Spring 2016

PQI Seminar Guen Prawiro-Atmodjio (University of Copenhagen): Pair Tunneling with Negative U at LAO/STO Complex Oxide Interface

The conducting interface at the LaAlO3/SrTiO3 (LAO/STO) interface has sparked large interest due to its many coexisting properties. Interfaces based on non-crystalline top layers show similar characteristics to their crystalline equivalent. Recently, we reported superconductivity in patterned devices of the disordered-LAO/STO interface where strong evidence was found for a superconducting phase originating from a Josephson-coupled array of superconducting puddles, possibly induced by inhomogeneous doping. We studied this system with nano-patterned gates to form a nano-scale constriction in a Hall bar, in which we were able to pinch off current flow. Close to pinch-off, a dot regime is formed with peaks of conductivity, that form diamonds of suppressed current when applying a bias voltage across the constriction. The conductance peaks split up when applying a large in-plane or out-of-plane magnetic field, indicating pairing of electrons without superconductivity (as reported in Cheng et al., Nature 2015). We study the system as a function of gate voltage, bias voltage, magnetic field and temperature and compare with a negative-U model that describes the behavior of pair tunneling, which differs from the conventional Coulomb-blockade scenario.

Katja Nowack (Cornell University): Imaging Current in Two-Dimensional Topological Insulators

The current in a two-dimensional topological insulator (2D TI) is expected to be carried by edge channels, which so far is only evidenced by transport measurements. At Stanford we have imaged the current density in HgTe quantum well as well as in InAs/GaSb quantum wells, the first and second experimentally realized TIs, and directly observed that current flows in the edges. The measurement principle is shown in the cartoon. A current (blue) flows at the edge of a device (green), producing a magnetic field (red) that is measured by detecting the flux through the SQUID’s  pickup loop (white). For a two-dimensional current density we can reconstruct the current distribution from the flux image. An example is shown on the right. The normalized x-component of the current density when the Fermi level in the device is tuned into the bulk energy gap. shows that the current flow along the edge of the device.

 

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