28,785 research outputs found
Unified Analysis of Cosmological Perturbations in Generalized Gravity
In a class of generalized Einstein's gravity theories we derive the equations
and general asymptotic solutions describing the evolution of the perturbed
universe in unified forms. Our gravity theory considers general couplings
between the scalar field and the scalar curvature in the Lagrangian, thus
includes broad classes of generalized gravity theories resulting from recent
attempts for the unification. We analyze both the scalar-type mode and the
gravitational wave in analogous ways. For both modes the large scale evolutions
are characterized by the same conserved quantities which are valid in the
Einstein's gravity. This unified and simple treatment is possible due to our
proper choice of the gauges, or equivalently gauge invariant combinations.Comment: 4 pages, revtex, no figure
Rotation misorientated graphene moire superlattices on Cu(111): classical molecular dynamics simulations and scanning tunneling microscopy studies
Graphene on copper is a system of high technological relevance, as Cu is one
of the most widely used substrates for the CVD growth of graphene. However,
very little is known about the details of their interaction. One approach to
gain such information is studying the superlattices emerging due to the
mismatch of the two crystal lattices. However, graphene on copper is a
low-corrugated system making both their experimental and theoretical study
highly challenging. Here, we report the observation of a new rotational Moire
superlattice of CVD graphene on Cu (111), characterized by a periodicity of
nm and corrugation of , as measured
by Scanning Tunneling Microscopy. To understand the observed superlattice we
have developed a newly parameterized Tersoff-potential for the graphene/Cu
(111) interface fitted to nonlocal van der Waals density functional theory
(DFT) calculations. The interfacial force field with time-lapsed CMD provides
superlattices in good quantitative agreement with the experimental results, for
a misorientation angle of without any further parameter
adjustment. Furthermore, the CMD simulations predict the existence of two
non-equivalent high-symmetry directions of the Moir\'e pattern that could also
be identified in the experimental STM images.Comment: 7 pages, 2 figures, 2 table
Cosmological perturbations in a gravity with quadratic order curvature couplings
We present a set of equations describing the evolution of the scalar-type
cosmological perturbation in a gravity with general quadratic order curvature
coupling terms. Equations are presented in a gauge ready form, thus are ready
to implement various temporal gauge conditions depending on the problems. The
Ricci-curvature square term leads to a fourth-order differential equation for
describing the spacetime fluctuations in a spatially homogeneous and isotropic
cosmological background.Comment: 5 pages, no figure, To appear in Phys. Rev.
The Origin of Structures in Generalized Gravity
In a class of generalized gravity theories with general couplings between the
scalar field and the scalar curvature in the Lagrangian, we can describe the
quantum generation and the classical evolution of both the scalar and tensor
structures in a simple and unified manner. An accelerated expansion phase based
on the generalized gravity in the early universe drives microscopic quantum
fluctuations inside a causal domain to expand into macroscopic ripples in the
spacetime metric on scales larger than the local horizon. Following their
generation from quantum fluctuations, the ripples in the metric spend a long
period outside the causal domain. During this phase their evolution is
characterized by their conserved amplitudes. The evolution of these
fluctuations may lead to the observed large scale structures of the universe
and anisotropies in the cosmic microwave background radiation.Comment: 5 pages, latex, no figur
Cosmological Gravitational Wave in a Gravity with Quadratic Order Curvature Couplings
We present a set of equations describing the cosmological gravitational wave
in a gravity theory with quadratic order gravitational coupling terms which
naturally arise in quantum correction procedures. It is known that the
gravitational wave equation in the gravity theories with a general term
in the action leads to a second order differential equation with the only
correction factor appearing in the damping term. The case for a
term is completely different. The gravitational wave is described by a fourth
order differential equation both in time and space. However, curiously, we find
that the contributions to the background evolution are qualitatively the same
for both terms.Comment: 4 pages, revtex, no figure
COBE constraints on inflation models with a massive non-minimal scalar field
We derive power spectra of the scalar- and tensor-type structures generated
in an inflation model based on a massive non-minimally coupled scalar field
with the strong coupling assumption. We make analyses in both the
original-frame and the conformally transformed Einstein-frame. We derive
contributions of both structures to the anisotropy of the cosmic microwave
background radiation, and compare the contributions with the four-year COBE-DMR
data. Previous study showed that sufficient amount of inflation requires a
small coupling parameter. In such a case the spectra become near Zeldovich
spectra, and the gravitational wave contribution becomes negligible compared
with the scalar-type contribution which is testable in future CMBR experiments.Comment: 4 pages, no figure, To appear in Phys. Rev.
Single electron control in n-type semiconductor quantum dots using non-Abelian holonomies generated by spin orbit coupling
We propose that n-type semiconductor quantum dots with the Rashba and
Dresselhaus spin orbit interactions may be used for single electron
manipulation through adiabatic transformations between degenerate states. All
the energy levels are discrete in quantum dots and possess a double degeneracy
due to time reversal symmetryin the presence of the Rashba and/or Dresselhaus
spin orbit coupling terms. We find that the presence of double degeneracy does
not necessarily give rise to a finite non-Abelian (matrix) Berry phase. We show
that a distorted two-dimensional harmonic potential may give rise to
non-Abelian Berry phases. The presence of the non-Abelian Berry phase may be
tested experimentally by measuring the optical dipole transitions.Comment: accepted in Phys. Rev.
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