UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item Supercurrent and Andreev bound states in multi-terminal Josephson junctions(2022) Lee, Hanho; Manucharyan, Vladimir; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A Josephson junction (JJ) is known as a weak link connecting two superconductors, in which the non-dissipative supercurrent flows. More than two superconductors also can form a single composite JJ, called a “multi-terminal JJ”, by being connected through a common weak link. The supercurrent in multi-terminal JJs may depend on multiple superconducting phase differences defined across the junction. The multi-phase-dependence of the supercurrent is attributed to the sub-gap quasiparticle bound states, called Andreev bound states (ABSs), which carry the supercurrent across the junction. First, we investigate the supercurrent of three- and four-terminal JJs fabricated on hybrid two-dimensional Al/InAs (superconductor/semiconductor) heterostructures. The critical current of an N-terminaljunction is given as a (N-1)-dimensional hypersurface of the DC bias currents, which can be reduced to a set of critical current contours (CCCs) in low dimensional space. Non-trivial modifications of the geometry of the CCCs in response to magnetic field, electrical gating and phase biasing can be understood in the presence of the multi-phase-dependent ABSs. Second, we demonstrate the multi-phase-dependent ABSs in three-terminal JJs by tunneling spectroscopy measurements. Multi-loop superconducting quantum interference devices (SQUIDs) are realized to detect the multi-phase-dependence. The ABS energy spectrum mimics electronic band structure in solid, which makes multi-terminal JJs provide a new platform to study band topology in higher dimensional parameter space. Moreover, spin-splitting of ABS energies induced by the multi-phase and gapless energy spectrum facilitated by the presence of a discrete vortex, a nonzero winding of the superconducting phases, are investigated.Item MAJORANA AND ANDREEV BOUND STATES IN SEMICONDUCTOR-SUPERCONDUCTOR NANOSTRUCTURES(2018) Liu, Chun-Xiao; Sau, Jay D; Das Sarma, Sankar; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Majorana bound states have been a topic of active research over the last decades. In the perspective of theoretical physics, Majorana bound states, which are their own antiparticles, are zero-energy quasi-particle excitations in exotic superconductive systems. From a technological perspective, Majorana bound states can be utilized for the implementation of fault-tolerant quantum computation due to their topological properties. For example, two well-separated Majorana bound states can form a fermionic qubit state, the quantum information of its occupancy is stored in a nonlocal way, being robust against local decoherence. Also since Majorana bound states obey non-Abelian statistics, quantum gates can be implemented by braiding. Such gate operations are robust because small deviations in braiding trajectories do not affect the braiding results. So far the most promising platform for the realization of Majorana bound states is the one-dimensional semiconductor-superconductor nanostructures. The hallmark of the existence of Majorana bound states in such systems is a quantized zero-bias conductance peak in the tunneling spectroscopy for a normal-metal-superconductor junction. Although quantized zero-bias conductance peaks that resemble the theoretical prediction have been observed in several experimental measurements, confusing aspects of the data muddy the conclusion. One source of confusion results from the existence of another type of excitation in these systems, i.e., the topologically trivial near-zero-energy Andreev bound states. These excitations mimic many behaviors of the topological Majorana bound states. In this thesis, we first investigate the tunnel spectrsocopy signatures of both Majorana and Andreev bound states. Then we discuss multiple proposals for differentiating between Majorana and Andreev bound states. In Chapter 1, we give an overview for Majorana bound states in the context of both spinless p-wave superconductors and spin-orbit coupled nanowires in proximity with an s-wave superconductor. We also show how the existence of a zero-energy Majorana bound state leads to a quantized zero-bias conductance peak in tunneling spectroscopy at zero temperature. In Chapter 2, we discuss possible physical mechanisms responsible for the discrepancy between minimal theory of Majorana nanowire and real experimental observations. Specifically, we focus on the effect of dissipation inside the heterostructure. In Chapter 3, we show that a near-zero-energy Andreev bound state may arise quite generically in the semiconductor-superconductor nanowire in the presence of a smooth variation in chemical potential. Although such Andreev bound states are topologically trivial, they mimic the behaviors of the topological Majorana bound states in many aspects. In Chapter 4, we discuss multiple proposals for distinguishing between trivial Andreev bound states and topological Majorana bound states in the normal-metal-superconductor junction. In Chapter 5, we discuss a proposal for future experiments, i.e., a normal-superconductor-normal junction for a Coulomb blockaded superconductor. In this proposal, one can directly measure the topological invariant of the bulk superconductor. Finally Chapter 6 concludes the thesis.