Physics Commons^™

Developing Effective Theories: A Case Study In Monolayer Iron Selenide, Joseph O'Halloran

Theses and Dissertations

In this work, we outline the development of a series of models which have allowed us to investigate the low-temperature phases of monolayer FeSe. These models are built using group theoretic arguments to ensure the constraints provided by the crystal symmetry are satisfied. We investigate the interplay between stripe antiferromagnetism and spin-vortex crystal order in the framework of the Landau theory of phase transitions. By considering inversion symmetry breaking terms in the Landau free-energy, we show that the spin-vortex crystal is preferred when the non-symmorphic parent symmetry group $P4/nmm$ is reduced to a symmorphic subgroup.

We also consider symmetry constraints …

Magnesium Diboride Embedded Multi-Walled Carbon Nanotube Yarns, Ujjal Lamichhane

Theses and Dissertations

The discovery of superconductivity in a simple hexagonal binary compound of MgB2 at a relatively high critical temperature ~39 K renewed the interest in the field of superconductivity. Twisted laminar superconducting composite structures based on coiled multi-walled carbon nanotube (MWCNT) yarns were crafted by integrating magnesium and boron homogeneous mixture into the carbon nanotube sheets. After the ignition of the Mg/B/MWCNT system, under the controlled argon environment and uniform heating rate, the high exothermic reaction between magnesium (Mg) and boron (B) with stoichiometric ratio produced the MgB2/MWCNT superconducting composite yarns. The XRD analysis confirmed that the heated sample consists of …

Current Induced Depairing And Mixed State Behavior In The Bite/Fete Interfacial Superconductor, Charles Louis Dean Ii

Theses and Dissertations

The Bi2Te3/FeTe heterostructure intersects several phenomena and key classes of materials in condensed matter physics: topological insulators, superconductivity, magnetism, and the physics of interfaces. While neither the topological insulator (BiTe) nor the iron chalcogenide (FeTe) are themselves superconductors, superconductivity forms in a thin 7nm interfacial layer between the two. The restricted dimensionality and the extraordinarily conductive normal state, possibly sourced by the topologically protected surface states, have led to the observation of novel phenomena such as the Likharev vortex explosion and transitions in behavior resulting from the interplay between current induced depairing and the Berezinski-Kosterlitz-Thouless regime. The measured depairing current …

Depairing Current Density In Cuprate Superconductors, Manlai Liang

Theses and Dissertations

We report a study of the depairing critical current density in the electron-doped infinite-layer Sr_1-xLa_xCuO₂ and Nd_2-xCe_xCuO_4-\delta system. In this work, many new apparatuses were set up to measure the samples. Resistance-versus-temperature transition curves measured at high pulsed current densities show the classic proportionality between the transition-temperature shift and the two-thirds power of the applied current. The measurement provides an assessment of the superfluid density in the system, and a secondary route for obtaining the penetration depth purely through a transport measurements.