Physics

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    Multi-terminal Josephson effect
    (2021) Pankratova, Natalia; Manucharyan, Vladimir E; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Conventionally, a Josephson junction is an ubiquitous quantum device formed by a weak link between a pair of superconductors. In this work, we demonstrate the dc Josephson effect in mesoscopic junctions of more than two superconducting terminals. We report fabrication and characterization of the 3- and 4-terminal Josephson junctions built in a top-down fashion from hybrid semiconductor-superconductor InAs/Al epitaxial heterostructures. In general, the critical current of an N-terminal junction is an (N-1)-dimensional hypersurface in the space of bias currents, which can be reduced to a set of critical current contours (CCCs). The CCC is a key ground state characteristic of a multi-terminal Josephson junction, which is readily available from regular electron transport measurements. We investigate nontrivial modifications of the CCC's geometry in response to electrical gating, magnetic field, and phase bias. All observed effects are described by the scattering formulation of the Josephson effect generalized to the case of N>2. Our observations indicate superconducting phase coherence between all the terminals which establishes the Josephson effect in mesoscopic junctions of more than two superconductors. Such multi-terminal junctions could find their applications in a broad range of fields from topologically protected quantum computation to quantum metrology and others.
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    Electronic Transport in Dirac Materials:Graphene and a Topological Insulator(Bi2Se3)
    (2011) Cho, Sungjae; Fuhrer, Michael S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Materials with Dirac electronic spectra ("Dirac materials") have attracted much interest since the first successful electronic transport measurement in graphene in 2004. Dirac quasiparticles have novel physical properties such as absence of backscattering and Klein tunneling. Topological insulators are a more recently discovered class of materials that have a bulk band gap and gapless edge/surface states. The surface state in 3D topological insulators has a Dirac electronic spectrum like graphene, but is singly spin-degenerate, with spin-momentum locking. This thesis will describe electronic transport experiments in graphene and in Bi2Se3 ultrathin films, which are predicted to be either 2D topological insulators or conventional insulators. The basic quantum physics of a particle confined in a box is demonstrated using electrons in single and bilayer graphene as examples of massless and massive 2D Fermions, respectively. Ballistic metal-graphene-metal devices act as Fabry- Pérot cavities for electrons, and resonant states of the Fabry-Pérot cavity observed in electronic transport are used to measure the density of states as a function of particle number for massless and massive 2D Fermions. Nonlocal spin-valve experiments are demonstrated up to room temperature in mesoscopic graphene contacted by ferromagnetic electrodes. At low temperature the spin-valve signal shows changes in magnitude and sign with back-gate voltage, which may also result from quantum-coherent transport through Fabry Pérot cavities. The temperature- and magnetic-field-dependent longitudinal (ρxx) and Hall(ρxy) components of the resistivity of graphene were measured. Near the minimum conductivity point ρxx(H) is strongly enhanced and ρxy(H) is suppressed, indicating nearly equal electron and hole contributions to the current. The data are inconsistent with the standard two-fluid model, but consistent with the prediction for inhomogeneously distributed electron and hole regions of equal mobility. Ultrathin three quintuple layer (3QL) Bi2Se3 field effect transistors (FETs) were fabricated by mechanical exfoliation on 300 nm SiO2/Si susbtrates. Temperature and gate-voltage-dependent conductance measurements show a clear OFF state at negative gate voltage, with activated temperature-dependent conductance and energy barriers up to 250 meV, implying that 3QL-Bi2Se3 films are conventional insulators rather than 2D topological insulators, likely due to coupling of the topological surface states through the thin bulk.