1,755 research outputs found
Hartree-Fock based diagonalization: an efficient method for simulating disordered interacting electrons
We present an efficient numerical method for simulating the low-energy
properties of disordered many-particle systems. The method which is based on
the quantum-chemical configuration interaction approach consists in
diagonalizing the Hamiltonian in an energetically truncated basis build of the
low-energy states of the corresponding Hartree-Fock Hamiltonian. As an example
we investigate the quantum Coulomb glass, a model of spinless electrons in a
random potential interacting via long-range Coulomb interaction. We find that
the Coulomb interaction increases the conductance of strongly disordered
systems but reduces the conductance of weakly disordered systems.Comment: 7 pages, 3 eps figures included, invited talk at Conference on
Computational Physics (Granada, Sep 1998
Critical properties of the metal-insulator transition in anisotropic systems
We study the three-dimensional Anderson model of localization with
anisotropic hopping, i.e., weakly coupled chains and weakly coupled planes. In
our extensive numerical study we identify and characterize the metal-insulator
transition by means of the transfer-matrix method. The values of the critical
disorder obtained are consistent with results of previous studies,
including multifractal analysis of the wave functions and energy level
statistics. decreases from its isotropic value with a power law as a
function of anisotropy. Using high accuracy data for large system sizes we
estimate the critical exponent as . This is in agreement with
its value in the isotropic case and in other models of the orthogonal
universality class.Comment: 17 pages, 7 figures, requires svjour.csl and svepj.clo (included),
submitted to EPJ
Bursts of extensive air showers: chaos vs. stochasticity
Bursts of the count rate of extensive air showers (EAS) lead to the
appearance of clusters in time series that represent EAS arrival times. We
apply methods of nonlinear time series analysis to twenty EAS cluster events
found in the data set obtained with the EAS-1000 prototype array. In
particular, we use the Grassberger-Procaccia algorithm to compute the
correlation dimension of the time series in the vicinity of the clusters. We
find that four cluster events produce signs of chaos in the corresponding time
series. By applying a number of supplementary methods we assess that the nature
of the observed behaviour of the correlation dimension is likely to be
deterministic. We suggest a simple qualitative model that might explain an
origin of clusters in general and "possibly chaotic" clusters in particular.
Finally, we compare our conclusions with the results of similar investigations
performed by the EAS-TOP and LAAS groups.Comment: An extended version of the paper to be submitted to Astroparticle
Physics. Version 2: 22 pages, discussion extended, the main part shortened,
accepted for publication. Version 1 is still valid (up to a number of typos
Quantum Coulomb Glass within a Hartree-Fock Approximation
We study the influence of electron-electron interactions on the electronic properties of disordered materials. In particular, we consider the insulating side of a metal-insulator transition where screening breaks down and the electron-electron interaction remains long ranged. The investigations are based on the quantum Coulomb glass, a generalization of the classical Coulomb glass model of disordered insulators. The quantum Coulomb glass is studied by decoupling the Coulomb interaction by means of a Hartree-Fock approximation and exactly diagonalizing the remaining localization problem. We investigate the behavior of the Coulomb gap in the density of states when approaching the metal-insulator transition and study the influence of the interaction on the localization of the electrons. We find that the interaction leads to an enhancement of localization at the Fermi level
The effect of pressure on the thermodynamic properties of seawater
For many thermodynamic calculations in oceanography it is necessary to know the effect of pressure (or depth) on various thermodynamic properties. … Recently, a new equation of state for sea-water (Millero et al., 1980; Millero and Poisson, 1981) has been adopted by the UNESCO/ICES/IAPSO joint panel on oceanographic tables and standards. By appropriate differentiation of this equation of state, it is possible to determine the pressure derivatives for the specific volume of seawater solutions. To estimate the effect of pressure on the partial molal thermochemical properties (Millero and Leung, 1976), it is necessary to know the partial molal volumes of sea salt and water as a function of pressure. This can be accomplished by fitting the apparent molal volumes (c{\u3e,,) of seawater solutions to a function of pressure..
Comparative study of the growth of sputtered aluminum oxide films on organic and inorganic substrates
We present a comparative study of the growth of the technologically highly
relevant gate dielectric and encapsulation material aluminum oxide in inorganic
and also organic heterostructures. Atomic force microscopy studies indicate
strong similarities in the surface morphology of aluminum oxide films grown on
these chemically different substrates. In addition, from X-ray reflectivity
measurements we extract the roughness exponent \beta of aluminum oxide growth
on both substrates. By renormalising the aluminum oxide roughness by the
roughness of the underlying organic film we find good agreement with \beta as
obtained from the aluminum oxide on silicon oxide (\beta = 0.38 \pm 0.02),
suggesting a remarkable similarity of the aluminum oxide growth on the two
substrates under the conditions employed
Effects of Flexibility in Coarse-Grained Models for Bovine Serum Albumin and Immunoglobulin G
We construct a coarse-grained, structure-based, low-resolution, 6-bead flexible model of bovine serum albumin (BSA, PDB: 4F5S), which is a popular example of a globular protein in biophysical research. The model is obtained via direct Boltzmann inversion using all-atom simulations of a single molecule, and its particular form is selected from a large pool of 6-bead coarse-grained models using two suitable metrics that quantify the agreement in the distribution of collective coordinates between all-atom and coarse-grained Brownian dynamics simulations of solutions in the dilute limit. For immunoglobulin G (IgG), a similar structure-based 12-bead model has been introduced in the literature [Chaudhri et al., J. Phys. Chem. B 116, 8045 (2012)] and is employed here to compare findings for the compact BSA molecule and the more anisotropic IgG molecule. We define several modified coarse-grained models of BSA and IgG, which differ in their internal constraints and thus account for a variation of flexibility. We study denser solutions of the coarse-grained models with purely repulsive molecules (achievable by suitable salt conditions) and address the effect of packing and flexibility on dynamic and static behavior. Translational and rotational self-diffusivity is enhanced for more elastic models. Finally, we discuss a number of effective sphere sizes for the BSA molecule, which can be defined from its static and dynamic properties. Here, it is found that the effective sphere diameters lie between 4.9 and 6.1 nm, corresponding to a relative spread of about ±10% around a mean of 5.5 nm
Strongly Enhanced Thermal Stability of Crystalline Organic Thin Films Induced by Aluminum Oxide Capping Layers
We show that the thermal stability of thin films of the organic semiconductor
diindenoperylene (DIP) can be strongly enhanced by aluminum oxide capping
layers. By thermal desorption spectroscopy and in-situ X-ray diffraction we
demonstrate that organic films do not only stay on the substrate, but even
remain crystalline up to 460C, i.e. 270 deg. above their desorption point for
uncapped films (190C). We argue that this strong enhancement of the thermal
stability compared to uncapped and also metal-capped organic layers is related
to the very weak diffusion of aluminum oxide and the structurally well-defined
as-grown interfaces. We discuss possible mechanisms for the eventual breakdown
at high temperatures.Comment: 5 pages, 4 figures, submitted to Adv. Mat., for further information
see http://www.physchem.ox.ac.uk/~f
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