Researchers demonstrate new kind of quantum transport

In a recent study, researchers from the SUNY Polytechnic Institute, Stony Brook University and the Brookhaven National Laboratory in the US, along with Aalto University in Finland, have demonstrated a new electronic device that employs the unique ways in which electrons behave in graphene and high-temperature superconductors.

Developing new quantum devices relies on controlling how electrons behave. Graphene has fascinated researchers because its electrons behave as if they have no mass. Scientists have also been interested in high-temperature superconductors: ceramic materials where electron interactions yield a macroscopic quantum state where electrons pair with each other. They do so at a temperature above the usual superconducting temperature of metals, which approaches absolute zero.

The experiment, led by Sharadh Jois and Ji Ung Lee from SUNY, with support from Jose Lado, assistant professor at Aalto, demonstrated a new quantum device that combines graphene and an unconventional high-temperature superconductor. In particular, the team demonstrated that the electronic transport between graphene and the high-temperature superconductor was dominated by a unique transport process arising from the combination of two properties: graphene’s Klein tunnelling and the superconductor’s Andreev reflection. The team showed experimentally that this transport process is fully consistent with the existing theoretical predictions concerning hybrid Andreev-Klein electronic transport.

The study relies on two unique phenomena found in these materials: Klein tunnelling and Andreev reflection. Because graphene electrons behave as if they have no mass, they can move in situations where normal electrons could not. This phenomenon is known as Klein tunnelling. In turn, electrons in high-temperature superconductors form so-called Cooper pairs of two electrons. Cooper pairs can have unique mathematical structures, leading to unconventional superconducting states. When a Cooper pair forms exactly at the meeting point of a standard material such as a piece of metal and a superconductor, it can result in a phenomenon called Andreev reflection, in which the pair ‘kicks back’ another kind of particle into the metal.

“The demonstration of electronic transport stemming from graphene’s Klein tunnelling and unconventional superconducting pairing establishes a milestone in graphene-based quantum devices. This observation establishes the starting point to develop a whole new family of graphene-based superconducting quantum circuits that exploit unconventional superconductivity,” Lado said.

The unique electronic properties of graphene make it a promising platform for developing electronics that do not consume a lot of power. Conventional superconductors are key materials in a variety of quantum devices and they are particularly important in one of the strategies to build both qubit and topological quantum computers. By combining the two, the study’s results can open new fundamental physics in these materials and ultimately establish a new platform for quantum technology devices.

Image credit: iStock.com/Bartlomiej Wroblewski