315 research outputs found
Thermodynamics and equilibrium structure of Ne_38 cluster: Quantum Mechanics versus Classical
The equilibrium properties of classical LJ_38 versus quantum Ne_38
Lennard-Jones clusters are investigated. The quantum simulations use both the
Path-Integral Monte-Carlo (PIMC) and the recently developed
Variational-Gaussian-Wavepacket Monte-Carlo (VGW-MC) methods. The PIMC and the
classical MC simulations are implemented in the parallel tempering framework.
The VGW method is used to locate and characterize the low energy states of
Ne_38, which are then further refined by PIMC calculations. Unlike the
classical case, the ground state of Ne_38 is a liquid-like structure. Among the
several liquid-like states with energies below the two symmetric states (O_h
and C_5v), the lowest two exhibit strong delocalization over basins associated
with at least two classical local minima. Because the symmetric structures do
not play an essential role in the thermodynamics of Ne_38, the quantum heat
capacity is a featureless curve indicative of the absence of any structural
transformations. Good agreement between the two methods, VGW and PIMC, is
obtained.Comment: 13 pages, 9 figure
First-order corrections to semiclassical Gaussian partition functions for clusters of atoms
Gaussian approximations to the Boltzmann operator have proven themselves in
recent years as useful tools for the study of the thermodynamic properties of
rare gas clusters. They are, however, not necessarily correct at very low
temperatures. In this article we introduce a first-order correction term to the
frozen Gaussian imaginary time propagator and apply it to the argon trimer. Our
findings show that the correction term provides objective access to the quality
of the propagator's results and clearly defines the "best" Gaussian width
parameter. The strength of the correction monitored as a function of the
temperature indicates that the results of the Gaussian propagator become
questionable below a certain temperature. The interesting thermodynamic
transition from a bounded trimer to three body dissociation lies in the
temperature range for which the Gaussian approximation is predicted to be
accurate.Comment: 9 pages, 5 figures, 1 table, corrections in the list of refereces,
minor modifications in the text, to be published in Chemical Physic
Explanation of quantum dot blinking without long-lived trap hypothesis
A simple model explaining the experimental data on QDs luminescence blinking
is suggested. The model does not assume the presence of the long-lived electron
traps. The bleaching of the QD luminescence is a result of the Auger assisted
radiationless relaxation of the excitation through the deep surface states.
Possible ways of the experimental verification of the model are discussed
Gaussian resolutions for equilibrium density matrices
A Gaussian resolution method for the computation of equilibrium density
matrices rho(T) for a general multidimensional quantum problem is presented.
The variational principle applied to the ``imaginary time'' Schroedinger
equation provides the equations of motion for Gaussians in a resolution of
rho(T) described by their width matrix, center and scale factor, all treated as
dynamical variables.
The method is computationally very inexpensive, has favorable scaling with
the system size and is surprisingly accurate in a wide temperature range, even
for cases involving quantum tunneling. Incorporation of symmetry constraints,
such as reflection or particle statistics, is also discussed.Comment: 4 page
An NMR relaxometry study of heteronuclear effects upon proton transfer in hydrogen bonds
The inherent quantum-mechanical nature of the proton transfer process in hydrogen bonds has been investigated through its effects on the nuclear spin-lattice relaxation rate. The fast magnetic field-cycling techniques employed allowed a direct measure of the rate characterising this dynamic process, which is closely related to the potential energy environment experienced by the mobile proton.
Various heteronuclear effects from magnetic and non-magnetic nuclei outside the hydrogen bond were characterised. The contribution to proton tunnelling from the displacement of heavy atoms in the molecule is an important consideration within a complete description of the process. This interdependence was accurately measured for the carboxyl-group oxygen atoms in benzoic acid dimers through the isotope effect. Careful comparison of O and O-enriched benzoic acid relaxation allowed this relationship to be measured from the difference in low-temperature tunnelling rates.
Fluctuating dipolar interactions caused by proton transfer motion couples the Zeeman states of different nuclear species. The cross-relaxation occurring through this natural coupling was explored as a function of field in 2,4,6-trifluorobenzoic acid and C-enriched pure benzoic acid. Characterising the strength of this interaction endeavoured to broaden the comprehension of heteronuclear coupling and served as confirmation of the model used.
Beyond the carboxylic acid dimer, this investigation also showed dynamic disorder in intermolecular short, strong hydrogen bonds of pyridine-3,5-dicarboxylic acid. This proton transfer mechanism was found to be strongly dependent on the molecular vibrational modes creating a pathway between two potential minima. A finite change in entropy between the proton sites ensured that greatest proton mobility occurred at intermediate temperature, between relatively stable configurations at the extremes of temperature.
A study of different sources of molecular dynamics within one compound showed the efficiency of field-cycling NMR at separating their contributions to relaxation. Dynamic rates from the proton transfer and methyl group rotation in 4-methylbenzoic acid were reliably extracted to the extent of identifying separate contributions from a small percentage of molecules around impurity centres
Active feedback of a Fabry-Perot cavity to the emission of a single InAs/GaAs quantum dot
We present a detailed study of the use of Fabry-Perot (FP) cavities for the
spectroscopy of single InAs quantum dots (QDs). We derive optimal cavity
characteristics and resolution limits, and measure photoluminescence linewidths
as low as 0.9 GHz. By embedding the QDs in a planar cavity, we obtain a
sufficiently large signal to actively feed back on the length of the FP to lock
to the emission of a single QD with a stability below 2% of the QD linewidth.
An integration time of approximately two seconds is found to yield an optimum
compromise between shot noise and cavity length fluctuations.Comment: 7 pages, 3 figure
Surface effects in the crystallization process of elastic flexible polymers
Investigating thermodynamic properties of liquid-solid transitions of
flexible homopolymers with elastic bonds by means of multicanonical Monte Carlo
simulations, we find crystalline conformations that resemble ground-state
structures of Lennard-Jones clusters. This allows us to set up a structural
classification scheme for finite-length flexible polymers and their freezing
mechanism in analogy to atomic cluster formation. Crystals of polymers with
"magic length" turn out to be perfectly icosahedral
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