A complex arrangement of electronic states within materials can manifest exotic quantum-mechanical effects. These systems are often referred to as quantum materials. Increased understanding of quantum materials has historically lead to the development of new technologies. It is therefore extremely important to develop and test precise methods for calculating the behaviour of electronic states within a material. For decades, the workhorse of electronic structure calculations has been density functional theory (DFT). DFT is often referred to as a first-principles method because it allows for the calculation of the distribution of electrons throughout a material with only specification of the lattice geometry and atomic components. From the results of a DFT calculation, it is possible to study the orbital character of electronic wavefunctions, topology of electronic band structure, and some aspects of superconductivity. This provides insight into many quantum properties of a system which may otherwise be difficult or impossible to ascertain from experiments. DFT is, however, sometimes limited by the approximations necessary for practical implementation. Further methods have been developed to systematically correct the limitations of DFT. In particular, the combination of DFT with dynamical mean-field theory (DFT+DMFT) is among the most widely accepted methods for correcting the inadequacy of DFT in handling strong electron-electron correlations. In this thesis, I use methods from DFT and DFT+DMFT to study the quantum properties of materials. / Thesis / Master of Science (MSc)

This thesis aims to contribute to the understanding of quantum materials by employing a combination of experimental techniques, such as angle-resolved photoemission spectroscopy and magnetic and transport measurements. Further collaborative support in the form of first-principles calculations is included and discussed in tandem. In this thesis, a lanthanide-based semimetal of the ZrSiS type, is investigated. Multiple nodal lines which remain gapless are observed along the X-R direction of the Brillouin zone. We also present a nodal line that is observed further below the Fermi level and aligned in the G-M direction; this nodal line appears to be sensitive to light source polarization. A surface state at the X point is also observed. Finally, this thesis includes some discussion on the development of a sample growth laboratory, along with the presentation and characterization of grown Bi2Se3 samples. With potential applications in the fields of spintronics and novel microelectronic devices, the experimental realization and understanding of quantum materials is key to a deeper understanding of materials physics.

Topological insulators (TIs) are a class of quantum materials, which behave as insulators in the bulk, yet possess gapless spin-polarized surface states, which are robust against nonmagnetic impurities. The unique properties of TIs make them attractive not only for studying various fundamental phenomena in condensed matter and particle physics, but also as promising candidates for applications ranging from spintronics to quantum computation. Within the topological insulator realm, a great deal of focus has been placed on discovering new quantum materials, however, ideal multi-modal quantum materials have yet to be found. Here we study alpha-PdBi2, KFe2Te2, and DySb compounds including others within these families with high-resolution angle-resolved photoemission spectroscopy (ARPES) complimented by first principles calculations. We observe unique phase changes and phenomena across their transition temperatures. Our work paves a new direction in material discovery and application related to their unique electronic properties.

Thesis advisor: Ilija Zeljkovic / The splendid world of quantum materials is being unveiled in modern condensed matter physics, thanks to the advanced material synthesis methods, refined experimental probing techniques and deeper theoretical understanding. Unconventional superconductivity and topological phenomena are two of the main themes in this realm. Many outstanding problems are waiting to be solved and there is also a great potential in future technological applications. Among many routes of studying the quantum materials, creating thin film structures provides a special opportunity to learn the physical properties in low dimensions, to explore the effect of substrate and strain and to make novel electronic devices.In this thesis, I will present successful molecular beam epitaxy thin film synthesis of: (1) unconventional superconductor FeSe, (2) topological insulator Bi2Se3 doped with magnetic Fe atoms and (3) kagome structure magnets FeSn and Fe3Sn2. For (1), I will describe the finding of a dislocation network, its impact on the spatially-modulated strain field and its interesting interplay with the spontaneous symmetry-broken nematic phase. This is a new finding in the FeSe/SrTiO3 heterostructure and also provides fresh insights in the understandings of nematicity. For (2), I will show how we cross-check the doping ratio using different characterization techniques. Our observation indicates the possible formation of Fe clusters or impurity phases and sets the foundation for future synthesis of similar structures. For (3), I will demonstrate the novel selective synthesis of FexSny thin films. A plethora of spectral features were found in Fe3Sn2, implying a link with the Weyl physics. The FexSny thin films can potentially be a platform for the exploration of correlated, topological quantum phases in low dimensions. / Thesis (PhD) — Boston College, 2022. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

A Kitaev quantum spin liquid is a phase of matter predicted to host excitations that can be used to preform fault-tolerant quantum computation. Though the theoretical prediction of such a state is on firm footing, its realisation in real materials has proven to be elusive. Recent developments have suggested honeycomb materials consisting of 3d transition metal ions as possible candidates. The focus of this thesis is the magnetic properties of one such material, K2Ni2–xCoxTeO6. It is part of a family of layered two dimensional materials consisting of honeycomb structured transition metal layers sandwiched between layers of alkali ions. A characterisation of the magnetic properties of K2Ni2–xCoxTeO6 has been carried out with the techniques of muon spin rotation/relaxation/resonance and bulk magnetisation as a function of the chemical composition. Further investigations of the detailed atomic structure and spin order using neutron scattering was also initiated. The results of such characterisations are presented and discussed in this thesis. / En Kitaev kvantspinvätska är en fas av materia som har förespåtts kunna husera exciterade tillstånd som kan användas for att konstruera en kvantdator. Även om de teoretiska rönen är väl underbyggda, har ett förverkligande av en sådan fas i verkliga material varit svår att åstadkomma. Nya rön har pekat ut bikakematerial bestående av 3d övergångsmetaller som potentiella kandidater. Därav fokuserar denna avhandling på ett sådant material, K2Ni2–xCoxTeO6. Det är en del av en familj av liknande material bestående av tvådimensionella lager av bikakeformade övergångsmetaller mellan lager av alkaliska joner. En karaktärisering av de magnetiska egenskaperna av K2Ni2–xCoxTeO6 har utförts genom att analysera data från myon spin rotation/dämpning/resonans samt magnetiserings mätningar som funktion av materialets kemiska samansättning. Ytterligare mätningar av den atomära strukturen och spinordning påbörjades också med hjälp av neutronspridningstekniker. I denna avhandling presenteras och diskuteras resultaten av dessa karaktäriseringar.

In this thesis, I examine three very different solid-state systems that are all poor conductors when compared to elemental metals. The physics of canonical metals, such as the alkali and noble metals, is well known and is usually idealized in the free-or nearly free-electron picture. Their electron band structures are characterized by parabolic-like bands that cross the Fermi energy and possibly d-bands with flatter dispersions a few eV away. These well-behaved systems lend themselves to the use of simple analytic relations. Each of the three systems that I examine here differs significantly from the nearly-free parabolic band-picture of the electronic structure and require more complex analyses. In the first system of quasicrystals and approximants, we will discover that the electrons are undergoing anomalous diffusion depending on the size and symmetry of the lattices. Of course, as is well known, the details of these atomic lattice are what determine the nature of electronic band structures and how electrons may propagate in solids. In the second system, I find great agreement between my NbAs measurements and calculations on the closely related NbP compound. Incidental to this, I find that a reading of band structures shows that claims of measuring the linear band dispersion in Weyl/Dirac semimetals are not supported by the experimental and theoretical band structures. Finally, in the metallic regime of Nd_(1−x)TiO_3, we find that the Fermi liquid b coefficient is not within the bounds allowed by present models in samples with x = 0.2 and x = 0.15. It is suggested that the approximations used in current models may be why theory and experiment disagree. / Thesis / Doctor of Philosophy (PhD)

abstract: Two dimensional (2D) Janus Transition Metal Dichalcogenides (TMDs) are a new class of atomically thin polar materials. In these materials, the top and the bottom atomic layer are made of different chalcogen atoms. To date, several theoretical studies have shown that a broken mirror symmetry induces a colossal electrical field in these materials, which leads to unusual quantum properties. Despite these new properties, the current knowledge in their synthesis is limited only through two independent studies; both works rely on high-temperature processing techniques and are specific to only one type of 2D Janus material - MoSSe. Therefore, there is an urgent need for the development of a new synthesis method to (1) Extend the library of Janus class materials. (2) Improve the quality of 2D crystals. (3) Enable the synthesis of Janus heterostructures. The central hypothesis in this work is that the processing temperature of 2D Janus synthesis can be significantly lowered down to room temperatures by using reactive hydrogen and sulfur radicals while stripping off selenium atoms from the 2D surface. To test this hypothesis, a series of controlled growth studies were performed, and several complementary characterization techniques were used to establish a process–structure-property relationship. The results show that the newly proposed approach, namely Selective Epitaxy and Atomic Replacement (SEAR), is effective in reducing the growth temperature down to ambient conditions. The proposed technique benefits in achieving highly crystalline 2D Janus layers with an excellent optical response. Further studies herein show that this technique can form highly sophisticated lateral and vertical heterostructures of 2D Janus layers. Overall results establish an entirely new growth technique for 2D Janus.layers, which pave ways for the realization of exciting quantum effects in these materials such as Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state, Majorana fermions, and topological p-wave superconductors. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2020