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Title:

Time-dependent density-functional approach for exciton binding energies

Description:

Optical processes in insulators and semiconductors, including excitonic effects, can be described in principle exactly using time-dependent density-functional theory (TDDFT). Starting from a linearization of the TDDFT semiconductor Bloch equations in a two-band model, we derive a simple formalism for calculating excitonic binding energies. This ...

Optical processes in insulators and semiconductors, including excitonic effects, can be described in principle exactly using time-dependent density-functional theory (TDDFT). Starting from a linearization of the TDDFT semiconductor Bloch equations in a two-band model, we derive a simple formalism for calculating excitonic binding energies. This formalism leads to a generalization of the standard Wannier equation for excitons, featuring a nonlocal effective electron-hole interaction determined by long-range and dynamical exchange-correlation (XC) effects. We calculate excitonic binding energies in several direct-gap semiconductors, using exchange-only and model XC kernels. ; Comment: 4 pages, 1 figure Minimize

Year of Publication:

2009-02-27

Document Type:

text

Subjects:

Condensed Matter - Materials Science ; Condensed Matter - Other Condensed Matter

Condensed Matter - Materials Science ; Condensed Matter - Other Condensed Matter Minimize

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Title:

Time-dependent density-functional theory for electronic excitations in materials: basics and perspectives

Description:

Time-dependent density-functional theory (TDDFT) is widely used to describe electronic excitations in complex finite systems with large numbers of atoms, such as biomolecules and nanocrystals. The first part of this paper will give a simple and pedagogical explanation, using a two-level system, which shows how the basic TDDFT formalism for excit...

Time-dependent density-functional theory (TDDFT) is widely used to describe electronic excitations in complex finite systems with large numbers of atoms, such as biomolecules and nanocrystals. The first part of this paper will give a simple and pedagogical explanation, using a two-level system, which shows how the basic TDDFT formalism for excitation energies works. There is currently an intense effort underway to develop TDDFT methodologies for the charge and spin dynamics in extended systems, to calculate optical properties of bulk and nanostructured materials, and to study transport through molecular junctions. The second part of this paper highlights some challenges and recent advances of TDDFT in these areas. Two examples are discussed: excitonic effects in insulators and intersubband plasmon excitations in doped semiconductor quantum wells. ; Comment: 15 pages, 2 figures, International Conference on Materials Discovery and Databases: Materials Informatics and DFT Minimize

Year of Publication:

2008-08-14

Document Type:

text

Subjects:

Condensed Matter - Materials Science

Condensed Matter - Materials Science Minimize

DDC:

541 Physical chemistry *(computed)*

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Title:

Time-dependent density-functional theory for ultrafast interband excitations

Description:

We formulate a time-dependent density functional theory (TDDFT) in terms of the density matrix to study ultrafast phenomena in semiconductor structures. A system of equations for the density matrix components, which is equivalent to the time-dependent Kohn-Sham equation, is derived. From this we obtain a TDDFT version of the semiconductor Bloch ...

We formulate a time-dependent density functional theory (TDDFT) in terms of the density matrix to study ultrafast phenomena in semiconductor structures. A system of equations for the density matrix components, which is equivalent to the time-dependent Kohn-Sham equation, is derived. From this we obtain a TDDFT version of the semiconductor Bloch equations, where the electronic many-body effects are taken into account in principle exactly. As an example, we study the optical response of a three-dimensional two-band insulator to an external short-time pulsed laser field. We show that the optical absorption spectrum acquires excitonic features when the exchange-correlation potential contains a $1/q^{2}$ Coulomb singularity. A qualitative comparison of the TDDFT optical absorption spectra with the corresponding results obtained within the Hartree-Fock approximation is made. Minimize

Year of Publication:

2007-09-19

Document Type:

text

Subjects:

Condensed Matter - Mesoscale and Nanoscale Physics

Condensed Matter - Mesoscale and Nanoscale Physics Minimize

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Title:

A quantitative test for superoxide radicals produced in biological systems.

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Description:

The preparation and properties of a partially succinoylated cytochrome c, suited for the detection of superoxide anion radicals in liver microsomes, is reported. By succinoylation of 45% of the primary amino groups of horse heart cytochrome c the activity towards solubilized NADPH--cytochrome P-450 reductase was diminished by 99% compared with n...

The preparation and properties of a partially succinoylated cytochrome c, suited for the detection of superoxide anion radicals in liver microsomes, is reported. By succinoylation of 45% of the primary amino groups of horse heart cytochrome c the activity towards solubilized NADPH--cytochrome P-450 reductase was diminished by 99% compared with native cytochrome c. The capacities of cytochrome b5 and cytochrome c oxidase to reduce the succinoylated ferricytochrome c and oxidize succinoylated ferrocytochrome c respectively were decreased to a similar extent. However, the bimolecular rate constant for the reduction of the partially succinoylated ferricytochrome c by O2-. was estimated to be one-tenth of the value for the reaction of O2-. with native ferricytochrome c a pH 7.7. On this basis the quantification of O2-. generated by NADPH-supplemented liver microsomes became possible. The initial rates of succinoylated ferricytochrome c reduction determined at various finite concentrations of the cytochrome c derivative can be extrapolated to obtain true rates of O2-. generation in a homogeneous system. The problems encountered in the quantitative determination of O2-. produced in biological membranes, e.g. microsomes, are discussed. Minimize

Year of Publication:

1982-06-01

Document Type:

Text

Language:

en

Subjects:

Research Article

Research Article Minimize

DDC:

500 Natural sciences & mathematics *(computed)*

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Title:

Receptor occupancy regulates Ca2+ entry and intracellular Ca2+ redistribution in activated human platelets.

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Description:

Fura-2-loaded human platelets were used to study Ca2+ release from intracellular compartments, as well as Ca2+ influx from the extracellular space. We investigated the response towards the endoperoxide/thromboxane-receptor agonist. U46619, and the inhibitor of the endoplasmic-reticulum Ca(2+)-ATPase, thapsigargin. U46619 dose-dependently deplete...

Fura-2-loaded human platelets were used to study Ca2+ release from intracellular compartments, as well as Ca2+ influx from the extracellular space. We investigated the response towards the endoperoxide/thromboxane-receptor agonist. U46619, and the inhibitor of the endoplasmic-reticulum Ca(2+)-ATPase, thapsigargin. U46619 dose-dependently depleted intracellular Ca2+ stores, followed by active sequestration of released Ca2+. Ca2+ influx induced by U46619 largely relies on receptor occupancy. Removing the thromboxane analogue from its receptor by using the endoperoxide/thromboxane-receptor antagonist BM 13177 largely blunted U46619-mediated Ca2+ influx. The Ca(2+)-ATPase inhibitor thapsigargin evoked a gradual rise in intracellular Ca2+, which was potentiated by a preceding activation of platelets with the receptor agonist U46619. This agonist-sensitizing effect also depends on receptor occupancy. Removing U46619 from its receptor by addition of the endoperoxide/thromboxane-receptor antagonist BM13177 suppressed the sensitizing effect completely. Furthermore, interrupting downstream receptor signalling events by raising intracellular levels of cyclic nucleotides (cyclic AMP, cyclic GMP) again suppressed the U46619-sensitizing effect on thapsigargin-induced Ca2+ release. This study indicates that the process of Ca2+ release followed by resequestration in response to a platelet agonist by its own is not sufficient to produce the sensitizing effect. Rather, a continuously occupied receptor triggering sustained downstream signalling events seems to be required for sensitization. The presence of a receptor agonist may induce an increased cycling of Ca2+ between the agonist-responsive and the thapsigargin-dischargeable compartment, leading to faster and more intense accumulation of Ca2+ in the cytosolic compartment after inhibition of the Ca(2+) ATPase. Suggestively, receptor occupancy increases the Ca(2+)-releasing potency of thapsigargin by coupling the thapsigargin-sensitive Ca(2+)-storing compartments with an agonist-responsive compartment that exhibits a high leakage rate in stimulated platelets. Minimize

Year of Publication:

1994-12-15

Document Type:

Text

Language:

en

Subjects:

Research Article

Research Article Minimize

DDC:

570 Life sciences; biology *(computed)*

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Title:

Thematic Mini-Review Series Thrombospondins in physiology and disease: new tricks for old dogs

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Year of Publication:

2012-04

Document Type:

Text

Language:

en

Subjects:

Article

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Rights:

© 2012 Elsevier B.V. All rights reserved.

© 2012 Elsevier B.V. All rights reserved. Minimize

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Title:

Model systems for mono-oxygenases, with results with iron(II)-sulphur complexes as models for cytochrome P-450.

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Year of Publication:

1971-11

Document Type:

Text

Language:

en

Subjects:

Research Article

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Title:

Response properties of III-V dilute magnetic semiconductors: interplay of disorder, dynamical electron-electron interactions and band-structure effects

Description:

A theory of the electronic response in spin and charge disordered media is developed with the particular aim to describe III-V dilute magnetic semiconductors like GaMnAs. The theory combines a detailed k.p description of the valence band, in which the itinerant carriers are assumed to reside, with first-principles calculations of disorder contri...

A theory of the electronic response in spin and charge disordered media is developed with the particular aim to describe III-V dilute magnetic semiconductors like GaMnAs. The theory combines a detailed k.p description of the valence band, in which the itinerant carriers are assumed to reside, with first-principles calculations of disorder contributions using an equation-of-motion approach for the current response function. A fully dynamic treatment of electron-electron interaction is achieved by means of time-dependent density functional theory. It is found that collective excitations within the valence band significantly increase the carrier relaxation rate by providing effective channels for momentum relaxation. This modification of the relaxation rate, however, only has a minor impact on the infrared optical conductivity in GaMnAs, which is mostly determined by the details of the valence band structure and found to be in agreement with experiment. ; Comment: 15 pages, 9 figures Minimize

Year of Publication:

2011-01-27

Document Type:

text

Subjects:

Condensed Matter - Materials Science ; Condensed Matter - Disordered Systems and Neural Networks

Condensed Matter - Materials Science ; Condensed Matter - Disordered Systems and Neural Networks Minimize

DDC:

530 Physics *(computed)*

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Title:

Non-adiabatic electron dynamics in time-dependent density-functional theory

Description:

Time-dependent density-functional theory (TDDFT) treats dynamical exchange and correlation (xc) via a single-particle potential, Vxc(r,t), defined as a nonlocal functional of the density n(r',t'). The popular adiabatic local-density approximation (ALDA) for Vxc(r,t) uses only densities at the same space-time point (r,t). To go beyond the ALDA, t...

Time-dependent density-functional theory (TDDFT) treats dynamical exchange and correlation (xc) via a single-particle potential, Vxc(r,t), defined as a nonlocal functional of the density n(r',t'). The popular adiabatic local-density approximation (ALDA) for Vxc(r,t) uses only densities at the same space-time point (r,t). To go beyond the ALDA, two local approximations have been proposed based on quantum hydrodynamics and elasticity theory: (a) using the current as basic variable (C-TDDFT) [G. Vignale, C. A. Ullrich, and S. Conti, Phys. Rev. Lett. 79, 4878 (1997)], (b) working in a co-moving Lagrangian reference frame (L-TDDFT) [I. V. Tokatly, Phys. Rev. B 71, 165105 (2005)]. This paper illustrates, compares, and analyzes both non-adiabatic theories for simple time-dependent model densities in the linear and nonlinear regime, for a broad range of time and frequency scales. C- and L-TDDFT are identical in certain limits, but in general exhibit qualitative and quantitative differences in their respective treatment of elastic and dissipative electron dynamics. In situations where the electronic density rapidly undergoes large deformations, it is found that non-adiabatic effects can become significant, causing the ALDA to break down. ; Comment: 15 pages, 15 figures Minimize

Year of Publication:

2006-02-13

Document Type:

text

Subjects:

Condensed Matter - Mesoscale and Nanoscale Physics

Condensed Matter - Mesoscale and Nanoscale Physics Minimize

DDC:

541 Physical chemistry *(computed)*

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Title:

Memory function formalism approach to electrical conductivity and optical response of dilute magnetic semiconductors

Description:

A combination of the memory function formalism and time-dependent density-functional theory is applied to transport in dilute magnetic semiconductors. The approach considers spin and charge disorder and electron-electron interaction on an equal footing. Within the weak disorder limit and using a simple parabolic approximation for the valence ban...

A combination of the memory function formalism and time-dependent density-functional theory is applied to transport in dilute magnetic semiconductors. The approach considers spin and charge disorder and electron-electron interaction on an equal footing. Within the weak disorder limit and using a simple parabolic approximation for the valence band we show that Coulomb and exchange scattering contributions to the resistivity in GaMnAs are of the same order of magnitude. The positional correlations of defects result in a significant increase of Coulomb scattering, while the suppression of localized spin fluctuations in the ferromagnetic phase contributes substantially to the experimentally observed drop of resistivity below T_c. A proper treatment of dynamical screening and collective excitations is essential for an accurate description of infrared absorption. ; Comment: Proceedings of the 13th Brazilian Workshop on Semiconductors Physics Minimize

Year of Publication:

2007-04-16

Document Type:

text

Subjects:

Condensed Matter - Materials Science

Condensed Matter - Materials Science Minimize

DDC:

530 Physics *(computed)*

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