1,721,033 research outputs found
Tuning the optical absorption and exciton bound states of germanene by chemical functionalization
No full textAbstract We present a comprehensive study of buckled honeycomb germanene functionalized with alternately bonded side groups hydroxyl (–H), methyl (–CH3) and trifluoro methyl (–CF3). By means of most modern theoretical and computational methods we determine the atomic geometries versus the functionalizing groups. The quasiparticle excitation effects on the electronic structure are taken into account by means of exchange-correlation treatment within the GW framework. The Bethe–Salpeter equation is solved ab initio to derive optical spectra including excitonic and quasiparticle effects. Band edge excitons are investigated in detail. The binding properties are compared with those resulting from model studies. The functionalization leads to significantly modified band structures compared with pristine germanene. The Dirac bands near the K point are destroyed and direct gaps appear at the point. Together with the many-body effects, quasiparticle gaps of 2.3, 1.8 or 1.0 eV result for –H, –CH3 and –CF3 functionalization. Totally different absorption spectra are found for in-plane and out-of-plane light polarization. Strongly bound excitons are visible below the quasiparticle band edge with binding energies of about 0.5, 0.4 or 0.3 eV. The nature of these band-gap excitons is investigated via their wave function, the contribution of various interband combinations and the dipole selection rules
Electronic and Optical Properties of Alkaline Earth Metal Fluoride Crystals with the Inclusion of Many-Body Effects: A Comparative Study on Rutile MgF2 and Cubic SrF2
Full text linkWe conducted a systematic investigation using state-of-the-art techniques on the electronic and optical properties of two crystals of alkaline earth metal fluorides, namely rutile MgF2 and cubic SrF2. For these two crystals of different symmetry, we present density functional theory (DFT), many-body perturbation theory (MBPT), and Bethe–Salpeter equation (BSE) calculations. We calculated a variety of properties, namely ground-state energies, band-energy gaps, and optical absorption spectra with the inclusion of excitonic effects. The quantities were obtained with a high degree of convergence regarding all bulk electronic and optical properties. Bulk rutile MgF2 has distinguished ground-state and excited-state properties with respect to the other cubic fluoride SrF2 and the other members of the alkaline earth metal fluoride family. The nature of the fundamental gaps and estimates of the self-energy and excitonic effects for the two compounds are presented and discussed in detail. Our results are in good accordance with the measurements and other theoretical–computational data. A comparison is made between the excitation and optical properties of bulk rutile MgF2, cubic SrF2, and the corresponding clusters, for which calculations have recently been published, confirming strong excitonic effects in finite-sized systems
Honeycomb silicon on alumina: Massless Dirac fermions in silicene on substrate
No full textWe predict the stability of a graphenelike silicene sheet on one monolayer of aluminum oxide. We find that the honeycomb buckled structure of silicene is not broken upon interaction with one monolayer of Al2O3 in the kagome geometry. As a consequence, the electronic band structure shows unperturbed cones with massless Dirac fermions embedded into Al2O3-derived bands. The heterostructure conserves the topological character of the freestanding silicene. The optical properties of the silicene/Al2O3 bilayers clearly maintain the characteristic infrared universal limit of pi alpha. A quantized absorbance N center dot pi alpha (with N silicene layers) also occurs for stacking of multilayers
Transitions in Xenes between excitonic, topological and trivial insulator phases: influence of screening, band dispersion and external electric field
Full text linkUsing a variational approach, the binding energies of the lowest bound
excitons in Xenes under varying electric field are investigated. The internal
exciton motion is described both by Dirac electron dispersion and in
effective-mass approximation, while the screened electron-hole attraction is
modeled by a Rytova-Keldysh potential with a 2D electronic polarizability
. The most important parameters as spin-orbit-induced gap
, Fermi velocity and are taken from ab initio
density functional theory calculations. In addition, is
approximated in two different ways. The relation of and is ruled by
the screening. The existence of an excitonic insulator phase with
sensitively depends on the chosen . The values of and
are strongly modified by a vertical external electric bias
, which defines a transition from the topological into a trivial insulator
at , with the exception of plumbene. Within the Dirac approximation,
but also within the effective mass description of the kinetic energy, the
treatment of screening dominates the appearance or non-appearance of an
excitonic insulator phase. Gating does not change the results: the prediction
done at zero electric field is confirmed when a vertical electric field is
applied. Finally, Many-Body perturbation theory approaches based on the Green's
function method, applied to stanene, confirm the absence of an excitonic
insulator phase, thus validating our results obtained by ab initio modeling of
Parameterfreie Untersuchungen an MnO in verschiedenen Kristallstrukturen und unter Berücksichtigung nichtlokaler Austauscheffekte
Full text linkIn der Diplomarbeit wird das Materialsystem Manganoxid (MnO) im Rahmen verschiedener Näherungen innerhalb der Dichtefunktionaltheorie untersucht. MnO, wie auch andere Übergangsmetalloxide, gehören zu den so genannten stark korrelierten Systemen. Diese Systeme werden im Rahmen der Standardnäherungen zur Beschreibung von Elektronenaustausch und -korrelation, wie der Generalized Gradient Approximation (GGA) nicht hinreichend gut beschrieben. Daher werden in der Diplomarbeit verschiedene erweiterte Ansätze zur Beschreibung von Austausch und Korrelation, durch Hinzunahme eines effektiven Austauschparameters U (GGA+U-Näherung) beziehungsweise durch Berücksichtigung eines Teils "echten" Hartree-Fock-Austausches (HSE03-Näherung), untersucht. Weiterhin werden die magnetischen Eigenschaften von MnO im Rahmen der Molekularfeldnäherung untersucht, wobei speziell die effektiven Kopplungsstärken der magnetischen Mn-Ionen sowie die Phasenübergangstemperaturen im Rahmen dieser Theorie bestimmt werden
Vielteilchenwechselwirkungen von Elektronen in einem Hubbard-Cluster
Full text linkAn important method in many-particle physics is related to the Hubbard model which can be used to investigate the behavior of fermions, particularly to study exchange and correlation of electron-
electron interaction. For a special cluster, the molecule consisting of two atoms, the electrons interact via one hopping integral, and for a small number of N = 1; 2; 3 electrons, the model has the advantage that it can be solved exactly. Therefore, some important physical quantities like the Green's function, the spectral function and the self-energy are calculated and discussed. Exchange and correlation are fully treated. The self-energies obtained by two approximate methods, namely the Hartree-Fock and the GW approximation, are compared with the exact self-energy. The comparison allows a discussion of the validity of the approximations versus the ratio of Hubbard parameter and hopping integral
Parameterfreie Untersuchung der elektronischen Struktur von graphitartigem Kohlenstoff
Full text linkDas Ziel dieser Diplomarbeit war die Untersuchung und der Vergleich der elektronischen Struktur sowie kollektiver Anregungen (Plasmonen) in einwandigen Kohlenstoff-Nanoröhrchen (SWCNT), Graphene und Graphit. Dazu wurden ab-initio Rechnungen im Rahmen der (zeitabhängigen) Dichtefunktional-Theorie durchgeführt und die Resultate sowohl mit einfachen theoretischen Modellen als auch mit Experimenten zur Elektronen-Energieverlust-Spektroskopie verglichen. Unter anderem wurde die Plasmonendispersion in Graphene erstmalig mit Hilfe von ab-initio Rechnungen untersucht, sowie mit Messungen an isolierten SWCNT verglichen: die Übereinstimmung ist ausgezeichnet und in beiden Systemen findet man eine fast lineare Dispersion. Zusätzlich wurde die Verlustfunktion von Graphit für Impulsüberträge außerhalb der ersten Brillouin-Zone untersucht und erstmals eine starke Anisotropie der dielektrischen Funktion in der Nähe von Bragg-Reflexen festgestellt: infinitesimale Änderungen im Impulsübertrag können dann das Verlustspektrum völlig verändern. Dieser Effekt wurde auf starke Lokalfeldeffekte in Graphit zurückgeführt sowie Vorschläge zur experimentellen Überprüfung angegeben
Tuning Gaps and Schottky Contacts of Graphene/Phosphorene Heterostructures by Vertical Electric Field and Strain
Full text linkWe present a comprehensive study of the structural and electronic properties of a graphene/
phosphorene (G/P) heterostructure in the framework of density functional theory, including van der
Waals interaction in the exchange–correlation functional. While the G(4 1)/P(3 1) superlattice
usually used in the literature is subject to a strain as high as about 7%, the in-plane strain could
be drastically reduced to under 1% in the G(4 13)/P(3 12) heterostructure investigated here.
Adapting the lattice constants of the rectangular lattices, the equilibrium configuration in the xy
plane of phosphorene relative to the graphene layer is optimized. This results in an equilibrium
interlayer distance of 3.5 Å and a binding energy per carbon atom of 37 meV, confirming the presence
of weak van der Waals interaction between the graphene and the phosphorene layers. The electronic
properties of the heterostructure are evaluated under different values of interlayer distance, strain
and applied vertical electric field. We demonstrate that G/P heterostructures form an n-type Schottky
contact, which can be transformed into p-type under external perturbations. These findings, together
with the possibility to control the gaps and barrier heights, suggest that G/P heterostructures are
promising for novel applications in electronics and may open a new avenue for the realization of
innovative optoelectronic devices
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