Generation of Anisotropic Massless Dirac Fermions and Asymmetric Klein Tunneling in Few-Layer Black Phosphorus Superlattices

Artificial lattices have been employed in many two-dimensional systems, including those of electrons, atoms and photons, in a quest for massless Dirac particles with flexibility and controllability. Periodically patterned molecule assembly and electrostatic gating as well as moir\'e pattern induced by substrate, have produced electronic states with linear dispersions from isotropic two-dimensional electron gas (2DEG). Here we demonstrate that massless Dirac fermions with tunable anisotropic characteristics can, in general, be generated in highly anisotropic 2DEG under slowly varying external periodic potentials. For patterned few-layer black phosphorus superlattices, the new chiral quasiparticles exist exclusively in an isolated energy window and inherit the strong anisotropic properties of pristine black phosphorus. These states exhibit asymmetric Klein tunneling with the direction of incidence for wave packet with perfect transmission deviating from normal incidence by more than 50{\deg} under an appropriate barrier orientation

Electron-Phonon Coupling from GW Perturbation Theory and Electronic and Magnetic Properties of Novel Two-Dimensional Materials

Condensed matter physics is a very broad and fast-developing field, which studies emerging phenomena, interactions, phases, and symmetries in materials, such as solids. Predictive first-principles, or ab initio, methodologies play a significant role in understanding various phenomena and new physics. This dissertation is aimed at developing new ab initio methodologies for the investigation of important novel phenomena and applying various ab initio methods combined with analytical approaches to a broad range of condensed matter systems, including the high-transition-temperature superconductor Ba1−xKxBiO3, the two-dimensional (2D) ferromagnet Cr2Ge2Te6, Dirac fermions generated in few-layer black phosphorus, defects in hexagonal boron nitride, and non-trivial topological surface states of antimony.This dissertation is divided into two parts. Part I is focused on methods development, and Part II is a collection of theoretical and computational studies of novel materials. The dissertation is organized as follows:Part I: Electronic structure methodologies for condensed matterIn Chapter 1, we review some important ab initio methods to lay the foundation for the development of a new ab initio method − named GW perturbation theory (GWPT) − in Chapter 2, and for various applications to the materials studied in Part II. In Chapter 1, we review the basics of density functional theory (DFT), the GW method, the general phonon formalism and electron-phonon (e-ph) coupling formalism, density-functional perturbation theory (DFPT), and the Wannier representation of e-ph coupling.In Chapter 2, we present a new ab initio method, which we named the GW perturbation theory (GWPT). This method is a linear-response theory of the GW method, and it gives efficient and accurate access to all e-ph matrix elements at the many-electron level in the full Brillouin zone and between any pairs of electronic states. We discuss its general formalism, implementation and verification in this Chapter.In Chapter 3, we develop a general renormalized spin-wave theory (RSWT) by including full sublattice dependence. This RSWT method includes magnon-magnon interactions, and therefore can give quantitative predictions of magnetic transition temperatures, especially in 2D. This method is solved numerically and self-consistently. We discuss its formalism, implementation, and behavior in this Chapter.Part II: Studies of superconductivity, and electronic and magnetic interactions in novel materialsIn Chapter 4, we apply our newly developed GWPT method to study superconductivity in Ba1−xKxBiO3, which shows an experimental superconducting transition temperature (Tc) of 30−32 K at optimal doping. Our GWPT calculations show that many-electron correlations significantly enhance the e-ph interactions compared to DFPT values for states near the Fermi surface and renormalize the e-ph coupling constant lambda by a factor of 2.4, nicely explaining the high Tc as well as the doping dependence observed in this family of material.In Chapter 5, we present a collaborative work with experimental groups on the discovery of the 2D van der Waals ferromagnet Cr2Ge2Te6, probed using the scanning magneto-optic Kerr effect (MOKE) technique. We apply our RSWT method to this system, and our calculation nicely reproduces and explains the experimentally observed strong dimensionality effect in this 2D ferromagnet. Furthermore, our theory reveals an intriguing interplay between anisotropy and dimensionality, which leads to an unprecedented magnetic-field control of ferromagnetism in this system.In Chapter 6, we propose a strategy for the generation of novel anisotropic Dirac fermions in few-layer black phosphorus by applying inversely designed superlattice potentials. We show that these novel quasiparticles exhibit asymmetric Klein tunneling, in which the perfect transmission direction significantly differs from the normal incidence direction. These unusual states are highly tunable and accessible with experimentally achievable conditions. The findings revealed in this Chapter provide new platforms for device design.In Chapter 7, we present a collaborative work with an experimental group to study the electron-irradiation-induced triangular and hexagonal defects in hexagonal boron nitride, observed in transmission electron microscopy (TEM) measurements. We use DFT to calculate the formation enthalpy of different structures (as well as the edges and corners), to provide an overall diagram of preferred structures under different conditions at equilibrium. Our theory provides important insights into the formation of these defects.In Chapter 8, we present a collaborative work with experimental groups to study the unusual behavior of photoelectrons from the topological surface states of Sb(111), measured with spin- and angle-resolved photoemission spectroscopy (spin-ARPES). Our theory, using the ab initio tight-binding method, reproduces well the observed spin textures. Our theoretical analysis shows that the unexpected spin-polarization behavior comes from the interplay between strong spin-orbit coupling (SOC) and the symmetry requirement of the electron wavefunction in high symmetry regions of the Brillouin zone

Two-gap superconductivity and decisive role of rare-earth dd electrons in infinite-layer nickelates

The discovery of superconductivity in infinite-layer nickelates, with transition temperature ($T_c$) up to 23 K, provides an exciting new avenue to study correlated electrons and emergent phases. Superconductivity in the nickelates has been mostly perceived to be unconventional and originated from the Ni $d$-electrons due to its analog to cuprate superconductors. The conventional mechanism for superconductivity - phonon-mediated pairing - was presumably ruled out because density functional theory (DFT) calculations reported a very weak electron-phonon coupling in the nickelates. Here, by including electron self-energy effects on the electronic structure and electron-phonon coupling (with $\textit{ab initio}$ $GW$ calculations), we discover that infinite-layer Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ is a dominantly two-gap phonon-mediated superconductor. We show electron correlations alter the character of its multi-band Fermi surface and also strongly enhance the electron-phonon coupling, leading to a large $T_c$ in agreement with experiment. The computed electron-phonon coupling constant $\lambda$ is enhanced by an unprecedented factor of 5.5 as compared to DFT. Solutions of the anisotropic Eliashberg equations yield two dominant $s$-wave gaps - a large gap on states of rare-earth Nd $d$-electron and interstitial orbital characters but a small gap on those of transition-metal Ni $d$-electron character. The superconducting quasiparticle density of states prominently reflects the two-gap nature and explains well tunneling experiments. Our results demonstrate that the phonon mechanism accounts for superconductivity in the nickelates, revealing an unforeseen two-gap $s$-wave nature as well as providing new insights to demystify the similarities and distinctions between the nickelate and cuprate superconductors.Comment: Main text and supplementary material

Two-dimensional single-valley exciton qubit and optical spin magnetization generation

Creating and manipulating coherent qubit states are actively pursued in two-dimensional (2D) materials research. Significant efforts have been made towards the realization of two-valley exciton qubits in monolayer transition-metal dichalcogenides (TMDs), based on states from their two distinct valleys in k-space. Here, we propose a new scheme to create qubits in 2D materials utilizing a novel kind of degenerate exciton states in a single valley. Combining group theoretic analysis and ab initio GW plus Bethe-Salpeter equation (GW-BSE) calculations, we demonstrate such novel qubit states in substrate-supported monolayer bismuthene -- which has been successfully grown using molecular beam epitaxy. In each of the two distinct valleys in the Brillouin zone, strong spin-orbit coupling along with $C_{3v}$ symmetry leads to a pair of degenerate 1s exciton states with opposite spin configurations. Specific coherent linear combinations of the two degenerate excitons in a single valley can be excited with specific light polarizations, enabling full manipulation of the exciton qubits and their spin configurations. In particular, a net spin magnetization can be generated. Our finding opens new routes to create and manipulate qubit systems in 2D materials.Comment: 27 pages, 5 figure

Rank Optimization of Personalized Search

Augmenting the global ranking based on the linkage structure of the Web is one of the popular approaches in data engineering community today for enhancing the search and ranking quality of Web information systems. This is typically done through automated learning of user interests and re-ranking of search results through semantic based personalization. In this paper, we propose a query context window (QCW) based framework for Selective uTilization of search history in personalized leArning and re-Ranking (STAR). We conduct extensive experiments to compare our STAR approach with the popular directory-based search methods (e.g., Google Directory search) and the general model of most existing re-ranking schemes of personalized search. Our experimental results show that the proposed STAR framework can effectively capture user-specific query-dependent personalization and improve the accuracy of personalized search over existing approaches