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 Univ.
of Illinois physics
professor Nadya Mason, the group published its findings in 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 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.
