Unexpected Topological Quantum States

Princeton-led team of physicists have discovered that, under certain conditions, interacting electrons can create what are called “topological quantum states,” which, has implications for many technological fields of study, especially information technology. This diagram of a scanning tunneling microscope shows the magic-angle twisted bilayer graphene. Credit: Kevin Nuckolls, Department of Physics, Princeton University

Physicists have discovered that, under certain conditions, interacting electrons can create what are called topological quantum states.

To get the desired quantum effect, they placed two sheets of graphene on top of each other with the top layer angled slightly. This twisting creates a moiré pattern, which resembles and is named after a common French textile design. The important point, however, is the angle at which the top layer of graphene is positioned: precisely 1.1 degrees, the “magic” angle that produces the quantum effect.

The researchers generated extremely low temperatures and created a slight magnetic field. They then used a machine called a scanning tunneling microscope, which relies on a technique called quantum tunneling rather than light to view the atomic and subatomic world. They directed the microscope’s conductive metal tip on the surface of the magic-angle twisted graphene and were able to detect the energy levels of the electrons.

They found that the magic-angle graphene changed how electrons moved on the graphene sheet. When electrons have the same energy — are in a flat band material — they interact with each other very strongly. One of these “exotic” things, the researchers discovered, was the creation of unexpected and spontaneous topological states.

Specifically, they discovered that the interaction between electrons creates what are called topological insulators. These are unique devices that act as insulators in their interiors, which means that the electrons inside are not free to move around and therefore do not conduct electricity. However, the electrons on the edges are free to move around, meaning they are conductive. Moreover, because of the special properties of topology, the electrons flowing along the edges are not hampered by any defects or deformations. They flow continuously and effectively circumvent the constraints — such as minute imperfections in a material’s surface — that typically impede the movement of electrons. (SciTechDaily)

The paper has been published in the journal Nature.

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