The thesis presents experimental and theoretical results about the surface dynamics and the surface Dirac fermion (DF) spectral function of the strong topological insulators Bi2Te3 and Bi2Se3. The experimental results reveal the presence of a strong Kohn anomaly in the measured surface phonon dispersion of a low-lying optical mode, and the absence of surface Rayleigh acoustic phonons. Fitting the experimental data to theoretical models employing phonon Matsubara functions allowed the extraction of the matrix elements of the coupling Hamiltonian and the modifications to the surface phonon propagator that are encoded in the phonon self-energy. This allowed, for the first time, calculation of phonon mode-specific DF coupling λν(q) from experimental data, with average coupling significantly higher than typical values for metals, underscoring the strong coupling between optical surface phonons and surface DFs in topological insulators. Finally, to connect to experimental results obtained from photoemission spectroscopies, an electronic (DF) Matsubara function was constructed using the determined electron-phonon matrix elements and the optical phonon dispersion. This allowed calculation of the DF spectral function and density of states, allowing for comparison with photoemission and scanning tunneling spectroscopies. The results set the necessary energy resolution and extraction methodology for calculating λ from the DF perspective.
PbTe is a member of the group IV-VI semiconducting compounds that distinctly crystallize in three closely related structures (cubic, rhombohedral, orthorhombic). The determination of the structure depends on a delicate balance between levels of ionicity and covalency. Classified as an incipient ferroelectric, PbTe lies very close to the cubic/rhombohedral phase boundary, crystallizing in a paraelectric rocksalt cubic configuration, but with a proclivity to distort into a ferroelectric state, although never doing so. This instability leads to anomalous characteristics in the lattice dynamics of PbTe and is thought to contribute to its naturally low thermal conductivity, which makes it so effective as a thermoelectric compound. Specifically, recent inelastic neutron scattering (INS) measurements reveal that the soft transverse optical (TO) mode of the material is far more dispersive than expected (less dispersive trajectory measured in past INS studies) and features a split-peak lineshape at zone center, both indicative of a "giant" anharmonicity. The zone center frequency of this mode is the typical indicator of the nearness to a ferroelectric transition. To more deeply understand the microscopic mechanisms that link the structural instability of PbTe with its unusual phonon behavior, an ultrafast experimental approach is utilized in this thesis to characterize the material. In this tactic, the compound is impulsively photoexcited away from its ground state and a unique perspective is offered on the interactions that govern its relaxation back to equilibrium. Several variations of time-resolved pump-probe methods are employed. One highlighted result proceeds from a series of fluence, temperature, and pressure dependent IR pump-IR probe measurements on PbTe, which show previously unreported, prominent reflectivity oscillations from the rocksalt-structured material. The oscillations increase in amplitude with higher fluence and temperature while blueshifting in frequency with higher pressure. It is determined that these oscillations originate from the photoexcitation of a TO-TA (transverse acoustic) combination via the second-order Raman mechanism. Another result stems from time and momentum resolved x-ray measurements on PbTe, which show evidence that electron-phonon interactions play a significant role in the material's equilibrium ferroelectric instability. Here, we characterize the dispersion of correlated pairs of phonons with equal and opposite momenta produced by the absorption of pump pulses near the energy of the direct band gap. From the dispersion data away from zone center coupled with constrained density functional theory (CDFT) calculations, we find that photoexcitation leads to the TO branch hardening near zone center and softening near zone edge (X point). This is further tied to the weakening of long-range forces along the cubic direction and in the band picture, a reduction of the Peierls-like electronic instability. All of this reinforces the paraelectric state. It is thus determined that electron-phonon coupling drives ferroelectric instability in PbTe. Furthermore, from the dispersion data very near zone center, we find a highly dispersive feature that is identified as a heavily screened longitudinal optical (LO) phonon mode. This feature is attributed to the high photoexcitation density in the measurements and is reminiscent of LO mode softening seen in past INS studies of PbTe. It is concluded that this similarity is due to the effective equivalence between photoexcitation in our measurements and relatively high carrier doping in the samples of the past studies. This is significant since it reconciles the inconsistency between the dispersion results of the aforementioned recent INS measurements (measured on less doped samples) with those seen in the older studies.
Topological insulators are insulating in the bulk, but process metallic states present around its boundary owing to the topological origin of the band structure. The metallic edge or surface states are immune to weak disorder or impurities, and robust against the deformation of the system geometry. This book, the first of its kind on topological insulators, presents a unified description of topological insulators from one to three dimensions based on the modified Dirac equation. A series of solutions of the bound states near the boundary are derived, and the existing conditions of these solutions are described. Topological invariants and their applications to a variety of systems from one-dimensional polyacetalene, to two-dimensional quantum spin Hall effect and p-wave superconductors, and three-dimensional topological insulators and superconductors or superfluids are introduced, helping readers to better understand this fascinating new field. This book is intended for researchers and graduate students working in the field of topological insulators and related areas. Shun-Qing Shen is a Professor at the Department of Physics, the University of Hong Kong, China.
This book summarizes the current status of theoretical and experimental progress in 2 dimensional graphene-like monolayers and few-layers of transition metal dichalcogenides (TMDCs). Semiconducting monolayer TMDCs, due to the presence of a direct gap, significantly extend the potential of low-dimensional nanomaterials for applications in nanoelectronics and nano-optoelectronics as well as flexible nano-electronics with unprecedented possibilities to control the gap by external stimuli. Strong quantum confinement results in extremely high exciton binding energies which forms an interesting platform for both fundamental studies and device applications. Breaking of spatial inversion symmetry in monolayers results in strong spin-valley coupling potentially leading to their use in valleytronics. Starting with the basic chemistry of transition metals, the reader is introduced to the rich field of transition metal dichalcogenides. After a chapter on three dimensional crystals and a description of top-down and bottom-up fabrication methods of few-layer and single layer structures, the fascinating world of two-dimensional TMDCs structures is presented with their unique atomic, electronic, and magnetic properties. The book covers in detail particular features associated with decreased dimensionality such as stability and phase-transitions in monolayers, the appearance of a direct gap, large binding energy of 2D excitons and trions and their dynamics, Raman scattering associated with decreased dimensionality, extraordinarily strong light-matter interaction, layer-dependent photoluminescence properties, new physics associated with the destruction of the spatial inversion symmetry of the bulk phase, spin-orbit and spin-valley couplings. The book concludes with chapters on engineered heterostructures and device applications such as a monolayer MoS2 transistor. Considering the explosive interest in physics and applications of two-dimensional materials, this book is a valuable source of information for material scientists and engineers working in the field as well as for the graduate students majoring in materials science.
This book reviews the structure and electronic, magnetic, and other properties of various MoS2 (Molybdenum disulfide) nanostructures, with coverage of synthesis, Valley polarization, spin physics, and other topics. MoS2 is an important, graphene-like layered nano-material that substantially extends the range of possible nanostructures and devices for nanofabrication. These materials have been widely researched in recent years, and have become an attractive topic for applications such as catalytic materials and devices based on field-effect transistors (FETs) and semiconductors. Chapters from leading scientists worldwide create a bridge between MoS2 nanomaterials and fundamental physics in order to stimulate readers' interest in the potential of these novel materials for device applications. Since MoS2 nanostructures are expected to be increasingly important for future developments in energy and other electronic device applications, this book can be recommended for Physics and Materials Science and Engineering departments and as reference for researchers in the field.
The latest volume in the well-established AMN series, this ready reference provides an up-to-date, self-contained summary of recent developments in the technologies and systems for thermoelectricity. Following an initial chapter that introduces the fundamentals and principles of thermoelectricity, subsequent chapters discuss the synthesis and integration of various bulk thermoelectric as well as nanostructured materials. The book then goes on to discuss characterization techniques, including various light and mechanic microscopy techniques, while also summarizing applications for thermoelectric materials, such as micro- and nano-thermoelectric generators, wearable electronics and energy conversion devices. The result is a bridge between industry and scientific researchers seeking to develop thermoelectric generators.
This series of books, which is published at the rate of about one per year, addresses fundamental problems in materials science. The contents cover a broad range of topics from small clusters of atoms to engineering materials and involve chemistry, physics, materials science, and engineering, with length scales ranging from Angstroms up to millimeters. The emphasis is on basic science rather than on applications. Each book focuses on a single area of current interest and brings together leading experts to give an up-to-date discussion of their work and the work of others. Each article contains enough references that the interested reader can access the relevant literature. Thanks are given to the Center for Fundamental Materials Research at Michigan State University for supporting this series. M.F. Thorpe, Series Editor E-mail: [email protected] East Lansing, Michigan, November 200 I v PREFACE The study of the atomic structure of crystalline materials began at the beginning of the twentieth century with the discovery by Max von Laue and by W.H. and W.L. Bragg that crystals diffract x-rays. At that time, even the existence of atoms was controversial.
This second edition deals in an elementary way with electrons in non-crystalline systems. It reflects advances in the theory of interactions in non-crystalline systems, provides a more detailed discussion of the "minimum metallic conductivity", and addresses the relevance of disorder in the new high-temperature semiconductors.
