Low Energy States in Hybrid Semiconductor-Superconductor Majorana DevicesYu, Peng (2020) Low Energy States in Hybrid Semiconductor-Superconductor Majorana Devices. Doctoral Dissertation, University of Pittsburgh. (Unpublished)
AbstractMajorana zero-energy modes (MZMs), which comprise an equal superposition of electrons and holes, are predicated to emerge as quasiparticles in condensed matter systems. MZMs has generated significant interest in recent years, mostly because they are anticipated to obey non-Abelian statistics and thus can be potentially exploited in topological quantum computation. Among various proposals for realizing MZMs, semiconductor nanowires with strong spin-orbit coupling and proximity induced superconductivity is a promising platform. Although several experiments have already reported the possible signature of MZMs in such a system, a conclusive proof of MZMs is still an ongoing effort. Moreover, none of them verified the long-standing prediction that MZMs should emerge in pairs with one at each end of a topological region. In this thesis, we first improved the required ingredients for generating MZMs. By optimizing the nanowire-superconductor interfaces as well as the NbTiN superconducting films, we achieved hard induced gaps and ballistic transport in InSb nanowires. We also optimized local bottom gates aimed to achieve better chemical potential control in nanowires. With those improvements, in a two-terminal device, we found zero bias conductance peaks (ZBCPs) in agreement with the Majorana theories. We also mapped out a phase diagram of the ZBCPs in the magnetic field and chemical potential space. While this data favors the Majorana origin of the ZBCPs, non-Majorana ZBCPs emerge as ubiquitous features in similar devices. Due to the similarities between these ZBCPs, we conclude it is impractical to unambiguously prove MZMs in a two-terminal geometry. In a three-terminal geometry, we gain the ability to probe the two ends of the nanowire-superconductor hybrid region by adding one more normal lead. We identified delocalized states near zero field, which emerged with correlated gate dependence on both ends. While the correlation between the ZBCPs on both ends at finite fields was not established, we demonstrated three-terminal geometry is a powerful method of diagnosing localization of wavefunctions. Future experiments can thus use this method to identify MZMs from localized trivial states. Once MZMs can be deterministically established, braiding experiments in nanowire networks could open the gate to topological quantum computation. Share
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