arXiv subjects

T. A. Niehaus

Publications and source records attributed to T. A. Niehaus.

13 recordsLinked to original sources

arXiv2015

Stacking dependence of carrier transport properties in multilayered black phosphorous

We present the effect of different stacking orders on carrier transport properties of multi-layer black phosphorous. We consider three different stacking orders AAA, ABA and ACA, with increasing number of layers (from 2 to 6 layers). We employ a hierarchical approach in density functional theory (DFT), with structural simulations performed with Generalized Gradient Approximation (GGA) and the bandstructure, carrier effective masses and optical properties evaluated with the Meta-Generalized Gradient Approximation (MGGA). The carrier transmission in the various black phosphorous sheets was carried out with the non-equilibrium Greens function (NEGF) approach. The results show that ACA stacking has the highest electron and hole transmission probabilities. The results show tunability for a wide range of band-gap, carrier effective masses and transmission with a great promise for lattice engineering (stacking order and layers) in black phosphorous.

cond-mat.mtrl-sci↗

arXiv2013

Spatial extension of excitons in triphenylene based polymers given by range-separated functionals

Motivated by an experiment in which the singlet-triplet gap in triphenylene based copolymers was effectively tuned, we used time dependent density functional theory (TDDFT) to reproduce the main results. By means of conventional and long-range corrected exchange correlation functionals, the luminescence energies and the exciton localization were calculated for a triphenylene homopolymer and several different copolymers. The phosphorescence energy of the pure triphenylene chain is predicted accurately by means of the optimally tuned long-range corrected LC-PBE functional and slightly less accurate by the global hybrid B3LYP. However, the experimentally observed fixed phosphorescence energy could not be reproduced because the localization pattern is different to the expectations: Instead of localizing on the triphenylene moiety - which is present in all types of polymers - the triplet state localizes on the different bridging units in the TDDFT calculations. This leads to different triplet emission energies for each type of polymer. Yet, there are clear indications that long-range corrected TDDFT has the potential to predict the triplet emission energies as well as the localization behavior more accurate than conventional local or semi-local functionals.

physics.chem-ph↗

arXiv2013

Extensions of the time-dependent density functional based tight-binding approach

The time-dependent density functional based tight-binding (TD-DFTB) approach is generalized to account for fractional occupations. In addition, an on-site correction leads to marked qualitative and quantitative improvements over the original method. Especially, the known failure of TD-DFTB for the description of σ-> π* and n -> π* excitations is overcome. Benchmark calculations on a large set of organic molecules also indicate a better description of triplet states. The accuracy of the revised TD-DFTB method is found to be similar to first principles TD-DFT calculations at a highly reduced computational cost. As a side issue, we also discuss the generalization of the TD-DFTB method to spin-polarized systems. In contrast to an earlier study [Trani et al., JCTC 7 3304 (2011)], we obtain a formalism that is fully consistent with the use of local exchange-correlation functionals in the ground state DFTB method.

cond-mat.mtrl-sci↗

arXiv2011

An efficient method for quantum transport simulations in the time domain

An approximate method based on adiabatic time dependent density functional theory (TDDFT) is presented, that allows for the description of the electron dynamics in nanoscale junctions under arbitrary time dependent external potentials. In this scheme, the density matrix of the device region is propagated according to the Liouville-von Neumann equation. The semi-infinite leads give rise to dissipative terms in the equation of motion which are calculated from first principles in the wide band limit. In contrast to earlier {\em ab-initio} implementations of this formalism, the Hamiltonian is here approximated by a density expansion in the spirit of the density functional based tight-binding (DFTB) method without introducing empirical parameters. Results are presented for two prototypical molecular devices and compared to calculations at the full TDDFT level. The issue of non-existence of a steady state under certain conditions is also briefly touched on.

cond-mat.mes-hall↗

arXiv2009

Atomistic simulations of self-trapped exciton formation in silicon nanostructures: The transition from quantum dots to nanowires

Using an approximate time-dependent density functional theory method, we calculate the absorption and luminescence spectra for hydrogen passivated silicon nanoscale structures with large aspect ratio. The effect of electron confinement in axial and radial directions is systematically investigated. Excited state relaxation leads to significant Stokes shifts for short nanorods with lengths less than 2 nm, but has little effect on the luminescence intensity. The formation of self-trapped excitons is likewise observed for short nanostructures only; longer wires exhibit fully delocalized excitons with neglible geometrical distortion at the excited state minimum.

cond-mat.mtrl-sci↗

arXiv2008

Efficient evaluation of the Fourier Transform over products of Slater-type orbitals on different centers

Using the shift-operator technique, a compact formula for the Fourier transform of a product of two Slater-type orbitals located on different atomic centers is derived. The result is valid for arbitrary quantum numbers and was found to be numerically stable for a wide range of geometrical parameters and momenta. Details of the implementation are presented together with benchmark data for representative integrals. We also discuss the assets and drawbacks of alternative algorithms available and analyze the numerical efficiency of the new scheme.

cond-mat.mtrl-sci↗

arXiv2008

Resonant electron heating and molecular phonon cooling in single C$_{60}$ junctions

We study heating and heat dissipation of a single \c60 molecule in the junction of a scanning tunneling microscope (STM) by measuring the electron current required to thermally decompose the fullerene cage. The power for decomposition varies with electron energy and reflects the molecular resonance structure. When the STM tip contacts the fullerene the molecule can sustain much larger currents. Transport simulations explain these effects by molecular heating due to resonant electron-phonon coupling and molecular cooling by vibrational decay into the tip upon contact formation.

cond-mat.mtrl-sci↗

arXiv2007

Dynamical generalization of a solvable family of two-electron model atoms with general interparticle repulsion

Holas, Howard and March [Phys. Lett. A {\bf 310}, 451 (2003)] have obtained analytic solutions for ground-state properties of a whole family of two-electron spin-compensated harmonically confined model atoms whose different members are characterized by a specific interparticle potential energy u($r_{12}$). Here, we make a start on the dynamic generalization of the harmonic external potential, the motivation being the serious criticism levelled recently against the foundations of time-dependent density-functional theory (e.g. [J. Schirmer and A. Dreuw, Phys. Rev. A {\bf 75}, 022513 (2007)]). In this context, we derive a simplified expression for the time-dependent electron density for arbitrary interparticle interaction, which is fully determined by an one-dimensional non-interacting Hamiltonian. Moreover, a closed solution for the momentum space density in the Moshinsky model is obtained.

quant-ph↗

arXiv2005

Influence of copper on the electronic properties of amorphous chalcogenides

We have studied the influence of alloying copper with amorphous arsenic sulfide on the electronic properties of this material. In our computer-generated models, copper is found in two-fold near-linear and four-fold square-planar configurations, which apparently correspond to Cu(I) and Cu(II) oxidation states. The number of overcoordinated atoms, both arsenic and sulfur, grows with increasing concentration of copper. Overcoordinated sulfur is found in trigonal planar configuration, and overcoordinated (four-fold) arsenic is in tetrahedral configuration. Addition of copper suppresses the localization of lone-pair electrons on chalcogen atoms, and localized states at the top of the valence band are due to Cu 3d orbitals. Evidently, these additional Cu states, which are positioned at the same energies as the states due to ([As4]-)-([S_3]+) pairs, are responsible for masking photodarkening in Cu chalcogenides.

cond-mat.dis-nn↗

arXiv2005

Importance of electronic self-consistency in the TDDFT based treatment of nonadiabatic molecular dynamics

A mixed quantum-classical approach to simulate the coupled dynamics of electrons and nuclei in nanoscale molecular systems is presented. The method relies on a second order expansion of the Lagrangian in time-dependent density functional theory (TDDFT) around a suitable reference density. We show that the inclusion of the second order term renders the method a self-consistent scheme and improves the calculated optical spectra of molecules by a proper treatment of the coupled response. In the application to ion-fullerene collisions, the inclusion of self-consistency is found to be crucial for a correct description of the charge transfer between projectile and target. For a model of the photoreceptor in retinal proteins, nonadiabatic molecular dynamics simulations are performed and reveal problems of TDDFT in the prediction of intra-molecular charge transfer excitations.

physics.chem-ph↗

arXiv2004

Quasiparticle energies for large molecules: a tight-binding GW approach

We present a tight-binding based GW approach for the calculation of quasiparticle energy levels in confined systems such as molecules. Key quantities in the GW formalism like the microscopic dielectric function or the screened Coulomb interaction are expressed in a minimal basis of spherically averaged atomic orbitals. All necessary integrals are either precalculated or approximated without resorting to empirical data. The method is validated against first principles results for benzene and anthracene, where good agreement is found for levels close to the frontier orbitals. Further, the size dependence of the quasiparticle gap is studied for conformers of the polyacenes ($C_{4n+2}H_{2n+4}$) up to n = 30.

cond-mat.mtrl-sci↗

arXiv2004

A new type of charged defect in amorphous chalcogenides

We report on density-functional-based tight-binding (DFTB) simulations of a series of amorphous arsenic sulfide models. In addition to the charged coordination defects previously proposed to exist in chalcogenide glasses, a novel defect pair, [As4]--[S3]+, consisting of a four-fold coordinated arsenic site in a seesaw configuration and a three-fold coordinated sulfur site in a planar trigonal configuration, was found in several models. The valence-alternation pairs S3+-S1- are converted into [As4]--[S3]+ pairs under HOMO-to-LUMO electronic excitation. This structural transformation is accompanied by a decrease in the size of the HOMO-LUMO band gap, which suggests that such transformations could contribute to photo-darkening in these materials.

cond-mat.mtrl-sci↗

arXiv2003

Simulation of the physical properties of the chalcogenide glass As$_2$S$_3$ using a density-functional-based tight-binding method

We have used a density-functional-based tight-binding method in order to create structural models of the canonical chalcogenide glass, amorphous (a-)As$_2$S$_3$. The models range from one containing defects that are both chemical (homopolar bonds) and topological (valence-alternation pairs) in nature to one that is defect-free (stoichiometric). The structural, vibrational and electronic properties of the simulated models are in good agreement with experimental data where available. The electronic densities of states obtained for all models show clean optical band gaps. A certain degree of electron-state localization at the band edges is observed for all models, which suggests that photoinduced phenomena in chalcogenide glasses may not necessarily be attributed to the excitation of defects of only one particular kind.

cond-mat↗