652 research outputs found
Evaporation and Step Edge Diffusion in MBE
Using kinetic Monte-Carlo simulations of a Solid-on-Solid model we
investigate the influence of step edge diffusion (SED) and evaporation on
Molecular Beam Epitaxy (MBE). Based on these investigations we propose two
strategies to optimize MBE-growth. The strategies are applicable in different
growth regimes: during layer-by-layer growth one can reduce the desorption rate
using a pulsed flux. In three-dimensional (3D) growth the SED can help to grow
large, smooth structures. For this purpose the flux has to be reduced with time
according to a power law.Comment: 5 pages, 2 figures, latex2e (packages: elsevier,psfig,latexsym
The dynamical equivalence of modified gravity revisited
We revisit the dynamical equivalence between different representations of
vacuum modified gravity models in view of Legendre transformations. The
equivalence is discussed for both bulk and boundary space, by including in our
analysis the relevant Gibbons-Hawking terms. In the f(R) case, the Legendre
transformed action coincides with the usual Einstein frame one. We then
re-express the R+f(G) action, where G is the Gauss-Bonnet term, as a second
order theory with a new set of field variables, four tensor fields and one
scalar and study its dynamics. For completeness, we also calculate the
conformal transformation of the full Jordan frame R+f(G) action. All the
appropriate Gibbons-Hawking terms are calculated explicitly.Comment: 17 pages; v3: Revised version. New comments added in Sections 3 & 5.
New results added in Section 6. Version to appear in Class. Quantum Gravit
Creation of the universe with a stealth scalar field
The stealth scalar field is a non-trivial configuration without any
back-reaction to geometry, which is characteristic for non-minimally coupled
scalar fields. Studying the creation probability of the de Sitter universe with
a stealth scalar field by the Hartle and Hawking's semi-classical method, we
show that the effect of the stealth field can be significant. For the class of
scalar fields we consider, creation with a stealth field is possible for a
discrete value of the coupling constant and its creation probability is always
less than that with a trivial scalar field. However, those creation rates can
be almost the same depending on the parameters of the theory.Comment: 7 pages; v2, references added; v3, creation of the open universe
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The conformal frame freedom in theories of gravitation
It has frequently been claimed in the literature that the classical physical
predictions of scalar tensor theories of gravity depend on the conformal frame
in which the theory is formulated. We argue that this claim is false, and that
all classical physical predictions are conformal-frame invariants. We also
respond to criticisms by Vollick [gr-qc/0312041], in which this issue arises,
of our recent analysis of the Palatini form of 1/R gravity.Comment: 9 pages, no figures, revtex; final published versio
A lattice gas model of II-VI(001) semiconductor surfaces
We introduce an anisotropic two-dimensional lattice gas model of metal
terminated II-IV(001) seminconductor surfaces. Important properties of this
class of materials are represented by effective NN and NNN interactions, which
result in the competition of two vacancy structures on the surface. We
demonstrate that the experimentally observed c(2x2)-(2x1) transition of the
CdTe(001) surface can be understood as a phase transition in thermal
equilbrium. The model is studied by means of transfer matrix and Monte Carlo
techniques. The analysis shows that the small energy difference of the
competing reconstructions determines to a large extent the nature of the
different phases. Possible implications for further experimental research are
discussed.Comment: 7 pages, 2 figure
Supersonic strain front driven by a dense electron-hole plasma
We study coherent strain in (001) Ge generated by an ultrafast
laser-initiated high density electron-hole plasma. The resultant coherent pulse
is probed by time-resolved x-ray diffraction through changes in the anomalous
transmission. The acoustic pulse front is driven by ambipolar diffusion of the
electron-hole plasma and propagates into the crystal at supersonic speeds.
Simulations of the strain including electron-phonon coupling, modified by
carrier diffusion and Auger recombination, are in good agreement with the
observed dynamics.Comment: 4 pages, 6 figure
Non-Abelian Black Holes in Brans-Dicke Theory
We find a black hole solution with non-Abelian field in Brans-Dicke theory.
It is an extension of non-Abelian black hole in general relativity. We discuss
two non-Abelian fields: "SU(2)" Yang-Mills field with a mass (Proca field) and
the SU(2)SU(2) Skyrme field. In both cases, as in general relativity,
there are two branches of solutions, i.e., two black hole solutions with the
same horizon radius. Masses of both black holes are always smaller than those
in general relativity. A cusp structure in the mass-horizon radius
(-) diagram, which is a typical symptom of stability change in
catastrophe theory, does not appear in the Brans-Dicke frame but is found in
the Einstein conformal frame. This suggests that catastrophe theory may be
simply applied for a stability analysis as it is if we use the variables in the
Einstein frame. We also discuss the effects of the Brans-Dicke scalar field on
black hole structure.Comment: 31 pages, revtex, 21 figure
Symmetry properties of the metric energy-momentum tensor in classical field theories and gravity
We derive a generic identity which holds for the metric (i.e. variational)
energy-momentum tensor under any field transformation in any generally
covariant classical Lagrangian field theory. The identity determines the
conditions under which a symmetry of the Lagrangian is also a symmetry of the
energy-momentum tensor. It turns out that the stress tensor acquires the
symmetry if the Lagrangian has the symmetry in a generic curved spacetime. In
this sense a field theory in flat spacetime is not self-contained. When the
identity is applied to the gauge invariant spin-two field in Minkowski space,
we obtain an alternative and direct derivation of a known no-go theorem: a
linear gauge invariant spin-2 field, which is dynamically equivalent to
linearized General Relativity, cannot have a gauge invariant metric
energy-momentum tensor. This implies that attempts to define the notion of
gravitational energy density in terms of the metric energy--momentum tensor in
a field-theoretical formulation of gravity must fail.Comment: Revised version to match the published version in Class. Quantum Gra
Some aspects of the cosmological conformal equivalence between ``Jordan Frame'' and ``Einstein Frame''
The conformal equivalence between Jordan frame and Einstein frame can be used
in order to search for exact solutions in general theories of gravity in which
scalar fields are minimally or nonminimally coupled with geometry. In the
cosmological arena a relevant role is played by the time parameter in which
dynamics is described. In this paper we discuss such issues considering also if
cosmological Noether symmetries in the ``point--like'' Lagrangian are
conformally preserved. Through this analysis and through also a careful
analysis of the cosmological parameters \Omega and \Lambda, it is possible to
contribute to the discussion on which is the physical system.Comment: 25 pages, latex, submitted to Class. Quantum Gra
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