8,690 research outputs found
Scaling Behavior of Ricci Curvature at Short Distance near Two Dimensions
We study the renormalization of the Ricci curvature as an example of
generally covariant operators in quantum gravity near two dimensions. We find
that it scales with a definite scaling dimension at short distance. The Ricci
curvature singularity at the big bang can be viewed as such a scaling
phenomenon. The problem of the spacetime singularity may be resolved by the
scale invariance of the spacetime at short distance.Comment: 9pages, LaTe
The Validity of the Adiabatic Contraction Approximation for Dark Matter Halos
We use high resolution numerical simulations to investigate the adiabatic
contraction of dark matter halos with a Hernquist density profile. We test the
response of the halos to the growth of additional axisymmetric disk potentials
with various central concentrations and the spherically symmetric potential of
a softened point mass. Adding the potentials on timescales that are long
compared to the dynamical time scale of the halo, the contracted halos have
density profiles that are in excellent agreement with analytical predictions
based on the conservation of the adiabatic invariant . This is
surprising as this quantity is strictly conserved only for particles on
circular orbits and in spherically symmetric potentials. If the same potentials
are added on timescales that are short compared to the dynamical timescale, the
result depends strongly on the adopted potential. The adiabatic approximation
still works for disk potentials. It does, however, fail for the central
potential.Comment: 7 pages, 3 figures, 1 table. Added reference. Accepted for
publication in ApJ
Poincar\'{e} gauge theory of gravity
A Poincar\'{e} gauge theory of (2+1)-dimensional gravity is developed.
Fundamental gravitational field variables are dreibein fields and Lorentz gauge
potentials, and the theory is underlain with the Riemann-Cartan space-time. The
most general gravitational Lagrangian density, which is at most quadratic in
curvature and torsion tensors and invariant under local Lorentz transformations
and under general coordinate transformations, is given. Gravitational field
equations are studied in detail, and solutions of the equations for weak
gravitational fields are examined for the case with a static, \lq \lq spin"less
point like source. We find, among other things, the following: (1)Solutions of
the vacuum Einstein equation satisfy gravitational field equations in the
vacuum in this theory. (2)For a class of the parameters in the gravitational
Lagrangian density, the torsion is \lq \lq frozen" at the place where \lq \lq
spin" density of the source field is not vanishing. In this case, the field
equation actually agrees with the Einstein equation, when the source field is
\lq \lq spin"less. (3)A teleparallel theory developed in a previous paper is
\lq \lq included as a solution" in a limiting case. (4)A Newtonian limit is
obtainable, if the parameters in the Lagrangian density satisfy certain
conditions.Comment: 27pages, RevTeX, OCU-PHYS-15
Free boson formulation of boundary states in W_3 minimal models and the critical Potts model
We develop a Coulomb gas formalism for boundary conformal field theory having
a symmetry and illustrate its operation using the three state Potts model.
We find that there are free-field representations for six conserving
boundary states, which yield the fixed and mixed physical boundary conditions,
and two violating boundary states which yield the free and new boundary
conditions. Other violating boundary states can be constructed but they
decouple from the rest of the theory. Thus we have a complete free-field
realization of the known boundary states of the three state Potts model. We
then use the formalism to calculate boundary correlation functions in various
cases. We find that the conformal blocks arising when the two point function of
is calculated in the presence of free and new boundary conditions
are indeed the last two solutions of the sixth order differential equation
generated by the singular vector.Comment: 25 page
Mechanically-Induced Transport Switching Effect in Graphene-based Nanojunctions
We report a theoretical study suggesting a novel type of electronic switching
effect, driven by the geometrical reconstruction of nanoscale graphene-based
junctions. We considered junction struc- tures which have alternative
metastable configurations transformed by rotations of local carbon dimers. The
use of external mechanical strain allows a control of the energy barrier
heights of the potential profiles and also changes the reaction character from
endothermic to exothermic or vice-versa. The reshaping of the atomic details of
the junction encode binary electronic ON or OFF states, with ON/OFF
transmission ratio that can reach up to 10^4-10^5. Our results suggest the
possibility to design modern logical switching devices or mechanophore sensors,
monitored by mechanical strain and structural rearrangements.Comment: 10 pages, 4 figure
New nonlinear dielectric materials: Linear electrorheological fluids under the influence of electrostriction
The usual approach to the development of new nonlinear dielectric materials
focuses on the search for materials in which the components possess an
inherently large nonlinear dielectric response. In contrast, based on
thermodynamics, we have presented a first-principles approach to obtain the
electrostriction-induced effective third-order nonlinear susceptibility for the
electrorheological (ER) fluids in which the components have inherent linear,
rather than nonlinear, responses. In detail, this kind of nonlinear
susceptibility is in general of about the same order of magnitude as the
compressibility of the linear ER fluid at constant pressure. Moreover, our
approach has been demonstrated in excellent agreement with a different
statistical method. Thus, such linear ER fluids can serve as a new nonlinear
dielectric material.Comment: 11 page
Semiclassical treatment of fusion processes in collisions of weakly bound nuclei
We describe a semiclassical treatment of nuclear fusion reactions involving
weakly bound nuclei. In this treatment, the complete fusion probabilities are
approximated by products of two factors: a tunneling probability and the
probability that the system is in its ground state at the strong absorption
radius. We investigate the validity of the method in a schematic two-channel
application, where the channels in the continuum are represented by a single
resonant state. Comparisons with full coupled-channels calculations are
performed. The agreement between semiclassical and quantal calculations isquite
good, suggesting that the procedure may be extended to more sophisticated
discretizations of the continuum.Comment: 11 pages, 5 figure
Brownian molecular motors driven by rotation-translation coupling
We investigated three models of Brownian motors which convert rotational
diffusion into directed translational motion by switching on and off a
potential. In the first model a spatially asymmetric potential generates
directed translational motion by rectifying rotational diffusion. It behaves
much like a conventional flashing ratchet. The second model utilizes both
rotational diffusion and drift to generate translational motion without spatial
asymmetry in the potential. This second model can be driven by a combination of
a Brownian motor mechanism (diffusion driven) or by powerstroke (drift driven)
depending on the chosen parameters. In the third model, elements of both the
Brownian motor and powerstroke mechanisms are combined by switching between
three distinct states. Relevance of the model to biological motor proteins is
discussed.Comment: 11 pages, 8 figure
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