May 5 2017
MIT physicists have found that a flake of graphene, when brought in close proximity with two superconducting materials, can inherit some of those materials� superconducting qualities. As graphene is sandwiched between superconductors, its electronic state changes dramatically, even at its center. Pictured is the experimental concept and device schematic.
In the case of copper and silver, which are normal conductive materials, there are differing degrees of resistance to the flow of electric current. These resistances are caused by individual electrons that generate defects, thereby dissipating energy when they move.
On the contrary, superconductors have significant potential of conducting electricity without any resistance. The reason is the electrons get paired up and travel throughout the material together, thus eliminating friction.
At present, a team of physicists from MIT have discovered that if a graphene flake is placed in close proximity to two superconducting materials, it has the ability to assume certain superconducting qualities of the materials. Since graphene is enclosed in-between the superconducting materials, its electronic state drastically altered even at its center.
The research team discovered that electrons in graphene—which earlier acted as individual, scattering particles—now paired up in “Andreev states,” a basic electronic configuration in which traditional, nonsuperconducting material is enable to convey a “supercurrent.” A supercurrent is electric current flowing without the dissipation of energy.
The outcomes of the research have been reported in the Nature Physics journal this week, and are the first-of-its-kind analysis of Andreev states caused by superconductivity’s “proximity effect” in a two-dimensional material, for example, graphene.
In future, the graphene platform developed by the research team might be used to analyze exotic particles (e.g. Majorana fermions), that are perceived to emerge from Andreev states and can prove to be important particles for developing powerful, error-proof quantum computers.
There is a huge effort in the condensed physics community to look for exotic quantum electronic states. In particular, new particles called Majorana fermions are predicted to emerge in graphene that is connected to superconducting electrodes and exposed to large magnetic fields. Our experiment is promising, as we are unifying some of these ingredients.
Landry Bretheau, Postdoc, Department of Physics, MIT
Landry’s co-authors at MIT are postdoc Joel I-Jan Wang, visiting student Riccardo Pisoni, and associate professor of physics Pablo Jarillo-Herrero, as well as Kenji Watanabe and Takashi Taniguchi from the National Institute for Materials Science, Japan.
The superconducting proximity effect
Brian David Josephson, a British physicist, speculated in the year 1962 that two superconductors including a nonsuperconducting layer sandwiched between them have the ability to withstand a supercurrent of electron pairs without necessitating any external voltage.
In general, the supercurrent affiliated with Josephson effect has been evaluated in a number of experiments. However, Andreev states, or the microscopic building blocks of a supercurrent, have been detected in very few systems (e.g. silver wires) but not in a two-dimensional material.
Bretheau, Wang, and Jarillo-Herrero overcame this problem by using graphene, which is an ultrathin sheet of carbon atoms that are interlinked with one another, as the nonsuperconducting material. According to Bretheau, Graphene is an exceptionally “clean” system that involves lesser electron scattering. The extended atomic configuration of graphene allows researchers to evaluate the electronic Andreev states of graphene when it is in close proximity to superconductors. Researchers will also be able to regulate the density of electrons in graphene and analyze its impacts on the superconducting proximity effect.
The research team flaked off a highly thin graphene flake (with a width of just few hundred nanometers) from a larger graphite chunk, and positioned the graphene flake on a small platform formed of boron nitride crystal overlaying a graphite sheet. They placed an aluminum electrode on both ends of the graphene flake, where the electrode acts as a superconductor at lower temperatures. Then, they placed the entire structure into a dilution refrigerator and reduced the temperature to 20 mK, which is well inside the superconducting range of aluminum.
“Frustrated” states
During the experiments, the research team altered the magnitude of supercurrent flowing between the superconductors by exerting a modifying magnetic field to the whole structure. In addition, they altered the number of electrons in the material by directly applying an external voltage to graphene.
The team evaluated the density of electronic states in graphene under the altered conditions when the flake was in close proximity to both aluminum superconductors. The researchers employed tunneling spectroscopy—a usual method for measuring the density of electronic states in a conductive material—to investigate the central region of graphene to check for impacts due to the superconductors even in areas in which they had no physical contact with graphene.
The evaluations showed that the electrons in graphene, in contrast to behaving as individual particles—paired up, albeit in “frustrated” configurations, where their energies relied on the magnetic field.
Electrons in a superconductor dance harmoniously in pairs, like a ballet, but the choreography in the left and right superconductors can be different. Pairs in the central graphene are frustrated as they try to satisfy both ways of dancing. These frustrated pairs are what physicists know as Andreev states; they are carrying the supercurrent.
Landry Bretheau, Postdoc, Department of Physics, MIT
Bretheau and Wang discovered that the Andreev states alter their energy following a change in magnetic field. Andreev states markedly occur as result of higher density of electrons in graphene and a stronger supercurrent flow between the electrodes.
“[The superconductors] are actually giving graphene some superconducting qualities,” explained Bretheau. “We found these electrons can be dramatically affected by superconductors.”
Although the research team performed the experiments under lower magnetic fields, the researchers believe that the platform can act as a basis for evaluating the more exotic Majorana fermions that occur under high magnetic fields.
There are proposals for how to use Majorana fermions to build powerful quantum computers. These particles could be the elementary brick of topological quantum computers, with very strong protection against errors. Our work is an initial step in this direction.
Landry Bretheau, Postdoc, Department of Physics, MIT
The U.S. Department of Energy and the Gordon and Betty Moore Foundation partially supported this study.