwise in the other.

The two states of matter have never been demonstrated together.

The Cornell team learned that their experiment had realized a toy model for Graphene first proposed by professors Charles Kane and Eugene Mele at the University of Pennsylvania in 2005. The Kane-Mele model was the first theoretical model for 2D topological insulators.

Mak said that it was a surprise. We did the measurements after making this material. We saw the Hall effect and the 2D topological insulator. That's great. We talked to our theory friend at MIT. They did the calculations and figured out that the model they were looking for was realized. We didn't expect this.

In September, the team reported in Nature that MoTe2/WSe2 can switch between a range of quantum states, including a transition from a metal to a Mott insulator.

Mak and Shan's lab is investigating the full potential of the material by using it to create qubits, the basic element for quantum computing, by attaching it to superconductors and building quantum Hall interferometers. Mak is hopeful that they will find a way to raise the temperature at which the Hall effect occurs, which is at about 2 kelvin.

Lizhong Li and Zui Tao are PhD students at MIT, and they are one of the co-authors.

More information about the quantum Hall effect from intertwined bands. The article is titled "Tingxin Li et al."

Nature has a report on Continuous Mott transition in Semiconductor Superlattices. The DOI is 10.1038/s41586-021-03853-0.

Nature journal information.

The Semiconductor demonstrates elusive quantum physics model was retrieved fromphys.org.

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