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Sep 23, 2013
09/13
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T. A. Niehaus; D. Heringer; B. Torralva; Th. Frauenheim
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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...
Source: http://arxiv.org/abs/physics/0411104v2
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47
Sep 23, 2013
09/13
by
Y. Wang; R. Q. Zhang; Th. Frauenheim; T. A. Niehaus
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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...
Source: http://arxiv.org/abs/0903.3022v2
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36
Sep 22, 2013
09/13
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Y. Wang; C. -Y. Yam; G. H. Chen; Th. Frauenheim; T. A. Niehaus
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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...
Source: http://arxiv.org/abs/1101.5929v1
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57
Sep 23, 2013
09/13
by
S. I. Simdyankin; S. R. Elliott; Z. Hajnal; T. A. Niehaus; Th. Frauenheim
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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...
Source: http://arxiv.org/abs/cond-mat/0312224v1
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37
Sep 22, 2013
09/13
by
S. I. Simdyankin; T. A. Niehaus; G. Natarajan; Th. Frauenheim; S. R. Elliott
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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...
Source: http://arxiv.org/abs/cond-mat/0409441v1
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52
Sep 23, 2013
09/13
by
T. A. Niehaus; M. Rohlfing; F. Della Sala; A. Di Carlo; Th. Frauenheim
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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...
Source: http://arxiv.org/abs/cond-mat/0411024v1
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45
Sep 18, 2013
09/13
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S. I. Simdyankin; M. Elstner; T. A. Niehaus; Th. Frauenheim; S. R. Elliott
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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...
Source: http://arxiv.org/abs/cond-mat/0504207v1
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54
Sep 18, 2013
09/13
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M. Kaukonen; R. M. Nieminen; P. K. Sitch; G. Jungnickel; D. Porezag; Th. Frauenheim
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We present a study of the effects of Nitrogen and Boron doping on the growth of CVD diamond in the (100) mechanism. These are the first calculations of this type and show that, in accordance with recent experimental results, the presently accepted growth mechanism, that due to Harris, is cataylsed by the presence of subsurface Boron impurities. In contrast, we find that the Harris Mechanism cannot explain growth in the presence of subsurface N and suggest an alternative mechanism.
Source: http://arxiv.org/abs/cond-mat/9710054v1
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33
Sep 23, 2013
09/13
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G. Waldherr; J. Beck; M. Steiner; P. Neumann; A. Gali; Th. Frauenheim; F. Jelezko; J. Wrachtrup
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The nitrogen-vacancy (NV) center in diamond is supposed to be a building block for quantum computing and nanometer scale metrology at ambient conditions. Therefore, precise knowledge of its quantum states is crucial. Here, we experimentally show that under usual operating conditions the NV exists in an equilibrium of two charge states (70% in the expected negative (NV-) and 30% in the neutral one (NV0)). Projective quantum non-demolition measurement of the nitrogen nuclear spin enables the...
Source: http://arxiv.org/abs/1012.5017v2
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63
Sep 18, 2013
09/13
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G. Schulze; K. J. Franke; A. Gagliardi; G. Romano; C. S. Lin; A. Da Rosa; T. A. Niehaus; Th. Frauenheim; A. Di Carlo; A. Pecchia; J. I. Pascual
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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...
Source: http://arxiv.org/abs/0803.1358v1
