First observation of individual Andreev bound states in graphene

In a paper published in the February 6 online issue of Nature Physics, University of Illinois physicists, led by experimentalist Nadya Mason and theorist Paul Goldbart, report the first observation in graphene of individual, gate-tunable superconducting states known as Andreev bound states (ABS). (Read the UI News Bureau story.)

“We are clarifying the fundamental physics of superconducting junctions,” said Mason, “which could lead to improved electronic devices based on these junctions, including superconducting transistors and solid-state bits for quantum computers.”

While ABS have been theoretically predicted since the 1960s and have been invoked to explain supercurrents in superconductor–normal metal junctions, ABS have proven to be very difficult to isolate and measure invidually. Mason’s group succeeded in identifying and manipulating discrete ABS in graphene for the first time.

In the experiment, Mason’s group attached superconducting probes to an exfoliated graphene sheet, which is a two-dimensional semi-metal. Although graphene is not normally a superconductor, it can carry a superconducting current when connected to superconducting electrodes.

“Because of a work-function mismatch between the graphene film and the superconducting lead, a confined particle-in-a-box region, or ‘quantum dot,’ forms in the graphene,” said Mason. The individual ABS are formed on the discrete energy levels of this quantum dot.

“Now that these states have been isolated, we can study the exact mechanisms of superconductivity and coherence in normal metals. This discovery also opens the door to new applications for these unusual electronic states,” said Mason.