15,768 research outputs found
Edge Electron Gas
The uniform electron gas, the traditional starting point for density-based
many-body theories of inhomogeneous systems, is inappropriate near electronic
edges. In its place we put forward the appropriate concept of the edge electron
gas.Comment: 4 pages RevTex with 7 ps-figures included. Minor changes in
title,text and figure
Quantal Density Functional Theory of Degenerate States
The treatment of degenerate states within Kohn-Sham density functional theory
(KS-DFT) is a problem of longstanding interest. We propose a solution to this
mapping from the interacting degenerate system to that of the noninteracting
fermion model whereby the equivalent density and energy are obtained via the
unifying physical framework of quantal density functional theory (Q-DFT). We
describe the Q-DFT of \textit{both} ground and excited degenerate states, and
for the cases of \textit{both} pure state and ensemble v-representable
densities. This then further provides a rigorous physical interpretation of the
density and bidensity energy functionals, and of their functional derivatives,
of the corresponding KS-DFT. We conclude with examples of the mappings within
Q-DFT.Comment: 10 pages. minor changes made. to appear in PR
Variational Bayes with Intractable Likelihood
Variational Bayes (VB) is rapidly becoming a popular tool for Bayesian
inference in statistical modeling. However, the existing VB algorithms are
restricted to cases where the likelihood is tractable, which precludes the use
of VB in many interesting situations such as in state space models and in
approximate Bayesian computation (ABC), where application of VB methods was
previously impossible. This paper extends the scope of application of VB to
cases where the likelihood is intractable, but can be estimated unbiasedly. The
proposed VB method therefore makes it possible to carry out Bayesian inference
in many statistical applications, including state space models and ABC. The
method is generic in the sense that it can be applied to almost all statistical
models without requiring too much model-based derivation, which is a drawback
of many existing VB algorithms. We also show how the proposed method can be
used to obtain highly accurate VB approximations of marginal posterior
distributions.Comment: 40 pages, 6 figure
An experimental and analytical investigation of proprotor whirl flutter
The results of an experimental parametric investigation of whirl flutter are presented for a model consisting of a windmilling propeller-rotor, or proprotor, having blades with offset flapping hinges mounted on a rigid pylon with flexibility in pitch and yaw. The investigation was motivated by the need to establish a large data base from which to assess the predictability of whirl flutter for a proprotor since some question has been raised as to whether flutter in the forward whirl mode could be predicted with confidence. To provide the necessary data base, the parametric study included variation in the pylon pitch and yaw stiffnesses, flapping hinge offset, and blade kinematic pitch-flap coupling over a large range of advance ratios. Cases of forward whirl flutter and of backward whirl flutter are documented. Measured whirl flutter characteristics were shown to be in good agreement with predictions from two different linear stability analyses which employed simple, two dimensional, quasi-steady aerodynamics for the blade loading. On the basis of these results, it appears that proprotor whirl flutter, both forward and backward, can be predicted
Hysteretic resistance spikes in quantum Hall ferromagnets without domains
We use spin-density-functional theory to study recently reported hysteretic
magnetoresistance \rho_{xx} spikes in Mn-based 2D electron gases
[Jaroszy\'{n}ski et al. Phys. Rev. Lett. (2002)]. We find hysteresis loops in
our calculated Landau fan diagrams and total energies signaling
quantum-Hall-ferromagnet phase transitions. Spin-dependent exchange-correlation
effects are crucial to stabilize the relevant magnetic phases arising from
distinct symmetry-broken excited- and ground-state solutions of the Kohn-Sham
equations. Besides hysteretic spikes in \rho_{xx}, we predict hysteretic dips
in the Hall resistance \rho_{xy}. Our theory, without domain walls,
satisfactorily explains the recent data.Comment: 4 pages, 4 figures, published version (some changes to the text; same
figures as in v1
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