Transition metal dichalcogenides (TMDs or TMDCs) have garnered much interest recently due to their weakly layered structures, allowing for mechanical exfoliation down to a single atomic layer. As such, it is pertinent to re-examine the bulk properties of these materials in order to completely understand and predict what is happening in the few-layered limit. A large majority of these systems were first investigated in the 1950s and 1960s. As such, many of the current growth methods rely on these reports, making new growth techniques for lowering defects of importance as well. In this thesis, both topics are taken into consideration and discussed, though the latter remains to be investigated in much more detail and should be the work of future research efforts. Orthorhombic MoTe₂ and its isostructural compound WTe₂ were recently claimed to belong to a new class (type II) of Weyl semimetals characterized by a linear touching between hole and electron Fermi surfaces in addition to nodal lines. These compounds have recently been shown to display very large non-saturating magnetoresistances which have been attributed to nearly perfectly compensated densities of electrons and holes. Here, we present a detailed study on the temperature and angular dependence of the Shubnikov-de-Haas (SdH) effect in the semi-metal WTe₂ and MoTe₂. In WTe₂, we observe four fundamental SdH frequencies and attribute them to spin-orbit split, electron- and hole-like, Fermi surface (FS) cross-sectional areas. Their angular dependence seems consistent with ellipsoidal FSs with volumes suggesting a modest excess in the density of electrons with respect to that of the holes. We show that density functional theory (DFT) calculations fail to correctly describe the FSs of WTe₂. When their cross-sectional areas are adjusted to reflect the experimental data, the resulting volumes of the electron/hole FSs obtained from the DFT calculations would imply a pronounced imbalance between the densities of electrons and holes. We find evidence for field-dependent Fermi surface cross-sectional areas by fitting the oscillatory component superimposed onto the magnetoresistivity signal to several Lifshitz-Kosevich components. We also observe a pronounced field-induced renormalization of the effective masses. Taken together, our observations suggest that the electronic structure of WTe₂ evolves with the magnetic field due to the Zeeman splitting. This evolution is likely to contribute to its pronounced magnetoresistivity. For β-MoTe2, high quality single-crystals were synthesized by flux in excess tellurium. We find that its superconducting transition temperature depends on disorder as quantified by the ratio between the room- and low-temperature resistivities, (residual resistivity ratio, RRR. Similar to WTe₂, its magnetoresistivity does not saturate at high magnetic fields and can easily surpass 10₂⁶%, with the superimposed Shubnikov de Haas oscillations revealing a non-trivial Berry phase of ≃pi. The geometry of the Fermi surface, as extracted from the quantum oscillations, is markedly distinct from the calculated one. A broad anomaly seen in the heat capacity and in the Hall-effect indicates that the crystallographic and the electronic structures evolve upon cooling below 100 K, likely explaining the discrepancy between these recent predictions and our experimental observations. In α-MoTe₂, grown at lower temperatures in vapor transport, we focus on few-layered crystals mechanically exfoliated onto a 270 nm thick SiO₂ layer. Previous reports found that the field-effect mobility of transition metal dichalcogenides TMDs tends to increase, reaching a maximum value when crystals are composed of approximately 10 atomic layers. We show that the overall performance of a MoTe₂ based field-effect transistor is comparable to similar devices based on MoS₂ or MoSe₂. But with an optical gap quite close in value to the one of Si and an enhanced spin-orbit interaction (since Te is a 5p element) suggests that this compound might be particularly suitable for optoelectronic applications in a complimentary range of wavelengths. In the case of MoSe₂, we show that multi-layered (~ 10 atomic layers) field-effect transistors can display ambipolar behavior at room temperature when using a standard combination of metals, Au on Ti, for all the electrical contacts. The 4.33 eV work function of Ti is closely matched by the electron affinity of bulk MoSe₂, 4.45 ± 0.11 eV. This implies that the Fermi level of Ti is very close to the bottom of the conduction band of MoSe₂, and therefore that one should expect a rather small Schottky barrier for electron conduction through the Ti:Au contacts. One extracts through Hall effect measurements, Hall mobilities in excess of 250 cm₂²/(V s) for both holes and electrons at room temperature. These values are remarkable, since they are comparable or higher than most values reported so far for transition metal dichalcogenides at room temperature, but are obtained without the use of high k-dielectrics such as HfO₂, doping, or of a particular combination of metals for the electrical contacts. Our results suggest that improvements in fabrication, and on the quality of the starting material (with a lower amount of defects) could make field-effect transistors based on few atomic layers of synthetic MoSe₂ excellent candidates for complementary logic electronics. Finally, we report on alloys of MoTe₂ and WTe₂, Mo₁₋xWxTe₂, grown by a chemical vapor transport process with the goal of obtaining a phase diagram with respect to doping and temperature. These crystals have been analyzed for composition via EDS and investigated through high resolution transmission electron microscopy, scanning tunneling microscopy (STM) and ARPES. Transmission electron microscopy images clearly show that through W substitution we are able to synthesize both 2H, trigonal prismatic, and Td, orthorhombic structures. As opposed to other reports, we find a phase transition from the 2H- to the Td-phase at 10 % W doping, much lower than expected. This structure is characterized by a linear arrangement of atoms, as opposed to a hexagonal pattern. In addition, through examination of monolayers via TEM, we find that the W disperses randomly amongst the crystal as opposed to forming grain boundaries. Given the crystallinity and quality of the material, mapping the phase diagram should be of relative ease, however this work is still ongoing. As such, the phase diagram has not yet been completed and remains unreported in this manuscript. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester 2016. / July 13, 2016. / Fermi Surface, Magnetoresistance, MoTe2, Synthesis, TMDs, WTe2 / Includes bibliographical references. / Luis Balicas, Professor Co-Directing Dissertation; Nicholas Bonesteel, Professor Co-Directing Dissertation; Theo Siegriest, University Representative; Mark Riley, Committee Member; Irinel Chiorescu, Committee Member.

This dissertation presents studies on mononuclear single molecule magnets (SMMs) with magnetic properties arising from transition metal ions in trigonal bipyramidal (TBP) coordination environments. We use both experimental and theoretical methods to elucidate the effects of coordination geometry on the magnetic anisotropy of a SMM. The role of an axial magnetic anisotropy is to pin the magnetic moment of the metal ion in one of two preferred orientations, either parallel or anti-parallel to the magnetic easy-axis. For transition metals, maximization of the axial magnetic anisotropy requires stabilization of an unquenched orbital moment that can couple to a ligand field. SMMs with giant magnetic anisotropy play an important role both in terms of fundamental scientific reasons and potential application in information technologies. Thus the studies presented in this dissertation attempt to explore some of the interesting physics in these compounds. The presence of orbitally degenerate states and unquenched orbital momentum pushes the limits of spin-only model. To overcome this limitation, we propose a phenomenological spin-orbit model based on point charge approximation with the goal to investigate orbitally degenerate mononuclear compounds. As an application of our model, we consider two test compounds : Iron(II) and Nickel(II) ions in trigonal bipyramidal (TBP) environments, where we find that the high symmetry configuration supports a large magnetic anisotropy in the absence of Jahn-Teller distortion. The motivation for our phenomenological model stemmed from our detailed EPR measurements performed on a mononuclear Nickel(II) SMM in a TBP environment that revealed an unprecedented magnetic anisotropy, reaching the limits of applicability of the familiar spin-only description. The axial anisotropy estimated for this complex was found to the be the largest so far for a mononuclear Nickel(II) complex; and, importantly, only a very small degree of axial symmetry breaking was detected. This was most likely considered to be due to the unquenched orbital moment in the ground states of the Nickel(II) ion. To further confirm this prediction we performed theoretical studies of the SMM using the phenomenological spin-orbit model. This study showed the suppression of Jahn-Teller effects in trigonal bipyramidal Nickel(II) complex because of rigid, bulky axial ligands. To further understand the effects of using bulky ligands in TBP coordination environments we performed experimental and theoretical studies on a mononuclear Iron(II) SMM. Although the ground states of this complex is also orbitally degenerate, our investigation showed reduced axial magnetic anisotropy compared to Nickel(II) with a very small transverse component. Our phenomenological investigation of the ground states revealed that the magnitude of the first order contribution is strongly dependent on the bond angles, and the spin-orbit coupling constant also plays a significant role in achieving large magnetic anisotropy. Finally, we also explore the effects of different ligand types in Cobalt(II) mononuclear complexes in TBP coordination environments. In these Kramers systems we and that the combination of a 3-fold symmetric ligand and a trigonal space group gives rise to an increase in the easy-plane magnetic anisotropy, while keeping the rhombicity of the system close to zero. This is particularly interesting for quantum information processing, especially in relation to molecules with a large spin ground state characterized by a large easy-plane anisotropy. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester 2018. / June 29, 2018. / Includes bibliographical references. / Stephen Hill, Professor Directing Dissertation; Mykhailo Shatruk, University Representative; Jianming Cao, Committee Member; Nicholas Bonesteel, Committee Member; Jorge Piekarewicz, Committee Member.

Thesis (Ph.D.)--Brown University, 2008. / Advisor: Xinsheng S. Ling. Includes bibliographical references (leaves 111-125).

Two mathematical problems in disordered systems are studied: geodesics in first-passage percolation and conductivity of random resistor networks. In first-passage percolation, we consider a translation-invariant ergodic family {t(b): b bond of Z²} of nonnegative random variables, where t(b) represent bond passage times. Geodesics are paths in Z², infinite in both directions, each of whose finite segments is time-minimizing. We prove part of the conjecture that geodesics do not exist in any fixed half-plane and that they have to intersect all straight lines with rational slopes. In random resistor networks, we consider an independent and identically distributed family {C(b): b bond of a hierarchical lattice H} of nonnegative random variables, where C(b) represent bond conductivities. A hierarchical lattice H is a sequence {H(n): n = 0, 1, 2} of lattices generated in an iterative manner. We prove a central limit theorem for a sequence x(n) of effective conductivities, each of which is defined on lattices H(n), when a system is in a percolating regime. At a critical point, it is expected to have non-Gaussian behavior.

Thesis (Ph.D.) - University of St Andrews, June 2009. / Restricted until 1st December 2009.

<p>The following dissertation shows the results of a series of inelastic neutron scattering experiments on the geometrically frustrated pyrochlore system Tb2Sn2-xTixO7 for x=0, 0.1, 0.2, 0.5, 1, 1.5 and 2. Inelastic neutron scattering measurements were performed on the SEQUOIA direct geometry time-of-flight spectrometer at the Spallation Neutron Source of Oak Ridge National Laboratory. For the two end members, x=0, Tb2Sn2O7 and x=2, Tb2Ti2O7 , they display related, but different exotic ground states, with Tb2Sn2O7 displaying “soft” spin ice order below T_N~0.87K, while Tb2Ti2O7 enters a glassy antiferromagnetic spin ice state below T_g~0.2K.</p> <p>The first two chapters give a brief introduction to the physics of geometrically frustrated magnetism and neutron scattering. Chapter 3 studies the two end members Tb2Ti2O7 and Tb2Sn2O7 experimentally and theoretically. Inelastic neutron scattering measurements and appropriate crystal field calculations together probe the crystal field states associated with the J=6 states of Tb^3+ within the appropriate Fd3m pyrochlore environment. These crystal field states determine the size and anisotropy of the Tb^3+ magnetic moment in each material’s ground state, information that is an essential starting point for any description of the low temperature phase behavior and spin dynamics in Tb2Ti2O7 and Tb2Sn2O7. Chapter 4 treats neutron scattering, as well as accompanying AC magnetic susceptibility and muSR measurements performed by our collaborators on a series of solid solutions Tb2Sn2-xTixO7 showing a novel, dynamic spin liquid state for all x other than the end members x=0 and x=2. This state is the result of disorder in the low lying Tb^3+ crystal field environments which de-stabilizes the mechanism by which quantum fluctuations contribute to ground state selection in Tb2Sn2-xTixO7.</p> / Master of Science (MSc)

<p> Atomically thin, semiconducting transition metal dichalogenides (TMDs), a special class of layered semiconductors, that can be shaped as a perfect two dimensional material, have garnered a lot of attention owing to their fascinating electronic properties which are achievable at the extreme nanoscale. In contrast to graphene, the most celebrated two-dimensional (2D) material thus far; TMDs exhibit a direct band gap in the monolayer regime. The presence of a non-zero bandgap along with the broken inversion symmetry in the monolayer limit brands semiconducting TMDs as the perfect candidate for future optoelectronic and valleytronics-based device application. These remarkable discoveries demand exploration of different materials that possess similar properties alike TMDs. Recently, III-VI layered semiconducting materials (example: InSe, GaSe etc.) have also emerged as potential materials for optical device based applications as, similar to TMDs, they can be shaped into a perfect two-dimensional form as well as possess a sizable band gap in their nano-regime. The perfect 2D character in layered materials cause enhancement of strong Coulomb interaction. As a result, excitons, a coulomb bound quasiparticle made of electron-hole pair, dominate the optical properties near the bandgap. The basis of development for future optoelectronic-based devices requires accurate characterization of the essential properties of excitons. Two fundamental parameters that characterize the quantum dynamics of excitons are: a) the dephasing rate, &gamma;, which represents the coherence loss due to the interaction of the excitons with their environment (for example- phonons, impurities, other excitons, etc.) and b) excited state population decay rate arising from radiative and non-radiative relaxation processes. The dephasing rate is representative of the time scale over which excitons can be coherently manipulated, therefore accurately probing the source of exciton decoherence is crucial for understanding the basic unexplored science as well as creating technological developments. The dephasing dynamics in semiconductors typically occur in the picosecond to femtosecond timescale, thus the use of ultrafast laser spectroscopy is a potential route to probe such excitonic responses. </p><p> The focus of this dissertation is two-fold: firstly, to develop the necessary instrumentation to accurately probe the aforementioned parameters and secondly, to explore the quantum dynamics and the underlying many-body interactions in different layered semiconducting materials. A custom-built multidimensional optical non-linear spectrometer was developed in order to perform two-dimensional spectroscopic (2DFT) measurements. The advantages of this technique are multifaceted compared to regular one-dimensional and non-linear incoherent techniques. 2DFT technique is based on an enhanced version of Four wave mixing experiments. This powerful tool is capable of identifying the resonant coupling, probing the coherent pathways, unambiguously extracting the homogeneous linewidth in the presence of inhomogeneity and decomposing a complex spectra into real and imaginary parts. It is not possible to uncover such crucial features by employing one dimensional non-linear technique. </p><p> Monolayers as well as bulk TMDs and group III-VI bulk layered materials are explored in this dissertation. The exciton quantum dynamics is explored with three pulse four-wave mixing whereas the phase sensitive measurements are obtained by employing two-dimensional Fourier transform spectroscopy. Temperature and excitation density dependent 2DFT experiments unfold the information associated with the many-body interactions in the layered semiconducting samples. </p>

No description available.

<p> Surface plasmons are collective oscillations of free charge carriers confined in interface between two dielectrics, where the real part of the dielectric changes sign (e.g a metal-insulator interface such as gold film and air). The study of surface plasmon has been a popular research theme with potential applications utilizing the fact that the wavelength of plasmons can be many order smaller than that of the incident lights. The potential applications include transfer of information in hundreds of terahertz instead of upper limit of gigahertz in traditional wires, photodetectors with frequency range from terahertz to mid-IR, and nano-imaging. In our experiment, we use an IR near-field microscopy with resolution as low as 10nm but energy scale of micron range. This is achieved by shinning an AFM tip with infrared laser on top of the sample and collecting the scattered light from the sample. The spatial resolution proportional to where a is the size of the tip and the resolution can reach 10nm. This technique beats the diffraction limit of near-IR (10um) by over 1000x. The wavelength and amplitude damping of plasmon greatly depends on the property of free carriers in the material. While metals such as gold had been widely studied and shown promising results, a better platform with longer propagation length and shorter wavelength is needed for application. Graphenes supreme electronic transport property makes it apiii pears to be an excellent candidate for plasmonic. Graphene plasmon across a p-n junction will be discussed. Oxygen doping of graphene with different dosage via UV ozone is studied. Oxygen doping has shown promising results for graphene plasmon guide. Plasmon fringes are developed in the interior breaking the limit of boundary condition. The UV ozone treatment can be fine controlled and without damaging the graphene sheet. One can, in theory, mask and selectively dope to create a robust graphene plasmon circuit that is stable in room temperature. </p>

<p> Exchange spring based superconducting heterostructures and Josephson junctions are studied to search for evidence of odd-triplet superconductivity. Cooper pairs from a superconductor can leak into a nonhomogeneous ferromagnet a much greater distance than they leak into a homogeneous ferromagnet. This is a result of a conversion of the superconducting condensate at the superconductor-nonhomogeneous ferromagnet interface from the singlet and triplet states to the odd-triplet state. The odd-triplet state is insensitive to the exchange field of the ferromagnet. </p><p> To generate the nonhomogeneous magnetic region, an exchange spring is used. The exchange spring consists of coupled hard and soft magnetic layers that are used to produce a nonhomogeneous magnetization. The system studied consists of superconducting Niobium (Nb) and a Samarium-Cobalt/Permalloy (SmCo/Py) exchange spring.</p><p> Initial samples of Niobium had a critical temperature lower than that obtainable in our laboratory (&lt; 1.8 K). Preliminary work was done to find the cause of the suppressed critical temperature of Nb and to increase it. This work resulted in obtaining Niobium thin films with critical temperatures as high as 6 K.</p><p> Indirect evidence of the odd-triplet component is searched for by looking at the critical temperature of superconductor/exchange spring bi-layers. As the nonhomogeneity of the magnetization is increased, it is expected that the critical temperature will decrease as the condensate leaks further into the exchange spring. In Nb/Py/SmCo systems, this behavior was observed, along with a modulation in the resistance that is attributed to the anisotropic magnetoresistance of the permalloy layer. A decrease in the critical temperature with increasing nonhomogeneity of the exchange spring was also observed in Nb/SmCo/Py layers, provided the SmCo layer is not too thick.</p><p> Direct evidence of the odd-triplet component is searched for by looking at the modulation of the critical current through exchange spring based Josephson junctions as exchange spring magnetization becomes more nonhomogeneous. As the nonhomogeneity of the magnetization increases, the critical current through the junction should increase as well. Fabrication of Josephson junctions with exchange spring interlayers was performed at Oak Ridge National Laboratory, and the procedure is presented here. The critical current through these junctions was observed to increase with increasing nonhomogeneity of the exchange spring magnetization, although more tests are needed to verify this is due to the odd-triplet component of the superconducting condensate.</p>