Superconductivity and Vortex Dynamics in Nanostructures of Two-dimensional Crystals of Niobium Diselenide

Download or read book Superconductivity and Vortex Dynamics in Nanostructures of Two-dimensional Crystals of Niobium Diselenide written by Shaun Mills and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The confinement offered by superconducting nanostructures enables the study of the motion of individual Abrikosov vortices in reduced dimensions. At the present time, most superconducting nanostructures are fabricated from deposited films of metals, which are often polycrystalline or amorphous. The realization of a single-crystal nanostructure would provide the opportunity to explore the effects of the electronic band structure. Consider crystalline superconducting nanostructures of NbSe2. Questions such as the how the charge density wave order influences vortex pinning and the logarithmic vortex-vortex interaction, as well as how the crystal thickness dependence of the electronic band structure affects intrinsic vortex properties motivated our efforts to develop techniques to fabricate and measure superconducting nanostructures of 2D crystal NbSe2. Additionally, nanoscale transport devices are relevant to potential future superconducting electronics.In this dissertation, we begin by presenting a novel technique for the preparation of single-crystal nanostructures prepared from mechanically exfoliated few-layer crystals of NbSe2 using a process combining electron beam lithography and reactive plasma etching. Our technique allows for the preparation of ultra-thin, single-crystal superconducting nanostructures with a desired geometry for the study of vortex dynamics in extremely confined systems. A primary advantage of transport devices is the ability to directly manipulate individual vortices through the use of an applied current and other parameters.We first present magnetoresistance measurements on NbSe2 nanowires and show features related to vortex crossing, trapping, and pinning. The vortex crossing rate is found to vary non-monotonically with the applied field, which results in non-monotonic magnetoresistance variations in agreement with theoretical calculations in the London approximation. Above the lower critical field, Hc1, the crossing rate is also influenced by vortices trapped by sample boundaries or pinning centers, leading to sample-specific magnetoresistance patterns. We show that the local pinning potential can be modified by intentionally introducing surface adsorbates, making the magnetoresistance pattern a "magneto fingerprint" of the sample-specific configuration of vortex pinning centers in a superconducting nanowire.Building upon our work on NbSe2 nanowires, we next focus on NbSe2 nanoloops. The doubly-connected topology of nanoloops presents a unique opportunity for the manipulation of Abrikosov vortices; numerical calculations in the London limit suggest that an Abrikosov vortex can be trapped in a nanoloop above a critical magnetic field and generate a phase shift in the magnetoresistance oscillations. We measure magnetoresistance oscillations resulting from vortex crossing events in NbSe2 nanoloops and demonstrate experimentally that the crossing of vortices can be directed at a pair of constrictions in the loop, leading to more pronounced magnetoresistance oscillations than those in a uniform loop. We also observe for the first time a phase shift in the magnetoresistance oscillations resulting from vortex trapping in the nanoloop, and we manipulate the trapped vortex with an applied transport current. Results obtained in our NbSe2 devices provide a starting point for the manipulation of individual Abrikosov vortices, allowing further exploration of the fundamental properties of this topological object that will lead to a deeper understanding of the motion, including the quantum motion, of vortices. Longstanding, unresolved issues such as the Aharonov-Casher quantum interference of Abrikosov vortices may be solved.