Tunneling Spectroscopy of Quasiparticle Bound States in a Spinful Josephson Junction

The spectrum of a segment of InAs nanowire, confined between two superconducting leads, was measured as function of gate voltage and superconducting phase difference using a third normal-metal tunnel probe. Sub-gap resonances for odd electron occupancy---interpreted as bound states involving a confined electron and a quasiparticle from the superconducting leads, reminiscent of Yu-Shiba-Rusinov states---evolve into Kondo-related resonances at higher magnetic fields. An additional zero bias peak of unknown origin is observed to coexist with the quasiparticle bound states.Comment: Supplementary information available here: https://dl.dropbox.com/u/1742676/Chang_Sup.pd

Small-World Networks: Links with long-tailed distributions

Small-world networks (SWN), obtained by randomly adding to a regular structure additional links (AL), are of current interest. In this article we explore (based on physical models) a new variant of SWN, in which the probability of realizing an AL depends on the chemical distance between the connected sites. We assume a power-law probability distribution and study random walkers on the network, focussing especially on their probability of being at the origin. We connect the results to L\'evy Flights, which follow from a mean field variant of our model.Comment: 11 pages, 4 figures, to appear in Phys.Rev.

Superconductivity-enhanced bias spectroscopy in carbon nanotube quantum dots

We study low-temperature transport through carbon nanotube quantum dots in the Coulomb blockade regime coupled to niobium-based superconducting leads. We observe pronounced conductance peaks at finite source-drain bias, which we ascribe to elastic and inelastic cotunneling processes enhanced by the coherence peaks in the density of states of the superconducting leads. The inelastic cotunneling lines display a marked dependence on the applied gate voltage which we relate to different tunneling-renormalizations of the two subbands in the nanotube. Finally, we discuss the origin of an especially pronounced sub-gap structure observed in every fourth Coulomb diamond

Stable Equilibrium Based on L\'evy Statistics: Stochastic Collision Models Approach

We investigate equilibrium properties of two very different stochastic collision models: (i) the Rayleigh particle and (ii) the driven Maxwell gas. For both models the equilibrium velocity distribution is a L\'evy distribution, the Maxwell distribution being a special case. We show how these models are related to fractional kinetic equations. Our work demonstrates that a stable power-law equilibrium, which is independent of details of the underlying models, is a natural generalization of Maxwell's velocity distribution.Comment: PRE Rapid Communication (in press

Transport signatures of quasiparticle poisoning in a Majorana island

We investigate effects of quasiparticle poisoning in a Majorana island with strong tunnel coupling to normal-metal leads. In addition to the main Coulomb blockade diamonds, "shadow" diamonds appear, shifted by 1e in gate voltage, consistent with transport through an excited (poisoned) state of the island. Comparison to a simple model yields an estimate of parity lifetime for the strongly coupled island (~ 1 {\mu}s) and sets a bound for a weakly coupled island (> 10 {\mu}s). Fluctuations in the gate-voltage spacing of Coulomb peaks at high field, reflecting Majorana hybridization, are enhanced by the reduced lever arm at strong coupling. In energy units, fluctuations are consistent with previous measurements.Comment: includes supplementary materia

Viral Targeting of Host Hubs: Interactions between the Rabies Virus and LC8

As obligate intracellular parasites, viruses rely on host machinery for their own reproduction. Viruses are therefore required to interact with a wide variety of host proteins, despite limitations in viral genome size. The most parsimonious method of success is to hijack central and essential proteins, known as hub proteins. LC8 is a notable example of a hub protein, and has been shown to interact with more than 100 eukaryotic partners in various unrelated pathways. An increasing number of studies have noted interactions between LC8 and viruses, including the Ebola, rabies, and rotaviruses; however, LC8’s role within these systems is unclear. This thesis examines the structural and functional properties of hub proteins, with a particular focus on LC8 and its interactions with the rabies virus. Three chapters of original work include two primary research reports and one review/opinion piece. The first research report, Chapter 2, structurally characterizes the interaction between LC8 and the rabies virus phosphoprotein (RavP). We use insights gleaned from our structural studies to predict and test potential roles for LC8 in the rabies virus infection cycle, and demonstrate that LC8 is important for efficient viral polymerase activity. Chapter 3 is an in-depth study of the LC8 recognition motif, where we examine both the structural and functional plasticity of LC8, and identify and validate many new LC8 interactions. We also develop a tool that can be used to predict LC8 binding motifs in proteins of interest, which will greatly improve the ability of those outside of the LC8 field to recognize, test, and validate partner proteins. In Chapter 4 we present a variety of new ideas about what qualities describe a linear motif-binding hub protein. This work provides ideological and linguistic suggestions for important structural and functional features of hubs that underlie their plasticity. It then further describes how viruses take advantage of these features to more efficiently hijack host pathways. Finally, Chapter 5 discusses the impacts of my thesis work, and outlines some potential future projects. Each individual chapter builds on the previous one to create an expanding view of the importance of linear motif-binding hubs for infectious viruses