8,022 research outputs found
Random Walk over Basins of Attraction to Construct Ising Energy Landscapes
An efficient algorithm is developed to construct disconnectivity graphs by a
random walk over basins of attraction. This algorithm can detect a large number
of local minima, find energy barriers between them, and estimate local thermal
averages over each basin of attraction. It is applied to the SK spin glass
Hamiltonian where existing methods have difficulties even for a moderate number
of spins. Finite-size results are used to make predictions in the thermodynamic
limit that match theoretical approximations and recent findings on the free
energy landscapes of SK spin glasses.Comment: 2 Figures and 1 Table. To be published in Physical Review Letter
Direct Observation of Early-stage Quantum Dot Growth Mechanisms with High-temperature Ab Initio Molecular Dynamics
Colloidal quantum dots (QDs) exhibit highly desirable size- and
shape-dependent properties for applications from electronic devices to imaging.
Indium phosphide QDs have emerged as a primary candidate to replace the more
toxic CdSe QDs, but production of InP QDs with the desired properties lags
behind other QD materials due to a poor understanding of how to tune the growth
process. Using high-temperature ab initio molecular dynamics (AIMD)
simulations, we report the first direct observation of the early stage
intermediates and subsequent formation of an InP cluster from separated indium
and phosphorus precursors. In our simulations, indium agglomeration precedes
formation of In-P bonds. We observe a predominantly intercomplex pathway in
which In-P bonds form between one set of precursor copies while the carboxylate
ligand of a second indium precursor in the agglomerated indium abstracts a
ligand from the phosphorus precursor. This process produces an indium-rich
cluster with structural properties comparable to those in bulk zinc-blende InP
crystals. Minimum energy pathway characterization of the AIMD-sampled reaction
events confirms these observations and identifies that In-carboxylate
dissociation energetics solely determine the barrier along the In-P bond
formation pathway, which is lower for intercomplex (13 kcal/mol) than
intracomplex (21 kcal/mol) mechanisms. The phosphorus precursor chemistry, on
the other hand, controls the thermodynamics of the reaction. Our observations
of the differing roles of precursors in controlling QD formation strongly
suggests that the challenges thus far encountered in InP QD synthesis
optimization may be attributed to an overlooked need for a cooperative tuning
strategy that simultaneously addresses the chemistry of both indium and
phosphorus precursors.Comment: 40 pages, 9 figures, submitted for publicatio
The parity of specular Andreev reflection under mirror operation in zigzag graphene ribbon
It is known that the parity of reflection amplitude can either be even or odd
under the mirror operation. Up to now, all the parities of reflection amplitude
in the one-mode energy region are even under the mirror operation. In this
paper, we give an example of odd parity for Andreev reflection (AR) in a
three-terminal graphene-supercondutor hybrid systems. We found that the parity
is even for the Andreev retroreflection (ARR) and odd for specular Andreev
reflection (SAR). We attribute this remarkable phenomenon to the distinct
topology of the band structure of graphene and the specular Andreev reflection
involving two energy bands with different parity symmetry. As a result of odd
parity of SAR, the SAR probability of a four-terminal system with two
superconducting leads (two reflection interfaces) can be zero even when the
system is asymmetric due to the quantum interference of two ARs.Comment: 11 pages, 3 figure
Gate-controllable spin-battery
We propose a gate-controllable spin-battery for spin current. The
spin-battery consists of a lateral double quantum dot under a uniform magnetic
field. A finite DC spin-current is driven out of the device by controlling a
set of gate voltages. Spin-current can also be delivered in the absence of
charge-current. The proposed device should be realizable using present
technology at low temperature.Comment: 3 pages, 3 figures, accepted by Appl. Phys. Let
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