8,321 research outputs found
A new broken U(1)-symmetry in extreme type-II superconductors
A phase transition within the molten phase of the Abrikosov vortex system
without disorder in extreme type-II superconductors is found via large-scale
Monte-Carlo simulations. It involves breaking a U(1)-symmetry, and has a
zero-field counterpart, unlike vortex lattice melting. Its hallmark is the loss
of number-conservation of connected vortex paths threading the entire system
{\it in any direction}, driving the vortex line tension to zero. This tension
plays the role of a generalized ``stiffness'' of the vortex liquid, and serves
as a probe of the loss of order at the transition, where a weak specific heat
anomaly is found.Comment: 5 pages, 3 figure
Fractional-Period Excitations in Continuum Periodic Systems
We investigate the generation of fractional-period states in continuum
periodic systems. As an example, we consider a Bose-Einstein condensate
confined in an optical-lattice potential. We show that when the potential is
turned on non-adiabatically, the system explores a number of transient states
whose periodicity is a fraction of that of the lattice. We illustrate the
origin of fractional-period states analytically by treating them as resonant
states of a parametrically forced Duffing oscillator and discuss their
transient nature and potential observability.Comment: 10 pages, 6 figures (some with multiple parts); revised version:
minor clarifications of a couple points, to appear in Physical Review
Quantum dynamics in photonic crystals
Employing a recently developed method that is numerically accurate within a
model space simulating the real-time dynamics of few-body systems interacting
with macroscopic environmental quantum fields, we analyze the full dynamics of
an atomic system coupled to a continuum light-field with a gapped spectral
density. This is a situation encountered, for example, in the radiation field
in a photonic crystal, whose analysis has been so far been confined to limiting
cases due to the lack of suitable numerical techniques. We show that both
atomic population and coherence dynamics can drastically deviate from the
results predicted when using the rotating wave approximation, particularly in
the strong coupling regime. Experimental conditions required to observe these
corrections are also discussed.Comment: 5 pages, 2 figures Updated with published versio
Higher Order Force Gradient Symplectic Algorithms
We show that a recently discovered fourth order symplectic algorithm, which
requires one evaluation of force gradient in addition to three evaluations of
the force, when iterated to higher order, yielded algorithms that are far
superior to similarly iterated higher order algorithms based on the standard
Forest-Ruth algorithm. We gauge the accuracy of each algorithm by comparing the
step-size independent error functions associated with energy conservation and
the rotation of the Laplace-Runge-Lenz vector when solving a highly eccentric
Kepler problem. For orders 6, 8, 10 and 12, the new algorithms are
approximately a factor of , , and better.Comment: 23 pages, 10 figure
On the construction of high-order force gradient algorithms for integration of motion in classical and quantum systems
A consequent approach is proposed to construct symplectic force-gradient
algorithms of arbitrarily high orders in the time step for precise integration
of motion in classical and quantum mechanics simulations. Within this approach
the basic algorithms are first derived up to the eighth order by direct
decompositions of exponential propagators and further collected using an
advanced composition scheme to obtain the algorithms of higher orders. Contrary
to the scheme by Chin and Kidwell [Phys. Rev. E 62, 8746 (2000)], where
high-order algorithms are introduced by standard iterations of a force-gradient
integrator of order four, the present method allows to reduce the total number
of expensive force and its gradient evaluations to a minimum. At the same time,
the precision of the integration increases significantly, especially with
increasing the order of the generated schemes. The algorithms are tested in
molecular dynamics and celestial mechanics simulations. It is shown, in
particular, that the efficiency of the new fourth-order-based algorithms is
better approximately in factors 5 to 1000 for orders 4 to 12, respectively. The
results corresponding to sixth- and eighth-order-based composition schemes are
also presented up to the sixteenth order. For orders 14 and 16, such highly
precise schemes, at considerably smaller computational costs, allow to reduce
unphysical deviations in the total energy up in 100 000 times with respect to
those of the standard fourth-order-based iteration approach.Comment: 23 pages, 2 figures; submitted to Phys. Rev.
Unzipping of DNA with correlated base-sequence
We consider force-induced unzipping transition for a heterogeneous DNA model
with a correlated base-sequence. Both finite-range and long-range correlated
situations are considered. It is shown that finite-range correlations increase
stability of DNA with respect to the external unzipping force. Due to
long-range correlations the number of unzipped base-pairs displays two widely
different scenarios depending on the details of the base-sequence: either there
is no unzipping phase-transition at all, or the transition is realized via a
sequence of jumps with magnitude comparable to the size of the system. Both
scenarios are different from the behavior of the average number of unzipped
base-pairs (non-self-averaging). The results can be relevant for explaining the
biological purpose of correlated structures in DNA.Comment: 22 pages, revtex4, 14 eps figures; reprinted in the June 15, 2004
issue of Virtual Journal of Biological Physics Researc
Distribution of entanglement in light-harvesting complexes and their quantum efficiency
Recent evidence of electronic coherence during energy transfer in
photosynthetic antenna complexes has reinvigorated the discussion of whether
coherence and/or entanglement has any practical functionality for these
molecular systems. Here we investigate quantitative relationships between the
quantum yield of a light-harvesting complex and the distribution of
entanglement among its components. Our study focusses on the entanglement yield
or average entanglement surviving a time scale comparable to the average
excitation trapping time. As a prototype system we consider the
Fenna-Matthews-Olson (FMO) protein of green sulphur bacteria and show that
there is an inverse relationship between the quantum efficiency and the average
entanglement between distant donor sites. Our results suggest that longlasting
electronic coherence among distant donors might help modulation of the
lightharvesting function.Comment: Version accepted for publication in NJ
Effect of time delay on the onset of synchronization of the stochastic Kuramoto model
We consider the Kuramoto model of globally coupled phase oscillators with
time-delayed interactions, that is subject to the Ornstein-Uhlenbeck (Gaussian)
colored or the non-Gaussian colored noise. We investigate numerically the
interplay between the influences of the finite correlation time of noise
and the time delay on the onset of the synchronization process. Both
cases for identical and nonidentical oscillators had been considered. Among the
obtained results for identical oscillators is a large increase of the
synchronization threshold as a function of time delay for the colored
non-Gaussian noise compared to the case of the colored Gaussian noise at low
noise correlation time . However, the difference reduces remarkably for
large noise correlation times. For the case of nonidentical oscillators, the
incoherent state may become unstable around the maximum value of the threshold
(as a function of time delay) even at lower coupling strength values in the
presence of colored noise as compared to the noiseless case. We had studied the
dependence of the critical value of the coupling strength (the threshold of
synchronization) on given parameters of the stochastic Kuramoto model in great
details and presented results for possible cases of colored Gaussian and
non-Gaussian noises.Comment: 19 pages with 7 figure
Flux melting in BSCCO: Incorporating both electromagnetic and Josephson couplings
Multilevel Monte Carlo simulations of a BSCCO system are carried out
including both Josephson as well as electromagnetic couplings for a range of
anisotropies. A first order melting transition of the flux lattice is seen on
increasing the temperature and/or the magnetic field. The phase diagram for
BSCCO is obtained for different values of the anisotropy parameter .
The best fit to the experimental results of D. Majer {\it et al.} [Phys. Rev.
Lett. {\bf 75}, 1166 (1995)] is obtained for provided one
assumes a temperature dependence of the
penetration depth with . Assuming a dependence
the best fit is obtained for . For finite anisotropy the data is shown to collapse on a straight line
when plotted in dimensionless units which shows that the melting transition can
be satisfied with a single Lindemann parameter whose value is about 0.3. A
different scaling applies to the case. The energy jump is
measured across the transition and for large values of it is found to
increase with increasing anisotropy and to decrease with increasing magnetic
field. For infinite anisotropy we see a 2D behavior of flux droplets with a
transition taking place at a temperature independent of the magnetic field. We
also show that for smaller values of anisotropy it is reasonable to replace the
electromagnetic coupling with an in-plane interaction represented by a Bessel
function of the second kind (), thus justifying our claim in a previous
paper.Comment: 12 figures, revtex
- …