Molecular Beam Epitaxy Synthesis and Nanoscale Characterization of Topological Insulator Thin Films and Their Interface With High-temperature Superconductors:

Thesis advisor: Ilija ZeljkovicThe discovery of topological phases has ushered in an era of new materials with exotic electronicproperties; one particular area of excitement is realizing and studying topologically superconducting systems. These topological superconductors are theorized to host exotic excitations that can be applied towards making fault tolerant quantum computations. One way to achieve this is depositing thin films of topological insulators onto superconducting substrates. Molecular beam epitaxy offers precise control for fabricating thin film heterostructures down to the single layer limits. In this thesis I will present my work on the synthesis of thin film topological insulators grown epitaxially on both an iron based superconductor FeT e0.55Se0.45 as well as a cuprate superconductor Bi2Sr2CaCu2Ox+8. Additionally I will cover the scanning tunneling microscopy/spectroscopy characterization of the emergent phenomena on the surface as well as at the interface of these heterostructures. This work presents a viable platform for exploring the emergence of superconductivity in topologically insulating materials, as well as demonstrates the importance of a clean interface.Thesis (PhD) — Boston College, 2022.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Physics

Atomic-Scale Strain Manipulation of a Charge Density Wave

A charge density wave (CDW) is one of the fundamental instabilities of the Fermi surface occurring in a wide range of quantum materials. In dimensions higher than one, where Fermi surface nesting can play only a limited role, the selection of the particular wave vector and geometry of an emerging CDW should in principle be susceptible to controllable manipulation. In this work, we implement a simple method for straining materials compatible with low-temperature scanning tunneling microscopy/spectroscopy (STM/S), and use it to strain-engineer new CDWs in 2H-NbSe2. Our STM/S measurements combined with theory reveal how small strain-induced changes in the electronic band structure and phonon dispersion lead to dramatic changes in the CDW ordering wave vector and geometry. Our work unveils the microscopic mechanism of a CDW formation in this system, and can serve as a general tool compatible with a range of spectroscopic techniques to engineer novel electronic states in any material where local strain or lattice symmetry breaking plays a role.Comment: to appear in PNAS (2018

Plethora of tunable Weyl fermions in kagome magnet Fe3Sn2 thin films

Interplay of magnetism and electronic band topology in unconventional magnets enables the creation and fine control of novel electronic phenomena. In this work, we use scanning tunneling microscopy and spectroscopy to study thin films of a prototypical kagome magnet Fe3Sn2. Our experiments reveal an unusually large number of densely-spaced spectroscopic features straddling the Fermi level. These are consistent with signatures of low-energy Weyl fermions and associated topological Fermi arc surface states predicted by theory. By measuring their response as a function of magnetic field, we discover a pronounced evolution in energy tied to the magnetization direction. Electron scattering and interference imaging further demonstrates the tunable nature of a subset of related electronic states. Our experiments provide the first visualization of how in-situ spin reorientation drives changes in the electronic density of states of the Weyl fermion band structure. Combined with previous reports of massive Dirac fermions, flat bands and electronic nematicity, our work establishes Fe3Sn2 as a unique platform that harbors an extraordinarily wide array of topological and correlated electron phenomena

Plethora of tunable Weyl fermions in kagome magnet Fe3Sn2 thin films

Interplay of magnetism and electronic band topology in unconventional magnets enables the creation and fine control of novel electronic phenomena. In this work, we use scanning tunneling microscopy and spectroscopy to study thin films of a prototypical kagome magnet Fe3Sn2. Our experiments reveal an unusually large number of densely-spaced spectroscopic features straddling the Fermi level. These are consistent with signatures of low-energy Weyl fermions and associated topological Fermi arc surface states predicted by theory. By measuring their response as a function of magnetic field, we discover a pronounced evolution in energy tied to the magnetization direction. Electron scattering and interference imaging further demonstrates the tunable nature of a subset of related electronic states. Our experiments provide a direct visualization of how in-situ spin reorientation drives changes in the electronic density of states of the Weyl fermion band structure. Combined with previous reports of massive Dirac fermions, flat bands, and electronic nematicity, our work establishes Fe3Sn2 as an interesting platform that harbors an extraordinarily wide array of topological and correlated electron phenomena