751 research outputs found

### A simple electrostatic model applicable to biomolecular recognition

An exact, analytic solution for a simple electrostatic model applicable to
biomolecular recognition is presented. In the model, a layer of high dielectric
constant material (representative of the solvent, water) whose thickness may
vary separates two regions of low dielectric constant material (representative
of proteins, DNA, RNA, or similar materials), in each of which is embedded a
point charge. For identical charges, the presence of the screening layer always
lowers the energy compared to the case of point charges in an infinite medium
of low dielectric constant. Somewhat surprisingly, the presence of a
sufficiently thick screening layer also lowers the energy compared to the case
of point charges in an infinite medium of high dielectric constant. For charges
of opposite sign, the screening layer always lowers the energy compared to the
case of point charges in an infinite medium of either high or low dielectric
constant. The behavior of the energy leads to a substantially increased
repulsive force between charges of the same sign. The repulsive force between
charges of opposite signs is weaker than in an infinite medium of low
dielectric constant material but stronger than in an infinite medium of high
dielectric constant material. The presence of this behavior, which we name
asymmetric screening, in the simple system presented here confirms the
generality of the behavior that was established in a more complicated system of
an arbitrary number of charged dielectric spheres in an infinite solvent.Comment: 15 pages, 6 figure

### EMRI corrections to the angular velocity and redshift factor of a mass in circular orbit about a Kerr black hole

This is the first of two papers on computing the self-force in a radiation
gauge for a particle moving in circular, equatorial orbit about a Kerr black
hole. In the EMRI (extreme-mass-ratio inspiral) framework, with mode-sum
renormalization, we compute the renormalized value of the quantity
$h_{\alpha\beta}u^\alpha u^\beta$, gauge-invariant under gauge transformations
generated by a helically symmetric gauge vector; and we find the related order
$\frak{m}$ correction to the particle's angular velocity at fixed renormalized
redshift (and to its redshift at fixed angular velocity). The radiative part of
the perturbed metric is constructed from the Hertz potential which is extracted
from the Weyl scalar by an algebraic inversion\cite{sf2}. We then write the
spin-weighted spheroidal harmonics as a sum over spin-weighted spherical
harmonics and use mode-sum renormalization to find the renormalization
coefficients by matching a series in $L=\ell+1/2$ to the large-$L$ behavior of
the expression for $H := \frac12 h_{\alpha\beta}u^\alpha u^\beta$. The
non-radiative parts of the perturbed metric associated with changes in mass and
angular momentum are calculated in the Kerr gauge

### A modification of the Chen-Nester quasilocal expressions

Chen and Nester proposed four boundary expressions for the quasilocal
quantities using the covariant Hamiltonian formalism. Based on these four
expressions, there is a simple generalization that one can consider, so that a
two parameter set of boundary expressions can be constructed. Using these
modified expressions, a nice result for gravitational energy-momentum can be
obtained in holonomic frames.Comment: 11 page

### Entanglement Energetics in the Ground State

We show how many-body ground state entanglement information may be extracted
from sub-system energy measurements at zero temperature. A precise relation
between entanglement and energy fluctuations is demonstrated in the weak
coupling limit. Examples are given with the two-state system and the harmonic
oscillator, and energy probability distributions are calculated. Comparisons
made with recent qubit experiments show this type of measurement provides
another method to quantify entanglement with the environment.Comment: 7 pages, 3 figures, Conference proceeding for the Physics of Quantum
Electronics; Utah, USA, January 200

### Continuum coupled cluster expansion

We review the basics of the coupled-cluster expansion formalism for numerical
solutions of the many-body problem, and we outline the principles of an
approach directed towards an adequate inclusion of continuum effects in the
associated single-energy spectrum. We illustrate our findings by considering
the simple case of a single-particle quantum mechanics problem.Comment: 16 pages, 1 figur

### Macroscopic quantum jumps and entangled state preparation

Recently we predicted a random blinking, i.e. macroscopic quantum jumps, in
the fluorescence of a laser-driven atom-cavity system [Metz et al., Phys. Rev.
Lett. 97, 040503 (2006)]. Here we analyse the dynamics underlying this effect
in detail and show its robustness against parameter fluctuations. Whenever the
fluorescence of the system stops, a macroscopic dark period occurs and the
atoms are shelved in a maximally entangled ground state. The described setup
can therefore be used for the controlled generation of entanglement. Finite
photon detector efficiencies do not affect the success rate of the state
preparation, which is triggered upon the observation of a macroscopic
fluorescence signal. High fidelities can be achieved even in the vicinity of
the bad cavity limit due to the inherent role of dissipation in the jump
process.Comment: 14 pages, 12 figures, proof of the robustness of the state
preparation against parameter fluctuations added, figure replace

### Fermion Condensate and Vacuum Current Density Induced by Homogeneous and Inhomogeneous Magnetic Fields in (2+1)-Dimensions

We calculate the condensate and the vacuum current density induced by
external static magnetic fields in (2+1)-dimensions. At the perturbative level,
we consider an exponentially decaying magnetic field along one cartesian
coordinate. Non-perturbatively, we obtain the fermion propagator in the
presence of a uniform magnetic field by solving the Schwinger-Dyson equation in
the rainbow-ladder approximation. In the large flux limit, we observe that both
these quantities, either perturbative (inhomogeneous) and non-perturbative
(homogeneous), are proportional to the external field, in agreement with early
expectations.Comment: 8 pages, 2 figures. Accepted for publication in Phys. Rev.

### Microscopic Theory of the Single Impurity Surface Kondo Resonance

We develop a microscopic theory of the single impurity Kondo effect on a
metallic surface. We calculate the hybridization energies for the Anderson
Hamiltonian of a magnetic impurity interacting with surface and bulk states and
show that, contrary to the Kondo effect of an impurity in the bulk, the
hybridization matrix elements are strongly dependent on the momentum around the
Fermi surface. Furthermore, by calculating the tunneling conductance of a
scanning tunneling microscope (STM), we show that when the magnetic impurity is
located at a surface the Kondo effect can occur with equal strength between
bulk and surface states. We compare our results with recent experiments of Co
impurities in Cu(111) and Cu(100) surfaces and find good quantitative
agreement.Comment: New version of the original manuscript with extended discussions on
the problem of wavefunction orthogonality, the limitations of the theory,
more figures related to the STM experiments, and one correction to an earlier
result. Accepted for publication in Phys.Rev.

### Collective phenomena in quasi-two-dimensional fermionic polar molecules: band renormalization and excitons

We theoretically analyze a quasi-two-dimensional system of fermionic polar
molecules in a harmonic transverse confining potential. The renormalized energy
bands are calculated by solving the Hartree-Fock equation numerically for
various trap and dipolar interaction strengths. The inter-subband excitations
of the system are studied in the conserving time-dependent Hartree-Fock (TDHF)
approximation from the perspective of lattice modulation spectroscopy
experiments. We find that the excitation spectrum consists of both
inter-subband particle-hole excitation continuums and anti-bound excitons,
arising from the anisotropic nature of dipolar interactions. The excitonic
modes capture the majority of the spectral weight. We also evaluate the
inter-subband transition rates in order to investigate the nature of the
excitonic modes and find that they are anti-bound states formed from
particle-hole excitations arising from several subbands. Our results indicate
that the excitonic effects are present for interaction strengths and
temperatures accessible in current experiments with polar molecules.Comment: 21 pages, 12 figure

### Can quarkonia survive deconfinement ?

We study quarkonium correlators and spectral functions at zero and finite
temperature in QCD with only heavy quarks using potential models combined with
perturbative QCD. First, we show that this approach can describe the quarkonium
correlation function at zero temperature. Using a class of screened potentials
based on lattice calculations of the static quark-antiquark free energy we
calculate spectral functions at finite temperature. We find that all quarkonium
states, with the exception of the $1S$ bottomonium, dissolve in the deconfined
phase at temperatures smaller than $1.5T_c$, in contradiction with the
conclusions of recent studies. Despite this the temperature dependence of the
quarkonium correlation functions calculated on the lattice is well reproduced
in our model. We also find that even in the absence of resonances the spectral
function at high temperatures is significantly enhanced over the spectral
function corresponding to free quark antiquark propagation.Comment: Version accepted in Phys. Rev. D, 20 pages, 25 figure

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