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Observation of electron coherence and Fabry-Perot standing waves at a graphene edge.

Research paper by Monica M Allen, Oles O Shtanko, Ion Cosma IC Fulga, Joel I-Jan JI Wang, Daniyar D Nurgaliev, Kenji K Watanabe, Takashi T Taniguchi, Anton A Akhmerov, Pablo P Jarillo-Herrero, Leonid L Levitov, Amir A Yacoby

Indexed on: 19 Oct '17Published on: 19 Oct '17Published in: Nano Letters



Abstract

Electron surface states in solids are typically confined to the outermost atomic layers and, due to surface disorder, have negligible impact on electronic transport. Here we demonstrate a very different behavior for surface states in graphene. We probe the wavelike character of these states by Fabry-Perot (FP) interferometry and find that, in contrast to theoretical predictions, these states can propagate ballistically over micron-scale distances. This is achieved by embedding a graphene resonantor formed by gate-defined p-n junctions within a graphene SNS structure. By combining superconducting Aharanov-Bohm interferometry with Fourier methods, we visualize spatially resolved current flow and image FP resonances due to p-n-p cavity modes. The coherence of the standing wave edge states is revealed by observing a new family of FP resonances which coexist with the bulk resonances. The edge resonances have periodicity distinct from that of the bulk states, manifest in a repeated spatial redistribution of current on and off the FP resonances. This behavior is accompanied by a modulation of the multiple Andreev reflection amplitude on and off resonance, indicating that electrons propagate ballistically in a fully coherent fashion. These results, which were not anticipated by theory, provide a practical route to developing electron analog of optical FP resonators at the graphene edge.