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Physicists isolate bound states in graphene-superconductor junctions

Date:
February 15, 2011
Source:
University of Illinois at Urbana-Champaign
Summary:
Researchers have documented the first observations of some unusual physics when two prominent electric materials are connected: superconductors and graphene. When sandwiched between superconductors, graphene can adopt superconducting capacity because paired electrons from the superconductor are translated to Andreev bound states (ABS) in the graphene. The researchers isolated and manipulated individual ABS by confining them to a graphene quantum dot, which could be used as a qubit for quantum computing.

Illinois physics professor Nadya Mason led a team that isolated unique electron bound states that form in graphene-superconductor junctions.
Credit: Ivan Petrov

Illinois researchers have documented the first observations of some unusual physics when two prominent electric materials are connected: superconductors and graphene.

Led by University of Illinois physics professor Nadya Mason, the group published its findings in the journal Nature Physics.

When a current is applied to a normal conductor, such as metal or graphene, it flows through the material as a stream of single electrons. By contrast, electrons travel in pairs in superconductors. Yet when a normal material is sandwiched between superconductors, the normal metal can carry the supercurrent.

Normal metals can adopt superconducting capacity because the paired electrons from the superconductor are translated to special electron-hole pairs in the normal metal, called Andreev bound states (ABS).

"If you have two superconductors with a normal metal between, you can actually transport the superconductivity across the normal material via these bound states, even though the normal material doesn't have the electron pairing that the superconductors do," Mason said.

ABS are extremely difficult to measure or to observe directly. Researchers can measure conduction and overall magnitude of a current, but have not been able to study individual ABS to understand the fundamental physics contributing to these unique states.

Mason's group developed a method of isolating individual ABS by connecting superconducting probes to tiny, nanoscale flakes of graphene called quantum dots. This confined the ABS to discrete energy levels within the quantum dot, allowing the researchers to measure the superconducting ABS individually and even to manipulate them.

"Before this, it wasn't really possible to understand the fundamentals of what is transporting the current," Mason said. "Watching an individual bound state allows you to change one parameter and see how one mode changes. You can really get at a systematic understanding. It also allows you to manipulate ABS to use them for different things that just couldn't be done before."

Superconductor junctions have been proposed for use as superconducting transistors or bits for quantum computers, called qubits. Greater understanding of ABS may enable other applications as well. In addition, it may be possible to use the superconducting graphene quantum dots themselves as solid-state qubits.

"This is a unique case where we found something that we couldn't have discovered without using all of these different elements -- without the graphene, or the superconductor, or the quantum dot, it wouldn't have worked. All of these are really necessary to see this unusual state," Mason said.

The U.S. Department of Energy supported this work, conducted at the Frederick Seitz Materials Research Laboratory at Illinois.


Story Source:

The above story is based on materials provided by University of Illinois at Urbana-Champaign. Note: Materials may be edited for content and length.


Journal Reference:

  1. Travis Dirks, Taylor L. Hughes, Siddhartha Lal, Bruno Uchoa, Yung-Fu Chen, Cesar Chialvo, Paul M. Goldbart, Nadya Mason. Transport through Andreev bound states in a graphene quantum dot. Nature Physics, 2011; DOI: 10.1038/nphys1911

Cite This Page:

University of Illinois at Urbana-Champaign. "Physicists isolate bound states in graphene-superconductor junctions." ScienceDaily. ScienceDaily, 15 February 2011. <www.sciencedaily.com/releases/2011/02/110214142348.htm>.
University of Illinois at Urbana-Champaign. (2011, February 15). Physicists isolate bound states in graphene-superconductor junctions. ScienceDaily. Retrieved July 23, 2014 from www.sciencedaily.com/releases/2011/02/110214142348.htm
University of Illinois at Urbana-Champaign. "Physicists isolate bound states in graphene-superconductor junctions." ScienceDaily. www.sciencedaily.com/releases/2011/02/110214142348.htm (accessed July 23, 2014).

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