Physics
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Item ZERO-BIAS CONDUCTANCE PEAKS IN MAJORANA NANOWIRES: THE GOOD, THE BAD, AND THE UGLY(2022) Pan, Haining; Das Sarma, Sankar S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Majorana zero modes are neutral zero-energy localized excitations emerging in the low-dimensional condensed matter systems, which are their own antiparticles. These excitations are topological with an intrinsic ground-state degeneracy, belonging to the (SU$_2$)$_2$ algebra and obeying the non-Abelian anyonic braiding statistics, which provides a possibility to implement the fault-tolerant topological quantum computing. As a result, enormous experimental efforts have focused on the realization of the Majorana zero modes, especially in the one-dimensional semiconductor-superconductor Majorana nanowires during the past decade. Although experiments have observed the zero-bias conductance peaks, these experimentally observed peaks are not robustly quantized as theoretically predicted for the signature of Majorana zero modes, and many other hallmarks of Majorana zero modes are yet to be unambiguously confirmed in experiments, which makes the experimentally observed zero-bias conductance peaks being interpreted as the Majorana zero modes questionable. Therefore, in this dissertation, we carry out a detailed theoretical analysis of the experimental results, and classify the experimentally observed zero-bias conductance peaks into three types: the good (i.e., the actual topological Majorana zero modes), the bad (i.e., which is the partially-separated quasi-Majorana modes induced by the inhomogeneous potential and quantum dot in the nanowire), and the ugly (i.e., which is the trivial low energy fermionic state induced by random disorder). Our study concludes that almost all the current experimentally observed zero-bias conductance peaks in the publications are the ugly zero-bias conductance peaks, and future experiments should focus on the improvement of the material quality to reduce disorder.Item Ferrocene-based molecular electronics and nanomanufacturing of Pd nanowires.(2007-11-27) Wang, Lixin; Sita, Lawrence; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two test structures were tried out for molecular junction formation and subsequent I-V characteristics measurements. One is formed by insertion of certain dithiol molecules into an alkanethiol self-assembled monolayer (SAM), followed by tethering the free thiol end with gold nanoparticles. The test structure can then be measured with CP-AFM. The matrix SAM, mixed monolayer with inserted dithiol molecules, and final test structure with gold nanoparticles were prepared and characterized by ellipsometry, AFM and STM. However, the CP-AFM measurements were very irreproducible, even on an alkanethiol SAM. This problem was analyzed and believed to come from two possible causes, namely thermal drift and deformation of the metalized tips. The other test structure was from insertion of molecules into nanogaps made by electromigration technique. Two molecules were tested and drastically different properties were observed from junctions with each molecule. For Fc-OPE molecules, near perfect conductance peaks (>0.6G0) were observed in some junctions and analysis indicates that such molecular junction contains only one or two molecules inside the nanogap. The formation of conductance peaks was analyzed with Landauer formula and a simple metal-molecule-metal model. Computational calculation also predicted high conductance through such junctions and the existence of resonant peaks. The junctions with OPE molecules, however, showed poor conductance. Possible causes such as molecular structure and easiness of molecular junction formation were discussed. In the second part of this dissertation, a new method was developed to fabricate Pd nanowires on HOPG surface using a sacrificial Cu film. The morphology and composition of the nanowires were characterized by AFM, SEM and XPS. The formation of such Pd nanowires was explained with a galvanic displacement mechanism and some test experiments were carried out to prove such growth mechanism. It was also found that the size of the Pd nanowires can be directly controlled by the thickness of the Cu film that was initially deposited. However, attempts to make Au, Pt and Ag nanowires with the same method failed, and possible causes were discussed.