This work presents first-principles calculations of long-range intermolecular dispersion energies between two atoms or molecules as expressed in terms of the C6 dipole-dipole dispersion coefficients. In a series of publications, it has been shown by us that the complex linear polarization propagator method provides accurate ab initio and first-principles density functional theory values of the C6 dispersion coefficients in comparison with those reported in the literature. The selected samples for the investigation of dispersion interactions in the electronic ground state are the noble gases, n-alkanes, polyacenes, azabenzenes, and C60. It has been shown that the proposed method can also be used to determine dispersion energies for species in their respective excited electronic states. The C6 dispersion coefficients for the first π → π* excited state of the azabenzene molecules have been obtained with the adopted method in the multiconfiguration self-consistent field approximation. The dispersion energy of the π → π* excited state is smaller r than that of the ground state. It is found that the characteristic frequencies ω1 defined in the London approximation of n-alkanes vary in a narrow range and that makes it possible to construct a simple structure-to-property relation based on the number of -bonds for the dispersion interaction in these saturated compounds. However, this simple approach is not applicable for the interactions of the π-conjugated systems since their characteristic frequencies ω1 vary strongly depending on the systems. / <p>Report code: LIU-TEK-LIC-2006:2</p>

Calculations were performed on transition-metal complexes to (1) extrapolate the structure and bonding of the ground and phosphorescent states (2) determine the luminescence energies and (3) assist in difficult assignment of luminescent transitions. In the [Pt(SCN)4]2- complex, calculations determined that the major excited-state distortion is derived from a b2g bending mode rather than from the a1g symmetric stretching mode previously reported in the literature. Tuning of excimer formation was explained in the [Au(SCN)2]22- by interactions with the counterion. Weak bonding interactions and luminescent transitions were explained by calculation of Hg dimers, excimers and exciplexes formed with noble gases.

The evolution of electronics has become the staple thrust of modern scientific innovation: a need for advancing materials engineered for our equally rapidly advancing needs and computing requirements has fueled recent wealth of new materials. Here, I use the ideals of exotic materials design to answer this need, specifically for 2D materials. Two-dimensional (2D) van der Waals materials with in-plane anisotropy are of great interest for directional transport of charge and energy. I perform solid state synthesis to produce several such materials: an intrinsic antiferromagnet, superatomic semiconductors, and a polytype system with a component that displays the possibilities of Weyl nodes.The former, chromium sulfur bromide (CrSBr), is first synthesized, then fully studied structurally, compositionally, electronically, and magnetically. Second harmonic generation (SHG), more advanced than older techniques such as magneto-optical Kerr spectroscopy or Raman spectroscopy, allows us to fully understand the magnetic symmetry in this system as an interlayer antiferromagnetic and intralayer ferromagnetic in-plane anisotropic material. I also introduce published work in which we integrate CrSBr into different devices to show the utility of this fundamental research into a more practical application setting. It is used to stimulate more magnetic response from graphene — promising ultra-thin magnetic memory or sensory devices in future projects. Applying strain and external magnetic fields provides another tuning knob through which to access different functional modalities. In the latter third of this dissertation, we report a layered van der Waals semiconductor with in-plane anisotropy built upon the superatomic units of Mo₆S₃Br₆ (MSB), a robust construction with a direct gap of 1.64 eV. Next, MSB and Re₆Se₈Cl₂, another analogous superatomic vdW material, are potential candidates for optoelectronic applications; we qualify this by studying their Auger dynamics as a measure of quantum efficiency. Finally, layered van der Waals (vdW) materials belonging to the MM’Te₄ structure class have recently received intense attention due to their ability to host exotic electronic transport phenomena, such as in-plane transport anisotropy, Weyl nodes, and superconductivity. In summary, we have discovered two ternary exfoliatable vdW TMD polytypes with the composition TaFeTe₄, one of which (ꞵ) shows the prerequisite symmetry elements to be a type-II Weyl semimetal. This dissertation is a treatise to solid state synthesis, exploration into the more exotic spectrum of 2D materials, and robust and eclectic methods used to paint a full picture of different magnetic and electronic systems within.

Since the first successful isolation and subsequent characterization of graphene, the interest in two dimensional (2-D) materials has expanded exponentially. Despite the dozens of graphene-like van der Waals materials that have been found and their interesting properties, a significant obstacle in realizing their promise is their instability especially for monolayer and thin layers at elevated temperature. To overcome the obstacle of passivating the 2-D materials and study their properties at elevated temperature, we take advantage of the potential improvements afforded by assembling heterostructures by stacking the atomic thick 2-D materials together hexagonal boron nitride (ℎ-BN) which possess high chemical stability and thermal stability. In this dissertation, several experiments are described in detail in which we utilized h-BN encapsulation to passivate atomically-thin transition metal dichalcogenide and studied their properties at elevated temperature. In the first project we demonstrated that chemical vapor deposition (CVD)-grown flakes of high-quality monolayers of WS₂ can be stabilized at elevated temperatures by encapsulation with only top ℎ-BN layers in the presence of ambient air, N₂ or forming gas. The best passivation occurs for ℎ-BN covered samples with flowing N₂. In the second project, we demonstrated that encapsulating monolayer MoSe₂ and WS₂ with top and bottom ℎ-BN can improve their thermal stability at high temperature and increase their photoluminescence (PL). The increased PL likely occurs because impurities are laterally expelled from the TMD stack during heating. In the third project, we demonstrated the passivation of different modes of ℎ-BN encapsulation on thin layer FeSe sample by using temperature dependent Raman scattering. The complete encapsulation showed the best protection of thin layer FeSe. Finally, we utilized the temperature dependence of the Raman mode of thin-layer FeSe with complete encapsulation and applied a noncontact method to measure the thermal conductivity of the thin-layer FeSe.

The exploration of physical extremes drives technological innovation. Recent decades have seen a push towards materials engineering at the absolute limit of space with electronic systems that are a single atom thick. When electrons are confined to two-dimensional structures, exotic and often unexpected phenomena emerge due to enhanced interaction effects and crystalline anisotropies. The study of such unconventional phenomena offers the opportunity to extend knowledge of fundamental physics with an eye towards advancing the state of the art in control over quantum matter. In this thesis we use scanning tunneling microscopy to study the electronic structure of a collection of novel two-dimensional materials: twisted double-bilayer graphene (TDBG), mirror symmetric twisted trilayer graphene (TTG), small angle twisted double trilayer graphene (TDTG), and the van der Waals heavy Fermion material CeSiI. In TDBG, we directly image spontaneous symmetry breaking of the electronic states as a function of carrier density and attribute this to an intrinsic nematic instability of the metallic Fermi liquid. In TTG, we find evidence for a novel form of lattice relaxation, in which twist angle disorder leads to the formation of moiré lattice defects that can act to lock trilayer devices into a magic angle configuration while strongly modulating the local electronic structure, with implications for the superconducting state. In TDTG, we discover yet another form of lattice relaxation in which a global transformation of the stacking structure creates a net energy reduction, even while the stacking energy density in roughly half of the moiré lattice rises. Lastly, we show through quasiparticle interference spectroscopy and theoretical modeling that CeSiI hosts a nodal hybridization between itinerant conduction electrons and a lattice of local moments, giving rise to a strong angular dependence of the heavy Fermion mass enhancement in this van der Waals material.

Quantum computing and two dimensional van der Waals materials research have been two of the fastest growing fields of condensed matter physics research for the better part of the last two decades. In that time, advances in superconducting qubit design, materials and fabrication have improved their relaxation and coherence times by about 5 orders of magnitude. One of the key components that quantum devices such as qubits require are ultra low loss capacitance elements. Conventional parallel plate capacitors have been unable to fulfill this need due to bulk and inter-facial losses, necessitating the use of coplanar capacitors with extremely large footprints. In fact one of the driving forces behind increase coherence times has been the ever growing footprint of these coplanar capacitor pads, and the reduced electric field density and thus reduced surface losses that they provide. However, this style of capacitor creates a number of challenges when it comes to scaling the number of qubits in a system. First, the large geometric footprint of these pads limits the number of qubits that can be placed on a chip. Second, the dispersion of the electric field, above and below the plane of the capacitor pads can cause unwanted crosstalk between neighbouring qubits, again limiting the number of qubits that can be put on a chip without compromising coherence. Since the isolation of a single atomic layer of graphene in 2004 and the ability to create heterostructures of a variety of two dimensional materials, the field of van der Waals materials research has exploded at a similar rate. Single crystals of van der Waals materials, can be grown with extremely low defect densities, and then be stacked to create heterostructures with ultra-clean laminated interfaces. This work explores how van der Waals materials may be used to create low loss parallel plate capacitors. The parallel plate geometry confines the electric field between the crystalline materials and low loss interfaces of a van der Waals heterostructure, limiting both losses at the surfaces as well as undesired cross talk between qubits. We begin by studying the microwave losses in hexagonal boron nitride (hBN). Next we report a method to make low loss microwave contacts to air sensitive superconducting van der Waals materials like niobium diselinde (NbSe₂). Finally, we demostrate coherence in a transmon where the primary shunt capacitor is an all van der Waals parallel plate capacitor, achieving a 1000× reduction in geometric footprint, when compared to a conventional coplanar capacitor.

This dissertation provides a comprehensive exploration of the synthesis and properties of van der Waals materials for two-dimensional applications. We delve into the unique characteristics of these materials, which are composed of layers bound by weak van der Waals forces, in contrast to strong covalent or ionic bonds. Their layered structure allows for mechanical exfoliation, leading to the isolation of a monolayer or few-layer flakes. These structures retain the intrinsic properties of the bulk material while also exhibiting novel characteristics due to their reduced dimensionality. Chapter 1 briefly introduces van der Waals materials and their advantageous characteristics for 2D applications. It also provides a brief history of the discovery of atomically thin magnetic materials and their applications. Chapters 2 and 3 discuss the synthesis and characterization of a layered metallic antiferromagnet, TaFe1.14Te3. Using a combination of magnetic, electronic, and transport measurements supported by first-principles DFT calculations, we investigate the interplay between the magnetic and electronic properties in the layered van der Waals magnet. Chapter 4 explores a magnetic van der Waals semiconductor, KMnBi. KMnBi is part of the alkali metal manganese pnictide systems and is predicted to be a near-room temperature antiferromagnet and a small-gap semiconductor. Chapter 5 discusses the interplay between molecular symmetry and broad-band chiral absorbance in a series of [6]helicenes for practical chiroptical applications. Finally, Chapter 6 discusses the synthesis and characterizations of a systematically doped allotrope of carbon, graphullerite. We explore both solid-state and solution intercalation studies and the effects on their physical properties.

The 2D honeycomb lattice started as a theoretical construct, until its realization in a crystalline system enabled the study of a host novel exotic phenomena arising from its unique electronic structure. Since the isolation of graphene, the search for crystalline materials hosting interesting electronic structures has only increased with the excitement of correlated phenomena that can arise in the two-dimensional limit. This dissertation details the characterization of a van der Waals (vdW) material that realizes a novel flat band lattice model via frustrated hopping. Chapter 1 starts with an introduction into vdW materials and the electronic structure of frustrated lattices. Chapter 2 goes through some of the characterization methods that will be mentioned in this dissertation. Chapter 3 introduces the material that will be the subject of investigation in this thesis and establishes its as arising from a novel flat band lattice model via frustrated hopping. Chapter 4 discusses the electronic properties of newly synthesized analogs of this material. Chapter 5 introduces potential applications of this material in plasmonics. Chapter 6 covers a research story that is independent of the rest of this dissertation. It goes through the optical properties that arise from in-plane structural anisotropy in a superatomic vdW material.

First, I introduce the concept of heavy fermion systems and discuss the ease of tuning their properties with external parameters including pressure, chemical doping, and magnetic fields to induce new quantum states such as unconventional superconductivity. I then delve into the limited use of dimensionality as a tuning knob for quantum criticality and highlight the new possibilities available if heavy fermion behavior is discovered in the single-layer limit. Chapter 1 establishes the van der Waals material, CeSiI, as a heavy fermion system and is the first material where heavy fermion behavior exists down to the few-layer limit. The chapter further explores the bulk magnetic properties and electronic structure of CeSiI at high magnetic fields. The quasi-two-dimensional electronic character of CeSiI leads to anisotropic hybridization between local moments and conduction electrons, a phenomenon previously only realized in theoretical calculations. With the heavy fermion properties of CeSiI established, Chapter 2 investigates the effects of pressure and La-doping on CeSiI, aiming to push it from the antiferromagnetic region of the Doniach phase diagram towards a quantum critical point. Preliminary evidence suggests that CeSiI is too distant from quantum criticality. Instead, La-doping is utilized to explore single-ion Kondo physics at the dilute Ce limit in CeSiI. Additionally, CeGaI, with a crystal structure similar to CeSiI, is examined. Although no Kondo physics is observed, the magnetic and electronic properties remain coupled to each other. Chapter 3 delves into a separate project focusing on the study of polymers composed of perylene diimide and various organic linkers. It explores how the structure of the polymer influences its pseudocapacitance properties. The chapter demonstrates the significance of contortion in device performance, aiming to provide insights for future endeavors in developing environmentally friendly energy storage systems.

This thesis considers the problem of determining Casimir-Lifshitz forces in inhomogeneous media. The ground-state energy of the electromagnetic field in a piston-geometry is discussed. When the cavity is empty, the Casimir pressure on the piston is finite and independent of the small-scale physics of the media that compose the mirrors. However, it is demonstrated that, when the cavity is filled with an inhomogeneous dielectric medium, the Casimir energy is cut-off dependent. The local behavior of the stress tensor commonly used in calculations of Casimir forces is also determined. It is shown that the usual expression for the stress tensor is not finite anywhere within such a medium, whatever the temporal dispersion or index profile, and that this divergence is unlikely to be removed by modifying the regularisation. These findings suggest that the value of the Casimir pressure may be inextricably dependent on the detailed behavior of the mirror and the medium at large wave vectors. This thesis also examines two exceptions to this rule: first, the case of an idealised metamaterial is considered which, when introduced into a cavity, reduces the magnitude of the Casimir force. It is shown that, although the medium is inhomogeneous, it does not contribute additional scattering events but simply modifies the effective length of the cavity, so the predicted force is finite and can be stated exactly. Secondly, a geometric argument is presented for determining a Casimir stress in a spherical mirror filled with the inhomogeneous medium of Maxwell's fish-eye. This solution questions the idea that the Casimir force of a spherical mirror is repulsive, but prompts additional questions concerning regularisation and the role of non-local effects in determining Casimir forces.