29,410 research outputs found
Exciton and biexciton energies in bilayer systems
We report calculations of the energies of excitons and biexcitons in ideal
two-dimensional bilayer systems within the effective-mass approximation with
isotropic electron and hole masses. The exciton energies are obtained by a
simple numerical integration technique, while the biexciton energies are
obtained from diffusion quantum Monte Carlo calculations. The exciton binding
energy decays as the inverse of the separation of the layers, while the binding
energy of the biexciton with respect to dissociation into two separate excitons
decays exponentially
Evolution of InAs branches in InAs/GaAs nanowire heterostructures
Branched nanowireheterostructures of InAs∕GaAs were observed during Au-assisted growth of InAs on GaAsnanowires. The evolution of these branches has been determined through detailed electron microscopy characterization with the following sequence: (1) in the initial stage of InAsgrowth, the Au droplet is observed to slide down the side of the GaAsnanowire, (2) the downward movement of Aunanoparticle later terminates when the nanoparticle encounters InAsgrowing radially on the GaAsnanowire sidewalls, and (3) with further supply of In and As vapor reactants, the Aunanoparticles assist the formation of InAs branches with a well-defined orientation relationship with GaAs∕InAs core/shell stems. We anticipate that these observations advance the understanding of the kink formation in axial nanowireheterostructures.The Australian Research Council is acknowledged for
the financial support of this project. One of the authors
M.P. acknowledges the support of an International Postgraduate
Research Scholarship
Noisy pre-processing facilitating a photonic realisation of device-independent quantum key distribution
Device-independent quantum key distribution provides security even when the
equipment used to communicate over the quantum channel is largely
uncharacterized. An experimental demonstration of device-independent quantum
key distribution is however challenging. A central obstacle in photonic
implementations is that the global detection efficiency, i.e., the probability
that the signals sent over the quantum channel are successfully received, must
be above a certain threshold. We here propose a method to significantly relax
this threshold, while maintaining provable device-independent security. This is
achieved with a protocol that adds artificial noise, which cannot be known or
controlled by an adversary, to the initial measurement data (the raw key).
Focusing on a realistic photonic setup using a source based on spontaneous
parametric down conversion, we give explicit bounds on the minimal required
global detection efficiency.Comment: 5+16 pages, 4 figure
Hysteretic and chaotic dynamics of viscous drops in creeping flows with rotation
It has been shown in our previous publication
(Blawzdziewicz,Cristini,Loewenberg,2003) that high-viscosity drops in two
dimensional linear creeping flows with a nonzero vorticity component may have
two stable stationary states. One state corresponds to a nearly spherical,
compact drop stabilized primarily by rotation, and the other to an elongated
drop stabilized primarily by capillary forces. Here we explore consequences of
the drop bistability for the dynamics of highly viscous drops. Using both
boundary-integral simulations and small-deformation theory we show that a
quasi-static change of the flow vorticity gives rise to a hysteretic response
of the drop shape, with rapid changes between the compact and elongated
solutions at critical values of the vorticity. In flows with sinusoidal
temporal variation of the vorticity we find chaotic drop dynamics in response
to the periodic forcing. A cascade of period-doubling bifurcations is found to
be directly responsible for the transition to chaos. In random flows we obtain
a bimodal drop-length distribution. Some analogies with the dynamics of
macromolecules and vesicles are pointed out.Comment: 22 pages, 13 figures. submitted to Journal of Fluid Mechanic
Universality at integer quantum Hall transitions
We report in this paper results of experimental and theoretical studies of
transitions between different integer quantum Hall phases, as well as
transition between the insulating phase and quantum Hall phases at high
magnetic fields. We focus mainly on universal properties of the transitions. We
demonstrate that properly defined conductivity tensor is universal at the
transitions. We also present numerical results of a non-interacting electron
model, which suggest that the Thouless conductance is universal at integer
quantum Hall transitions, just like the conductivity tensor. Finite temperature
and system size effects near the transition point are also studied.Comment: 20 pages, 15 figure
Distinct order of Gd 4f and Fe 3d moments coexisting in GdFe4Al8
Single crystals of flux-grown tetragonal GdFe4Al8 were characterized by
thermodynamic, transport, and x-ray resonant magnetic scattering measurements.
In addition to antiferromagnetic order at TN ~ 155 K, two low-temperature
transitions at T1 ~ 21 K and T2 ~ 27 K were identified. The Fe moments order at
TN with an incommensurate propagation vector (tau,tau,0) with tau varying
between 0.06 and 0.14 as a function of temperature, and maintain this order
over the entire T<TN range. The Gd 4f moments order below T2 with a
ferromagnetic component mainly out of plane. Below T1, the ferromagnetic
components are confined to the crystallographic plane. Remarkably, at low
temperatures the Fe moments maintain the same modulation as at high
temperatures, but the Gd 4f moments apparently do not follow this modulation.
The magnetic phase diagrams for fields applied in [110] and [001] direction are
presented and possible magnetic structures are discussed.Comment: v2: 14 pages, 12 figures; PRB in prin
On the six-dimensional origin of the AGT correspondence
We argue that the six-dimensional (2,0) superconformal theory defined on M
\times C, with M being a four-manifold and C a Riemann surface, can be twisted
in a way that makes it topological on M and holomorphic on C. Assuming the
existence of such a twisted theory, we show that its chiral algebra contains a
W-algebra when M = R^4, possibly in the presence of a codimension-two defect
operator supported on R^2 \times C \subset M \times C. We expect this structure
to survive the \Omega-deformation.Comment: References added. 14 page
Evolving temporal association rules with genetic algorithms
A novel framework for mining temporal association rules by discovering itemsets with a genetic algorithm is introduced. Metaheuristics have been applied to association rule mining, we show the efficacy of extending this to another variant - temporal association rule mining. Our framework is an enhancement to existing temporal association rule mining methods as it employs a genetic algorithm to simultaneously search the rule space and temporal space. A methodology for validating the ability of the proposed framework isolates target temporal itemsets in synthetic datasets. The Iterative Rule Learning method successfully discovers these targets in datasets with varying levels of difficulty
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