1,148 research outputs found
The new physics of non-equilibrium condensates: insights from classical dynamics
We discuss the dynamics of classical Dicke-type models, aiming to clarify the
mechanisms by which coherent states could develop in potentially
non-equilibrium systems such as semiconductor microcavities. We present
simulations of an undamped model which show spontaneous coherent states with
persistent oscillations in the magnitude of the order parameter. These states
are generalisations of superradiant ringing to the case of inhomogeneous
broadening. They correspond to the persistent gap oscillations proposed in
fermionic atomic condensates, and arise from a variety of initial conditions.
We show that introducing randomness into the couplings can suppress the
oscillations, leading to a limiting dynamics with a time-independent order
parameter. This demonstrates that non-equilibrium generalisations of polariton
condensates can be created even without dissipation. We explain the dynamical
origins of the coherence in terms of instabilities of the normal state, and
consider how it can additionally develop through scattering and dissipation.Comment: 10 pages, 4 figures, submitted for a special issue of J. Phys.:
Condensed Matter on "Optical coherence and collective phenomena in
nanostructures". v2: added discussion of links to exact solution
Fast preparation of single hole spin in InAs/GaAs quantum dot in Voigt geometry magnetic field
The preparation of a coherent heavy-hole spin via ionization of a
spin-polarized electron-hole pair in an InAs/GaAs quantum dot in a Voigt
geometry magnetic field is investigated. For a dot with a 17 ueV bright-exciton
fine-structure splitting, the fidelity of the spin preparation is limited to
0.75, with optimum preparation occurring when the effective fine-structure of
the bright-exciton matches the in-plane hole Zeeman energy. In principle,
higher fidelities can be achieved by minimizing the bright-exciton
fine-structure splitting.Comment: 8 pages, 10 figs, published PRB 85 155310 (2012
Phonon-Induced Rabi-Frequency Renormalization of Optically Driven Single InGaAs/GaAs Quantum Dots
The authors thank the EPSRC (U.K.) EP/G001642, and the QIPIRC U.K. for financial support. A. N. is supported by the EPSRC and B.W. L. by the Royal Society.We study optically driven Rabi rotations of a quantum dot exciton transition between 5 and 50 K, and for pulse areas of up to 14 pi. In a high driving field regime, the decay of the Rabi rotations is nonmonotonic, and the period decreases with pulse area and increases with temperature. By comparing the experiments to a weak-coupling model of the exciton-phonon interaction, we demonstrate that the observed renormalization of the Rabi frequency is induced by fluctuations in the bath of longitudinal acoustic phonons, an effect that is a phonon analogy of the Lamb shift.Peer reviewe
Uncoupled excitons in semiconductor microcavities detected in resonant Raman scattering
We present an outgoing resonant Raman-scattering study of a GaAs/AlGaAs based microcavity embedded in a p-i-n junction. The p-i-n junction allows the vertical electric field to be varied, permitting control of exciton-photon detuning and quenching of photoluminescence which otherwise obscures the inelastic light scattering signals. Peaks corresponding to the upper and lower polariton branches are observed in the resonant Raman cross sections, along with a third peak at the energy of uncoupled excitons. This third peak, attributed to disorder activated Raman scattering, provides clear evidence for the existence of uncoupled exciton reservoir states in microcavities in the strong-coupling regime
Fermi-Edge Singularities in AlxGa1-xAs Quantum Wells : Extrinsic Versus Many-Body Scattering Processes
A Fano resonance mechanism is evidenced to control the formation of optical
Fermi-edge singularities in multi-subband systems such as remotely doped
AlxGa1-xAs heterostructures. Using Fano parameters, we probe the physical
nature of the interaction between Fermi-sea electrons and empty conduction
subbands. We show that processes of extrinsic origin like alloy-disorder
prevail easily at 2D over multiple diffusions from charged valence holes
expected by many-body scenarios.Comment: 4 pages, 3 figures, accepted for publication in Physical Review
Letter
Fr-TM-align: a new protein structural alignment method based on fragment alignments and the TM-score
©2008 Pandit and Skolnick; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This article is available from: http://www.biomedcentral.com/1471-2105/9/531doi:10.1186/1471-2105-9-531Background: Protein tertiary structure comparisons are employed in various fields of
contemporary structural biology. Most structure comparison methods involve generation of an
initial seed alignment, which is extended and/or refined to provide the best structural superposition
between a pair of protein structures as assessed by a structure comparison metric. One such
metric, the TM-score, was recently introduced to provide a combined structure quality measure
of the coordinate root mean square deviation between a pair of structures and coverage. Using the
TM-score, the TM-align structure alignment algorithm was developed that was often found to have
better accuracy and coverage than the most commonly used structural alignment programs;
however, there were a number of situations when this was not true.
Results: To further improve structure alignment quality, the Fr-TM-align algorithm has been
developed where aligned fragment pairs are used to generate the initial seed alignments that are
then refined using dynamic programming to maximize the TM-score. For the assessment of the
structural alignment quality from Fr-TM-align in comparison to other programs such as CE and TMalign,
we examined various alignment quality assessment scores such as PSI and TM-score. The
assessment showed that the structural alignment quality from Fr-TM-align is better in comparison
to both CE and TM-align. On average, the structural alignments generated using Fr-TM-align have
a higher TM-score (~9%) and coverage (~7%) in comparison to those generated by TM-align. Fr-
TM-align uses an exhaustive procedure to generate initial seed alignments. Hence, the algorithm is
computationally more expensive than TM-align.
Conclusion: Fr-TM-align, a new algorithm that employs fragment alignment and assembly provides
better structural alignments in comparison to TM-align. The source code and executables of Fr-
TM-align are freely downloadable at: http://cssb.biology.gatech.edu/skolnick/files/FrTMalign/
Intersublevel Polaron Dephasing in Self-Assembled Quantum Dots
Polaron dephasing processes are investigated in InAs/GaAs dots using
far-infrared transient four wave mixing (FWM) spectroscopy. We observe an
oscillatory behaviour in the FWM signal shortly (< 5 ps) after resonant
excitation of the lowest energy conduction band transition due to coherent
acoustic phonon generation. The subsequent single exponential decay yields long
intraband dephasing times of 90 ps. We find excellent agreement between our
measured and calculated FWM dynamics, and show that both real and virtual
acoustic phonon processes are necessary to explain the temperature dependence
of the polarization decay.Comment: 10 pages, 4 figures, submitted to Phys Rev Let
Persistence length of a polyelectrolyte in salty water: a Monte-Carlo study
We address the long standing problem of the dependence of the electrostatic
persistence length of a flexible polyelectrolyte (PE) on the screening
length of the solution within the linear Debye-Huckel theory. The
standard Odijk, Skolnick and Fixman (OSF) theory suggests ,
while some variational theories and computer simulations suggest . In this paper, we use Monte-Carlo simulations to study the conformation
of a simple polyelectrolyte. Using four times longer PEs than in previous
simulations and refined methods for the treatment of the simulation data, we
show that the results are consistent with the OSF dependence . The linear charge density of the PE which enters in the coefficient of
this dependence is properly renormalized to take into account local
fluctuations.Comment: 7 pages, 6 figures. Various corrections in text and reference
Reply to Comment on "Criterion that Determines the Foldability of Proteins"
We point out that the correlation between folding times and in protein-like heteropolymer models where
and are the collapse and folding transition temperatures
was already established in 1993 before the other presumed equivalent criterion
(folding times correlating with alone) was suggested. We argue that the
folding times for these models show no useful correlation with the energy gap
even if restricted to the ensemble of compact structures as suggested by
Karplus and Shakhnovich (cond-mat/9606037).Comment: 6 pages, Latex, 2 Postscript figures. Plots explicitly showing the
lack of correlation between folding time and energy gap are adde
Negative electrostatic contribution to the bending rigidity of charged membranes and polyelectrolytes screened by multivalent counterions
Bending rigidity of a charged membrane or a charged polyelectrolyte screened
by monovalent counterions is known to be enhanced by electrostatic effects. We
show that in the case of screening by multivalent counterions the electrostatic
effects reduce the bending rigidity. This inversion of the sign of the
electrostatic contribution is related to the formation of two-dimensional
strongly correlated liquids (SCL) of counterions at the charged surface due to
strong lateral repulsion between them. When a membrane or a polyelectrolyte is
bent, SCL is compressed on one side and stretched on the other so that
thermodynamic properties of SCL contribute to the bending rigidity.
Thermodynamic properties of SCL are similar to those of Wigner crystal and are
anomalous in the sense that the pressure, compressibility and screening radius
of SCL are negative. This brings about substantial negative correction to the
bending rigidity. For the case of DNA this effect qualitatively agrees with
experiment.Comment: 8 pages, 2 figure
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