Screened Exchange Corrections to the Random Phase Approximation from Many-Body Perturbation Theory

The random phase approximation (RPA) systematically overestimates the magnitude of the correlation energy and generally underestimates cohesive energies. This originates in part from the complete lack of exchange terms that would otherwise cancel Pauli exclusion principle violating (EPV) contributions. The uncanceled EPV contributions also manifest themselves in form of an unphysical negative pair density of spin parallel electrons close to electron-electron coalescence. We follow considerations of many-body perturbation theory to propose an exchange correction that corrects the largest set of EPV contributions, while having the lowest possible computational complexity. The proposed method exchanges adjacent particle/hole pairs in the RPA diagrams, considerably improving the pair density of spin-parallel electrons close to coalescence in the uniform electron gas (UEG). The accuracy of the correlation energy is comparable to other variants of second-order screened exchange (SOSEX) corrections although it is slightly more accurate for the spin-polarized UEG. Its computational complexity scales as O(N-5) or O(N-4) in orbital space or real space, respectively. Its memory requirement scales as O(N-2)

Effect of Layer-Stacking on the Electronic Structure of Graphene Nanoribbons

The evolution of electronic structure of graphene nanoribbons (GNRs) as a function of the number of layers stacked together is investigated using \textit{ab initio} density functional theory (DFT) including interlayer van der Waals interactions. Multilayer armchair GNRs (AGNRs), similar to single-layer AGNRs, exhibit three classes of band gaps depending on their width. In zigzag GNRs (ZGNRs), the geometry relaxation resulting from interlayer interactions plays a crucial role in determining the magnetic polarization and the band structure. The antiferromagnetic (AF) interlayer coupling is more stable compared to the ferromagnetic (FM) interlayer coupling. ZGNRs with the AF in-layer and AF interlayer coupling have a finite band gap while ZGNRs with the FM in-layer and AF interlayer coupling do not have a band gap. The ground state of the bi-layer ZGNR is non-magnetic with a small but finite band gap. The magnetic ordering is less stable in multilayer ZGNRs compared to single-layer ZGNRs. The quasipartcle GW corrections are smaller for bilayer GNRs compared to single-layer GNRs because of the reduced Coulomb effects in bilayer GNRs compared to single-layer GNRs.Comment: 10 pages, 5 figure

Alloyed surfaces: New substrates for graphene growth

We report a systematic ab-initio density functional theory investigation of Ni(111) surface alloyed with elements of group IV (Si, Ge and Sn), demonstrating the possibility to use it to grow high quality graphene. Ni(111) surface represents an ideal substrate for graphene, due to its catalytic properties and perfect matching with the graphene lattice constant. However, Dirac bands of graphene growth on Ni(111) are completely destroyed due to the strong hybridization between carbon p(z) and Ni d orbitals. Group IV atoms, namely Si, Ge and Sn, once deposited on Ni(111) surface, form an ordered alloyed surface with root 3 x root 3-R30 degrees reconstruction. We demonstrate that, at variance with the pure Ni(111) surface, alloyed surfaces effectively decouple graphene from the substrate, resulting untrained due to the nearly perfect lattice matching and preserves linear Dirac bands without the strong hybridization with Ni d states. The proposed surfaces can be prepared before graphene growth without resorting on post-growth processes which necessarily alter the electronic and structural properties of graphene. (C) 2017 Elsevier B.V. All rights reserved

Finding the hidden valence band of N 7 armchair graphene nanoribbons with angle resolved photoemission spectroscopy

To understand the optical and transport properties of graphene nanoribbons, an unambiguous determination of their electronic band structure is needed. In this work we demonstrate that the photoemission intensity of each valence sub-band, formed due to the quantum confinement in quasi-one-dimensional (1D) graphene nanoribbons, is a peaked function of the two-dimensional (2D) momentum. We resolve the long-standing discrepancy regarding the valence band effective mass (m*VB) of armchair graphene nanoribbons with a width of N=7 carbon atoms (7-AGNRs). In particular, angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy report m*VB ~ 0.2 and ~ 0.4 of the free electron mass (me), respectively. ARPES mapping in the full 2D momentum space identifies the experimental conditions for obtaining a large intensity for each of the three highest valence 1D sub-bands. Our detail map reveals that previous ARPES experiments have incorrectly assigned the second sub-band as the frontier one. The correct frontier valence sub-band for 7-AGNRs is only visible in a narrow range of emission angles. For this band we obtain an ARPES derived effective mass of 0.4 me, a charge carrier velocity in the linear part of the band of 0.63 X 10^6 m/s and an energy separation of only ~ 60 meV to the second sub-band. Our results are of importance not only for the growing research field of graphene nanoribbons but also for the community, which studies quantum confined systems

Low energy quasiparticle dispersion of graphite by angle-resolved photoemission spectroscopy

The low energy electron dispersion in graphite is measured by angle-resolved photoemission spectroscopy. The measured photoemission intensity maxima are compared to a tight-binding calculation of the electronic band structure. We observe a strong trigonal warping of the equi-energy contour which is well reproduced by the calculations. Furthermore we clearly show that the concept of Dirac Fermions breaks down for AB stacked graphite. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

First principles and angle resolved photoemission study of lithium doped metallic black phosphorous

First principles calculations demonstrate the metallization of phosphorene by means of Li doping filling the unoccupied antibonding pz states. The electron–phonon coupling in the metallic phase is strong enough to eventually lead to a superconducting phase at Tc= 17 K for LiP8 stoichiometry. Using angle-resolved photoemission spectroscopy we confirm that the surface of black phosphorus can be chemically functionalized using Li atoms which donate their 2s electron to the conduction band. The combined theoretical and experimental study demonstrates the semiconductor-metal transition indicating a feasible way to induce a superconducting phase in phosphorene and few-layer black phosphorus