40,966 research outputs found
An Underlying Theory for Gravity
A new direction to understand gravity has recently been explored by
considering classical gravity to be a derived interaction from an underlying
theory. This underlying theory would involve new degrees of freedom at a deeper
level and it would be structurally different from classical gravitation. It may
conceivably be a quantum theory or a non-quantum theory. The relation between
this underlying theory and Einstein's gravity is similar to the connection
between statistical mechanics and thermodynamics. We discuss the apparent lack
of evidence of any quantum nature of gravity in this context.Comment: Contributed paper to VIIth International Conference on Gravitation
and Cosmology, 14 - 19 December, 2011 GOA, INDIA. 4 page
Casimir Dispersion Forces and Orientational Pairwise Additivity
A path integral formulation is used to study the fluctuation-induced
interactions between manifolds of arbitrary shape at large separations. It is
shown that the form of the interactions crucially depends on the choice of the
boundary condition. In particular, whether or not the Casimir interaction is
pairwise additive is shown to depend on whether the ``metallic'' boundary
condition corresponds to a ``grounded'' or an ``isolated'' manifold.Comment: 6 pages, RevTe
Potential of mean force and the charge reversal of rodlike polyions
A simple model is presented to calculate the potential of mean force between
a polyion and a multivalent counterion inside a polyelectrolite solution. We
find that under certain conditions the electrostatic interactions can lead to a
strong attraction between the polyions and the multivalent counterions,
favoring formation of overcharged polyion-counterion complexes. It is found
that small concentrations of salt enhance the overcharging, while an excessive
amount of salt hinders the charge reversal. The kinetic limitations to
overcharging are also examined.Comment: To be published in the special issue of Molecular Physics in honor of
Prof. Ben Wido
Electron Addition Spectrum in the Supersymmetric t-J Model with Inverse-Square Interaction
The electron addition spectrum A^+(k,omega) is obtained analytically for the
one-dimensional (1D) supersymmetric t-J model with 1/r^2 interaction. The
result is obtained first for a small-sized system and its validity is checked
against the numerical calculation. Then the general expression is found which
is valid for arbitrary size of the system. The thermodynamic limit of
A^+(k,omega) has a simple analytic form with contributions from one spinon, one
holon and one antiholon all of which obey fractional statistics. The upper edge
of A^+(k,omega) in the (k,omega) plane includes a delta-function peak which
reduces to that of the single-electron band in the low-density limit.Comment: 5 pages, 1 figure, accepted for publication in Phys. Rev. Let
Combinatorial interpretation of Haldane-Wu fractional exclusion statistics
Assuming that the maximal allowed number of identical particles in state is
an integer parameter, q, we derive the statistical weight and analyze the
associated equation which defines the statistical distribution. The derived
distribution covers Fermi-Dirac and Bose-Einstein ones in the particular cases
q = 1 and q -> infinity (n_i/q -> 1), respectively. We show that the derived
statistical weight provides a natural combinatorial interpretation of
Haldane-Wu fractional exclusion statistics, and present exact solutions of the
distribution equation.Comment: 8 pages, 2 eps-figure
Finite Size Polyelectrolyte Bundles at Thermodynamic Equilibrium
We present the results of extensive computer simulations performed on
solutions of monodisperse charged rod-like polyelectrolytes in the presence of
trivalent counterions. To overcome energy barriers we used a combination of
parallel tempering and hybrid Monte Carlo techniques. Our results show that for
small values of the electrostatic interaction the solution mostly consists of
dispersed single rods. The potential of mean force between the polyelectrolyte
monomers yields an attractive interaction at short distances. For a range of
larger values of the Bjerrum length, we find finite size polyelectrolyte
bundles at thermodynamic equilibrium. Further increase of the Bjerrum length
eventually leads to phase separation and precipitation. We discuss the origin
of the observed thermodynamic stability of the finite size aggregates
Exact dynamical structure factor of the degenerate Haldane-Shastry model
The dynamical structure factor of the K-component (K = 2,3,4)
spin chain with the 1/r^2 exchange is derived exactly at zero temperature for
arbitrary size of the system. The result is interpreted in terms of a free
quasi-particle picture which is generalization of the spinon picture in the
SU(2) case; the excited states consist of K quasi-particles each of which is
characterized by a set of K-1 quantum numbers. Divergent singularities of
at the spectral edges are derived analytically. The analytic
result is checked numerically for finite systems.Comment: 4 pages, 1 figure, accepted for publication in Phys. Rev. Let
Conformational Instability of Rodlike Polyelectrolytes due to Counterion Fluctuations
The effective elasticity of highly charged stiff polyelectrolytes is studied
in the presence of counterions, with and without added salt. The rigid polymer
conformations may become unstable due to an effective attraction induced by
counterion density fluctuations. Instabilities at the longest, or intermediate
length scales may signal collapse to globule, or necklace states, respectively.
In the presence of added-salt, a generalized electrostatic persistence length
is obtained, which has a nontrivial dependence on the Debye screening length.
It is also found that the onset of conformational instability is a re-entrant
phenomenon as a function of polyelectrolyte length for the unscreened case, and
the Debye length or salt concentration for the screened case. This may be
relevant in understanding the experimentally observed re-entrant condensation
of DNA.Comment: 8 pages, 4 figure
Size dependent line broadening in the emission spectra of single GaAs quantum dots: Impact of surface charges on spectral diffusion
Making use of droplet epitaxy, we systematically controlled the height of
self-assembled GaAs quantum dots by more than one order of magnitude. The
photoluminescence spectra of single quantum dots revealed the strong dependence
of the spectral linewidth on the dot height. Tall dots with a height of ~30 nm
showed broad spectral peaks with an average width as large as ~5 meV, but
shallow dots with a height of ~2 nm showed resolution-limited spectral lines
(<120 micro eV). The measured height dependence of the linewidths is in good
agreement with Stark coefficients calculated for the experimental shape
variation. We attribute the microscopic source of fluctuating electric fields
to the random motion of surface charges at the vacuum-semiconductor interface.
Our results offer guidelines for creating frequency-locked photon sources,
which will serve as key devices for long-distance quantum key distribution.Comment: 6 pages, 6 figures; updated figs and their description
Observation of molecular vibrations in real time
Journal ArticleIn a recent Letter Rosker, Wise, and Tang1 have reported ultrafast optical measurements on two large dye molecules, malachite green and nile blue 690. Using the transmission-correlation technique with 40-fs light pulses, they measured photoinduced changes in optical transmission and found a response consisting of damped oscillations superimposed on two exponential decays. They tentatively interpreted the oscillations as quantum beats between two eigenstates of the isolated molecule. In this Comment we report Raman and infrared measurements on the same molecules and discuss the implications these measurements have for the interpretation of the data of Rosker, Wise, and Tang
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