Thesis advisor: Kenneth S. Burch / In condensed matter physics we study the behavior of crystals at finite density and low temperatures. By tuning and breaking the various materials, symmetries, and the topology of a crystal one can bring about brand new quantum phases of matter. These new phases of matter in turn produce emergent quasiparticles such as the cooper pair in superconductivity, the spinon in magnetic systems, and the Fermi arcs in Weyl semimetals. Of particular interest are systems in which superconductivity interacts with topology. These systems have been theoretically predicted to produce anyonic quasiparticles which may be used as qubits in a future fault-tolerant quantum computer. However, these ideas usually require the use of the superconducting proximity effect to inject cooper pairs into the topological system. This in turn requires interfacing two different materials which not only requires extremely clean interfaces, but also matching Fermi surfaces, comparable Fermi velocities, and more. The ideal candidate for topological superconductivity would therefore be a material that is both superconducting and topologically non-trivial. One promising candidate is the iron-based superconductor FeTe(1−x)Sex, specifically at the FeTe0.55Se0.45 (FTS) doping which also has non-trivial topology. In this dissertation, we address the fabrication of pristine interfaces using a new tool as well as new probes into the topology of FTS. In Chapter II we discuss the motivation, construction, and use of the “cleanroomin-a-glovebox”. This tool places an entire nanofabrication workflow into an inert argon atmosphere which has allowed us access to study a myriad of new materials and systems. A delightful offshoot of this glovebox is that it is a useful tool in training new scientists in fabrication techniques. The photolithography, Physical Vapor Deposition (PVD), and characterization tools in the glovebox are designed to be easy to use and thus afford new users a low-risk method of learning new techniques. In chapter III we discuss a specific example of a new quantum phase of matter e.g. topological superconductivity in FTS. There, I discuss the fabrication requirements to probe this elusive phase as well as the unique measurement technique used to provide evidence that FTS is a higher-order topological superconductor. The characterization of FTS continues in Chapter IV where we reveal some exciting new results in the FTS system. These new results are direct evidence for the topological nature of FTS, a feat which has only been shown in Angle-Resolved Photo Emission Spectroscopy (ARPES) and Scanning Tunneling Microscopy (STM). Chapter V concludes the dissertation with a summary of Chapters II, III, and IV. In addition, we give suggestions for future experiments to investigate the FTS system further as well as suggestions for insightful teaching programs with the cleanroom-in-a-glovebox. / Thesis (PhD) — Boston College, 2021. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Research into molecular electronics has exploded in recent years due to a proliferation of new and exciting techniques for producing atomic level structures (e-beam lithography, self-assembled monolayers, etc.); coupling these techniques with the ability to accurately manipulate atomic systems (such as with Scanning Tunneling Microscopes (STM), Atomic Force Microscopes (AFM), or Mechanically Controllable Break Junctions (MCBJ)) opens the possibility to create novel quantum coherent devices for both engineering applications, as well as research into fundamental physics. Along these lines, presented here is a series of experiments on superconducting point contacts which were aimed at understanding the dynamics of coupling superconducting effects to the mechanical degrees of freedom of a nanowire. In addition, another series of experiments presented here explore the nature of charge transport at high biases in superconducting point contacts. Specifically, an investigation of point contacts at high voltage biases revealed a suppression of one component of the total current, which is explained through a phenomenological model.

Thesis (M.S.)--Florida State University, 2006. / Advisor Juan C. Ordonez, Florida State University, College of Engineering, Dept. of Mechanical Engineering. Title and description from dissertation home page (viewed Sept. 19, 2006). Document formatted into pages; contains x, 267 pages. Includes bibliographical references.

Thesis advisor: Kevin Bedell / Over 20 years ago, Blagoev et. al. predicted an s-wave pairing instability in a ferromagnetic Fermi liquid (FFL) as a consequence of spin fluctuations [5]. Shortly after, it was discovered that, when magnetic interactions in the ferromagnetic superconductor UGe2 dominate, quasiparticles with parallel spin form pairs in odd-parity orbitals; i.e., a form of spin-triplet p-wave superconductivity emerges, in contrast to Blagoev et. al.'s prediction [6]. In this work, we return to this issue by introducing the effects of a gapped amplitude (or "Higgs") mode on the vertex corrections and subsequent form of Cooper pairing. As the Higgs mode only propagates in the presence of a finite spin current, such an amplitude mode results in strong momentum-dependence in the many-body vertex. This results in the emergence of an unconventional form of superconductivity mediated by unconventional low-energy modes in a weak itinerant ferromagnet. / Thesis (BS) — Boston College, 2021. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Scholar of the College. / Discipline: Physics. / Discipline: Mathematics.

This thesis investigates the many extraordinary physical properties of the candidate p-wave ferromagnetic superconductors UCoGe and URhGe, and proposes theoretical predictions for p-wave superconductors yet to be discovered. In particular, we carry out angular dependent quantum field theoretical calculations of the thermodynamic H - T phase diagram known as the upper critical field, or more appropriately for ferromagnetic superconductors the upper critical induction, for various p-wave superconducting order parameter symmetries including: The axial Anderson-Brinkman-Morel(ABM) state, the chiral Scharnberg-Klemm (SK) state, and the completely broken symmetry polar state (CBS), as well as for some other states with partially broken symmetry (PBS) superconducting order parameter symmetries. The most notable contribution of the work presented in this thesis is the application of the Klemm-Clem transformations to analytically calculate the full angular and temperature dependencies of the upper critical field for orthorhombic materials, which may prove to be useful to experimentalists in identifying these exotic states of matter experimentally. Second, this work formulates a double spin-split ellipsoidal Fermi surface (FS) model for ferromagnetic superconductors in the normal state, which introduces a field dependence to the effective mass in one crystallographic direction on the dominant Fermi surface and to the chemical potential, and is subsequently applied to the normal state of URhGe to explain theoretically the anomalous specific heat data of Aoki and Flouquet. Extension of this work to understanding the still elusive reentrant high-field superconducting phase of URhGe and the S-shaped upper critical field curve for external magnetic field parallel to the b-axis direction inUCoGe is discussed. Third, this work also presents theoretical fits to the upper critical field data of Kittika et al. for Sr2RuO4 using the helical p-wave states and including Pauli limiting effects of the three components of the triplet pair-spin fixed to the highly conducting layers by strong spin-orbit coupling.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

No description available.

No description available.

A model for the Josephson junction is constructed based on two macroscopic angular momentum vectors. These vectors, which interact via a Heisenberg-like Hamiltonian, are defined using Anderson's pseudospin concept in superconductivity. Along with this, a new state vector, which affords a more complete description of the constant-charge-imbalance mode of the junction, is explicitly constructed. The resulting equations of motion lead directly to the basic Josephson results and at the same time provide a simple physical picture for the dynamical behavior of the junction. Both the Anderson (n,(phi)) and Feynman two-state models of the junction are shown to be equivalent to a restricted form of the angular momentum approach. The process of formulating the junction problem in terms of pseudo-angular-momentum together with the above identification constitutes a microscopic derivation of the Feynman method. A perturbation theory calculation is carried out within the full pseudo-angular-momentum equations of motion to determine how this approach differs from the earlier ones.