195,531 research outputs found
Comment on "Fock-Darwin States of Dirac Electrons in Graphene-Based Artificial Atoms"
Chen, Apalkov, and Chakraborty (Phys. Rev. Lett. 98, 186803 (2007)) have
computed Fock-Darwin levels of a graphene dot by including only basis states
with energies larger than or equal to zero. We show that their results violate
the Hellman-Feynman theorem. A correct treatment must include both positive and
negative energy basis states. Additional basis states lead to new energy levels
in the optical spectrum and anticrossings between optical transition lines.Comment: 1 page, 1 figure, accepted for publication in PR
Finite circular plate on elastic foundation centrally loaded by rigid spherical indenter
The analytical solution of a finite circular plate on an elastic foundation centrally loaded by the rigid indenter is discussed. The procedure to use NASTRAN as a subroutine to iteratively converge to this solution numerically is described
States near Dirac points of rectangular graphene dot in a magnetic field
In neutral graphene dots the Fermi level coincides with the Dirac points. We
have investigated in the presence of a magnetic field several unusual
properties of single electron states near the Fermi level of such a
rectangular-shaped graphene dot with two zigzag and two armchair edges. We find
that a quasi-degenerate level forms near zero energy and the number of states
in this level can be tuned by the magnetic field. The wavefunctions of states
in this level are all peaked on the zigzag edges with or without some weight
inside the dot. Some of these states are magnetic field-independent surface
states while the others are field-dependent. We have found a scaling result
from which the number of magnetic field-dependent states of large dots can be
inferred from those of smaller dots.Comment: Physical review B in pres
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Information, VARs and DSGE Models
How informative is a time series representation of a given vector of observables about the structural shocks and impulse response functions in a DSGE model? In this paper we refer to this econometrician’s problem as “E-invertibility” and consider the corresponding information problem of the agents in the assumed DGP, the DSGE model, which we refer to as “A-invertibility” We consider how the general nature of the agents’ signal extraction problem under imperfect information impacts on the econometrician’s problem of attempting to infer the nature of structural shocks and associated impulse responses from the data. We also examine a weaker condition of recoverability. A general conclusion is that validating a DSGE model by comparing its impulse response functions with those of a data VAR is more problematic when we drop the common assumption in the literature that agents have perfect information as an endowment. We develop measures of approximate fundamentalness for both perfect and imperfect information cases and illustrate our results using analytical and numerical examples
Adaptive control of a manipulator with a flexible link
An adaptive controller for a manipulator with one rigid link and one flexible link is presented. The performance and robustness of the controller are demonstrated by numerical simulation results. In the simulations, the manipulator moves in a gravitational field and a finite element model represents the flexible link
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