Condensed Matter Seminar: "The superconducting proximity effect in Bi2Se3"
(sign-up)Jim Eckstein, University of Illinois at Urbana-Champaign
Physics - Condensed Matter
By combining materials growth, fabricated transport devices, and angle resolved photoemission spectroscopy we are exploring how superconducting pairs propagate in the topological insulator Bi2Se3. In regular arrays of niobium islands we find that a zero resistance state occurs via a BKT transition at low temperatures, and that this strongly depends on the contact conductance between the underlying Bi2Se3 film and the islands. The ratio of the carrier mean free path to the island spacing appears to control the temperature at which the array becomes coherent. The distance over which this happens is about 100nm which is close to the mean free path. One signature of the coherence of such an array is the observation of phase-locked microwave steps (Shapiro steps) upon illumination with GHz range microwaves. All of the junctions are acting like one. We’ve also developed a novel method of preparing samples for ARPES studies. We’ve made Bi2Se3 films of different thicknesses ranging from 4 to 10 molecular layers thick on top of superconducting niobium substrates and studied how the superconductivity from the substrate propagates through the film to the surface which is studied by ARPES at 1.5K. We find that if the Bi2Se3 layer is 4 quint layers thick, a strong gap is evident in the ARPES spectrum. However this attenuates rapidly as the films are made thicker. The gap appears to attenuate and fill in exponentially with a characteristic thickness of about 3 quint layers. Combining these studies a picture emerges of a strongly anisotropic proximity effect in which the layers in closest contact with the superconductor are strongly proximity coupled and the induced pairs propagate easily in plane but not between planes. This work has been carried out in collaboration with the groups of Tai Chiang and Alexey Bezryadin, and done by Yang Bai, Can Zhang, David Floetotto and Andrew Murphy.
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