45,622 research outputs found
Assessment of density-functional approximations: Long-range correlations and self-interaction effects
The complex nature of electron-electron correlations is made manifest in the very simple but nontrivial problem of two electrons confined within a sphere. The description of highly nonlocal correlation and self-interaction effects by widely used local and semilocal exchange-correlation energy density functionals is shown to be unsatisfactory in most cases. Even the best such functionals exhibit significant errors in the Kohn-Sham potentials and density profiles
Space Charge Effects in Ferroelectric Thin Films
The effects of space charges on hysteresis loops and field distributions in
ferroelectrics have been investigated numerically using the phenomenological
Landau-Ginzburg-Devonshire theory. Cases with the ferroelectric fully and
partially depleted have been considered. In general, increasing the number of
charged impurities results in a lowering of the polarization and coercive field
values. Squarer loops were observed in the partially depleted cases and a
method was proposed to identify fully depleted samples experimentally from
dielectric and polarization measurements alone. Unusual field distributions
found for higher dopant concentrations have some interesting implications for
leakage mechanisms and limit the range of validity of usual semiconductor
equations for carrier transport.Comment: 23 pages, 11 figure
Quantum conductance of homogeneous and inhomogeneous interacting electron systems
We obtain the conductance of a system of electrons connected to leads, within
time-dependent density-functional theory, using a direct relation between the
conductance and the density response function. Corrections to the
non-interacting conductance appear as a consequence of the functional form of
the exchange-correlation kernel at small frequencies and wavevectors. The
simple adiabatic local-density approximation and non-local density-terms in the
kernel both give rise to significant corrections in general. In the homogeneous
electron gas, the former correction remains significant, and leads to a failure
of linear-response theory for densities below a critical value.Comment: for resolution of the here published results see Phys. Rev. B 76,
125433 (2007
Comment on "Dynamical corrections to the DFT-LDA electron conductance in nanoscale systems"
In a recent paper Sai et al. [1] identified a correction R^{dyn}R=R_{s}+R^{dyn}R_{s}XCR^{dyn}$ in example systems of the type they considered should be considerably reduced, once a more appropriate form for the shear electron viscosity ¿ is used
Free Entropy Dimension in Amalgamated Free Products
We calculate the microstates free entropy dimension of natural generators in
an amalgamated free product of certain von Neumann algebras, with amalgamation
over a hyperfinite subalgebra. In particular, some `exotic' Popa algebra
generators of free group factors are shown to have the expected free entropy
dimension. We also show that microstates and non--microstates free entropy
dimension agree for generating sets of many groups. In the appendix by Wolfgang
Lueck, the first L^2-Betti number for certain amalgamated free products of
groups is calculated.Comment: The second revised version significantly generalized the main result
of the original one, (see the abstract) and contains a new appendix by
Wolfgang Lueck. The third and fourth revisions correct some minor mistakes.
The fifth version adds a result about embeddability of amalgmated free
product
Electrical Characterization of PbZr0.4Ti0.6O3 Capacitors
We have conducted a careful study of current-voltage (I-V) characteristics in
fully integrated commercial PbZr0.4Ti0.6O3 thin film capacitors with Pt bottom
and Ir/IrO2 top electrodes. Highly reproducible steady state I-V were obtained
at various temperatures over two decades in voltage from current-time data and
analyzed in terms of several common transport models including space charge
limited conduction, Schottky thermionic emission under full and partial
depletion and Poole-Frenkel conduction, showing that the later is the most
plausible leakage mechanism in these high quality films. In addition,
ferroelectric hysteresis loops and capacitance-voltage data were obtained over
a large range of temperatures and discussed in terms of a modified
Landau-Ginzburg-Devonshire theory accounting for space charge effects.Comment: 17 pages, 7 figure
- …