578 research outputs found
Dynamic Fluctuation Phenomena in Double Membrane Films
Dynamics of double membrane films is investigated in the long-wavelength
limit including the overdamped squeezing mode. We demonstrate that thermal
fluctuations essentially modify the character of the mode due to its nonlinear
coupling to the transversal shear hydrodynamic mode. The corresponding Green
function acquires as a function of the frequency a cut along the imaginary
semi-axis. Fluctuations lead to increasing the attenuation of the squeezing
mode it becomes larger than the `bare' value.Comment: 7 pages, Revte
Accretion Disk Instabilities, CDM models and their role in Quasar Evolution
We have developed a consistent analytical model to describe the observed
evolution of the quasar luminosity function. Our model combines black hole mass
distributions based on the Press - Schechter theory of the structure formation
in the Universe with quasar luminosity functions resulting from a physics-based
emission model that takes into account the time-dependent phenomena occurring
in the accretion disks. Quasar evolution and CDM models are mutually
constraining, therefore our model gives an estimation of the exponent, n, of
the power spectrum, P(k), which is found to be -1.8 < n < -1.6. We were able to
reject a generally assumed hypothesis of a constant ratio between Dark Matter
Halo and the Black Hole mass, since the observed data could not be fitted under
this assumption. We found that the relation between the Dark Matter Halos and
Black Hole masses is better described by M_{BH}=M_{DMH}^{0.668}. This model
provides a reasonable fit to the observed quasar luminosity function at
redshifts higher than ~2.0. We suggest that the disagreement at lower redshift
is due to mergers. Based on the agreement at high redshift, we estimated the
merger rate at lower redshift, and argue that this rate should depend on the
redshift, like (1+z)^3.Comment: 15 pages, 18 figures, Accepted for Publication in Ap
Surface Screening in the Casimir Force
We calculate the corrections to the Casimir force between two metals due to
the spatial dispersion of their response functions. We employ model-independent
expressions for the force in terms of the optical coefficients. We express the
non-local corrections to the Fresnel coefficients employing the surface
parameter, which accounts for the distribution of the surface
screening charge. Within a self-consistent jellium calculation, spatial
dispersion increases the Casimir force significatively for small separations.
The nonlocal correction has the opposite sign than previously predicted
employing hydrodynamic models and assuming abruptly terminated surfaces.Comment: 5 pages, 2 figure
Large Charge Four-Dimensional Non-Extremal N=2 Black Holes with R^2-Terms
We consider N=2 supergravity in four dimensions with small R^2 curvature
corrections. We construct large charge non-extremal black hole solutions in all
space, with either a supersymmetric or a non-supersymmetric extremal limit, and
analyze their thermodynamic properties. This generalizes some of the extremal
solutions presented in [arXiv:0902.0831]. The indexed entropy of the
non-extremal extension of the supersymmetric black hole, has the form of the
extremal entropy, with the charges replaced by a function of the charges, the
moduli at infinity and the non-extremality parameter. This is the same behavior
as in the case without R^2-terms.Comment: 13 pages. v2: stripped down to letter format, based on the background
given in [arXiv:0902.0831]. v3: up to date with CQG versio
Loschmidt echo and stochastic-like quantum dynamics of nano-particles
We investigate time evolution of prepared vibrational state (system) coupled
to a reservoir with dense spectrum of its vibrational states. We assume that
the reservoir has an equidistant spectrum, and the system - reservoir coupling
matrix elements are independent of the reservoir states. The analytical
solution manifests three regimes of the evolution for the system: (I) weakly
damped oscillations; (II) multicomponent Loschmidt echo in recurrence cycles;
(III) overlapping recurrence cycles. We find the characteristic critical values
of the system - reservoir coupling constant for the transitions between these
regimes. Stochastic dynamics occurs in the regime (III) due to inevoidably in
any real system coarse graining of time or energy measurements, or initial
condition uncertainty. Even though a specific toy model is investigated here,
when properly interpreted it yields quite reasonable description for a variety
of physically relevant phenomena.Comment: 8 pages, 3 figure
Present status of controversies regarding the thermal Casimir force
It is well known that, beginning in 2000, the behavior of the thermal
correction to the Casimir force between real metals has been hotly debated. As
was shown by several research groups, the Lifshitz theory, which provides the
theoretical foundation for the calculation of both the van der Waals and
Casimir forces, leads to different results depending on the model of metal
conductivity used. To resolve these controversies, the theoretical
considerations based on the principles of thermodynamics and new experimental
tests were invoked. We analyze the present status of the problem (in
particular, the advantages and disadvantages of the approaches based on the
surface impedance and on the Drude model dielectric function) using rigorous
analytical calculations of the entropy of a fluctuating field. We also discuss
the results of a new precise experiment on the determination of the Casimir
pressure between two parallel plates by means of a micromechanical torsional
oscillator.Comment: 14 pages, 1 figure, iopart.cls is used, to appear in J. Phys. A
(special issue: Proceedings of QFEXT05, Barcelona, Sept. 5-9, 2005
Regularization of the Coulomb scattering problem
Exact solutions of the Schr\"odinger equation for the Coulomb potential are
used in the scope of both stationary and time-dependent scattering theories in
order to find the parameters which define regularization of the Rutherford
cross-section when the scattering angle tends to zero but the distance r from
the center remains fixed. Angular distribution of the particles scattered in
the Coulomb field is investigated on the rather large but finite distance r
from the center. It is shown that the standard asymptotic representation of the
wave functions is not available in the case when small scattering angles are
considered. Unitary property of the scattering matrix is analyzed and the
"optical" theorem for this case is discussed. The total and transport
cross-sections for scattering of the particle by the Coulomb center proved to
be finite values and are calculated in the analytical form. It is shown that
the considered effects can be essential for the observed characteristics of the
transport processes in semiconductors which are defined by the electron and
hole scattering in the fields of the charged impurity centers.Comment: 20 pages, 6 figure
Deviations from Matthiessen's Rule for and
We have measured the change in the resistivity of thin films of and upon introducing point defects by electron
irradiation at low temperatures, and we find significant deviations from
Matthiessen's rule. For a fixed irradiation dose, the induced change in
resistivity {\it decreases} with increasing temperature. Moreover, for a fixed
temperature, the increase in resistivity with irradiation is found to be {\it
sublinear}. We suggest that the observed behavior is due to the marked
anisotropic scattering of the electrons together with their relatively short
mean free path (both characteristic of many metallic oxides including cuprates)
which amplify effects related to the Pippard ineffectiveness condition
Symmetries of Electrostatic Interaction between DNA Molecules
We study a model for pair interaction of DNA molecules generated by the
discrete dipole moments of base-pairs and the charges of phosphate groups, and
find noncommutative group of eighth order of symmetries that leave
invariant. We classify the minima using group and employ
numerical methods for finding them. The minima may correspond to several
cholesteric phases, as well as phases formed by cross-like conformations of
molecules at an angle close to , "snowflake phase". The results
depend on the effective charge of the phosphate group which can be modified
by the polycations or the ions of metals. The snowflake phase could exist for
above the threshold . Below there could be several cholesteric
phases. Close to the snowflake phase could change into the cholesteric
one at constant distance between adjacent molecules.Comment: 13 pages, 4 figure
Electromagnon excitations in modulated multiferroics
The phenomenological theory of ferroelectricity in spiral magnets presented
in [M. Mostovoy, Phys. Rev. Lett. 96, 067601 (2006)] is generalized to describe
consistently states with both uniform and modulated-in-space ferroelectric
polarizations. A key point in this description is the symmetric part of the
magnetoelectric coupling since, although being irrelevant for the uniform
component, it plays an essential role for the non-uniform part of the
polarization. We illustrate this importance in generic examples of modulated
magnetic systems: longitudinal and transverse spin-density wave states and
planar cycloidal phase. We show that even in the cases with no uniform
ferroelectricity induced, polarization correlation functions follow to the soft
magnetic behavior of the system due to the magnetoelectric effect. Our results
can be easily generalized for more complicated types of magnetic ordering, and
the applications may concern various natural and artificial systems in
condensed matter physics (e.g., magnon properties could be extracted from
dynamic dielectric response measurements).Comment: 5 page
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