24 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
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
Are Shockley-Read-Hall and ABC models valid for lead halide perovskites?
Metal halide perovskites are an important class of emerging semiconductors.
Their charge dynamics is poorly understood due to limited knowledge of defect
physics and charge recombination mechanisms. Nevertheless, classical ABC and
Shockley-Read-Hall (SRH) models are ubiquitously applied to perovskites without
considering their validity. Herein, an advanced technique mapping
photoluminescence quantum yield (PLQY) as a function of both the excitation
pulse energy and repetition frequency is developed and employed to examine the
validity of these models. While ABC and SRH fail to explain the charge dynamics
in a broad range of conditions, the addition of Auger recombination and
trapping to the SRH model enables a quantitative fitting of PLQY maps and
low-power PL decay kinetics, and extracting trap concentrations and efficacies.
Higher-power PL kinetics requires the inclusion of additional non-linear
processes. The PLQY mapping developed herein is suitable for a comprehensive
testing of theories and is applicable to any semiconductor.Comment: Supplementary Information available at
https://cloudstore.zih.tu-dresden.de/index.php/s/t5gBPJgwZiwfRR
Universal emission intermittency in quantum dots, nanorods, and nanowires
Virtually all known fluorophores, including semiconductor nanoparticles,
nanorods and nanowires exhibit unexplainable episodes of intermittent emission
blinking. A most remarkable feature of the fluorescence intermittency is a
universal power law distribution of on- and off-times. For nanoparticles the
resulting power law extends over an extraordinarily wide dynamic range: nine
orders of magnitude in probability density and five to six orders of magnitude
in time. The exponents hover about the ubiquitous value of -3/2. Dark states
routinely last for tens of seconds, which are practically forever on quantum
mechanical time scales. Despite such infinite states of darkness, the dots
miraculously recover and start emitting again. Although the underlying
mechanism responsible for this phenomenon remains an enduring mystery and many
questions remain, we argue that substantial theoretical progress has been made.Comment: 9 pages, 2 figures, Accepted versio