Search results for "Delocalized electron"
showing 10 items of 158 documents
Electronic structure of Gold, Aluminum and Gallium Superatom Complexes
2011
Using ab initio computational techniques on crystal determined clusters, we report on the similarities and differences of Al${}_{50}$(C${}_{5}$(CH${}_{3}{{)}_{5})}_{12}$, Ga${}_{23}$(N(Si(CH${}_{3}{)}_{3}$)${}_{2}$)${}_{11}$, and Au${}_{102}$(SC${}_{7}$O${}_{2}$H${}_{5}$)${}_{44}$ ligand-protected clusters. Each of the ligand-protected clusters in this study shows a similar stable character which can be described via an electronic shell model. We show here that the same type of analysis leads consistently to derivation of a superatomic electronic counting rule, independently of the metal and ligand compositions. One can define the cluster core as the set of atoms where delocalized single-an…
First-principles and semiempirical Hartree-Fock calculations for F centers in KNbO3 and Li impurities in KTaO3
1998
The LMTO method based on the density-functional theory and the semi-empirical INDO method based on the Hartree--Fock formalism are used for the supercell study of the F centers in cubic and orthorhombic ferroelectric KNbO3 crystals. Two electrons are found to be considerably delocalized even in the ground state of the defect. The absorption energies were calculated by means of the INDO method using the Delta-SCF scheme after a relaxation of atoms surrounding the F center. As an example of another type of point defect in perovskite, an isolated Li impurity in KTaO3 as well as interacting Li pairs are considered in the supercell approach, using the supercells of up to 270 atoms. The off-cente…
Delocalization-Localization Transition due to Anharmonicity
2008
Analytical and numerical calculations for a reduced Fermi-Pasta-Ulam chain demonstrate that energy localization does not require more than one conserved quantity. Clear evidence for the existence of a sharp delocalization-localization transition at a critical amplitude is given. Approaching the critical amplitude from above and below, diverging time scales occur. Above the critical amplitude, the energy packet converges towards a discrete breather. Nevertheless, ballistic energy transportation is present, demonstrating that its existence does not necessarily imply delocalization.
Observation of a charge delocalization from Se vacancies inBi2Se3: A positron annihilation study of native defects
2016
By means of positron annihilation lifetime spectroscopy, we have investigated the native defects present in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, which belongs to the family of topological insulators. We experimentally demonstrate that selenium vacancy defects $({\text{V}}_{\text{Se1}})$ are present in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ as-grown samples, and that their charge is delocalized as temperature increases. At least from 100 K up to room temperature both ${\text{V}}_{\text{Se1}}^{0}$ and ${\text{V}}_{\text{Se1}}^{+}$ charge states coexist. The observed charge delocalization determines the contribution of ${\text{V}}_{\text{Se1}}$ defects to the $n$-type conductivity of ${\mathrm{…
Location- and observation time-dependent quantum-tunneling
2009
We investigate quantum tunneling in a translation invariant chain of particles. The particles interact harmonically with their nearest neighbors, except for one bond, which is anharmonic. It is described by a symmetric double well potential. In the first step, we show how the anharmonic coordinate can be separated from the normal modes. This yields a Lagrangian which has been used to study quantum dissipation. Elimination of the normal modes leads to a nonlocal action of Caldeira-Leggett type. If the anharmonic bond defect is in the bulk, one arrives at Ohmic damping, i.e. there is a transition of a delocalized bond state to a localized one if the elastic constant exceeds a critical value $…
Can the Double Exchange Cause Antiferromagnetic Spin Alignment?
2020
The effect of the double exchange in a square-planar mixed-valence dn+1&minus
Unraveling exciton dynamics in amorphous silicon dioxide: Interpretation of the optical features from 8 to 11 eV.
2011
Physical review / B 83, 174201 (2011). doi:10.1103/PhysRevB.83.174201
Postpulse molecular alignment measured by a weak field polarization technique
2003
We report a direct nonintrusive observation of alignment and planar delocalization of ${\mathrm{C}\mathrm{O}}_{2}$ after an intense linearly polarized femtosecond laser pulse excitation. The effects are measured by a polarization technique involving a perturbative probe that itself does not induce appreciable alignment. We show that this technique allows one to measure a signal proportional to $⟨{cos}^{2}\ensuremath{\theta}⟩\ensuremath{-}1/3$, with $\ensuremath{\theta}$ the angle between the molecular axis and the laser polarization. Simulations that support this analysis allow one to characterize the experimentally observed alignment and planar delocalization quantitatively.
Exchange Interactions I: Mechanisms
1996
A most important phenomenon in molecular magnetism is the exchange interaction between magnetic centers. Its relevance as well as the terms and concepts required to its rationalization were stated long ago by physicists working in the quantum-mechanical theory of magnetism (Heisenberg, Dirac, van Vleck, Anderson, Zener, and many others). Depending on the extent of delocalization of the magnetic moments and on the metallic/non-metallic properties of the solid four kinds of exchange coupling were usually distinguished in the physical literature namely direct exchange, superexchange, indirect exchange and itinerant exchange [1]. The relations of these types of couplings are depicted in Figure …
Study of the dynamical approach to the interface localization–delocalization transition of the confined Ising model
2004
Confined magnetic Ising films in a L ? D geometry (), with short-range competing magnetic fields?(h) acting at opposite walls along the D-direction, exhibit a slightly rounded localization?delocalization transition of the interface between domains of different orientations that runs parallel to the walls. This transition is the precursor of a wetting transition that occurs in the limit of infinite film thickness () at the critical curve Tw(h). For T Tw(h)) such an interface is bounded (unbounded) to the walls, while right at Tw(h) the interface is freely fluctuating around the centre of the film. Starting from disordered configurations, corresponding to , we quench to the wetting critical t…