1,591 research outputs found
Effect of deconfinement on resonant transport in quantum wires
The effect of deconfinement due to finite band offsets on transport through
quantum wires with two constrictions is investigated. It is shown that the
increase in resonance linewidth becomes increasingly important as the size is
reduced and ultimately places an upper limit on the energy (temperature) scale
for which resonances may be observed.Comment: 6 pages, 6 postscript files with figures; uses REVTe
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Dielectric function decomposition by dipole interaction distribution: Application to triclinic K2Cr2O7
Here we present a general approach for the description for the frequency dependent dielectric tensor coefficients for optically anisotropic materials. Based on symmetry arguments we show that the components of the dielectric tensor are in general not independent of each other. For each excitation there exists an eigensystem, where its contribution to the dielectric tensor can be described by a diagonal susceptibility tensor. From the orientation of the eigensystem and the relative magnitude of the tensor elements, the dipole interaction distribution in real space can be deduced. In the limiting cases, the oriented dipole approach as well as the tensor of isotropic and uniaxial materials are obtained. The application of this model is demonstrated exemplarily on triclinic K2Cr2O7 and the orientation and directional distribution of the corresponding dipole moments in real space are determined. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft
Exploring the Monero Peer-to-Peer Network
As of September 2019, Monero is the most capitalized privacy-
preserving cryptocurrency, and is ranked tenth among all cryptocurren-
cies. Monero’s on-chain data privacy guarantees, i.e., how mixins are
selected in each transaction, have been extensively studied. However, de-
spite Monero’s prominence, the network of peers running Monero clients
has not been analyzed. Such analysis is of prime importance, since po-
tential vulnerabilities in the peer-to-peer network may lead to attacks on
the blockchain’s safety (e.g., by isolating a set of nodes) and on users’
privacy (e.g., tracing transactions flow in the network).
This paper provides the first step study on understanding Monero’s peer-
to-peer (P2P) network. In particular, we deconstruct Monero’s P2P pro-
tocol based on its source code, and develop a toolset to explore Monero’s
network, which allows us to infer its topology, size, node distribution,
and node connectivity. During our experiments, we collected 510 GB of
raw data, from which we extracted 21,678 IP addresses of Monero nodes
distributed in 970 autonomous systems. We show that Monero’s network
is highly centralized — 13.2% of the nodes collectively maintain 82.86%
of the network connections. We have identified approximately 2,758 ac-
tive nodes per day, which is 68.7% higher than the number reported by
the MoneroHash mining pool. We also identified all concurrent outgoing
connections maintained by Monero nodes with very high probability (on
average 97.98% for nodes with less than 250 outgoing connections, and
93.79% for nodes with more connections)
Exciton and negative trion dissociation by an external electric field in vertically coupled quantum dots
We study the Stark effect for an exciton confined in a pair of vertically
coupled quantum dots. A single-band approximation for the hole and a parabolic
lateral confinement potential are adopted which allows for the separation of
the lateral center-of-mass motion and consequently for an exact numerical
solution of the Schr\"odinger equation. We show that for intermediate tunnel
coupling the external electric field leads to the dissociation of the exciton
via an avoided crossing of bright and dark exciton energy levels which results
in an atypical form of the Stark shift. The electric-field-induced dissociation
of the negative trion is studied using the approximation of frozen lateral
degrees of freedom. It is shown that in a symmetric system of coupled dots the
trion is more stable against dissociation than the exciton. For an asymmetric
system of coupled dots the trion dissociation is accompanied by a positive
curvature of the recombination energy line as a function of the electric field.Comment: PRB - in prin
Influence of shock wave propagation on dielectric barrier discharge plasma actuator performance
Interest in plasma actuators as active flow control devices is growing rapidly due to their lack of mechanical parts, light weight and high response frequency. Although the flow induced by these actuators has received much attention, the effect that the external flow has on the performance of the actuator itself must also be considered, especially the influence of unsteady high-speed flows which are fast becoming a norm in the operating flight envelopes. The primary objective of this study is to examine the characteristics of a dielectric barrier discharge (DBD) plasma actuator when exposed to an unsteady flow generated by a shock tube. This type of flow, which is often used in different studies, contains a range of flow regimes from sudden pressure and density changes to relatively uniform high-speed flow regions. A small circular shock tube is employed along with the schlieren photography technique to visualize the flow. The voltage and current traces of the plasma actuator are monitored throughout, and using the well-established shock tube theory the change in the actuator characteristics are related to the physical processes which occur inside the shock tube. The results show that not only is the shear layer outside of the shock tube affected by the plasma but the passage of the shock front and high-speed flow behind it also greatly influences the properties of the plasma
Fantasy-driven versus contact-driven users of child sexual exploitation material: offender classification and implications for their risk assessment
Since the advent of the internet, convictions for the possession, display, trading and distribution of child sexual exploitation material (CSEM) have risen steadily, but little is known about the appropriate assessment and treatment of this offender group, especially in regards to their risk of reoffending. It has been suggested that a conceptual distinction of fantasy- vs. contact-driven CSEM users might be of merit. Sixty-eight offenders recruited from sex offender treatment providers were assessed via an anonymous computer survey including a variety of clinical and risk-related variables; the findings showed differences in the psychological profiles between CSEM users and contact child sex offenders. Numerical and spatial methods were employed to identify subgroups of CSEM users; these confirmed the two-fold distinction of fantasy vs. contact driven offending. The spatial representation of participants identified three dimensions as crucial in the classification of these subgroups: direct sexual contact with a minor, possession of fantasy-generating material, and social contact with other users with a sexual interest in minors, potentially differentiating distinct offender subgroups with different risks and needs. The current study informed the development of an empirical model of CSEM users that could aid in the assessment of risk of reoffending and cross-over to contact sex offending
New approximations for the cone of copositive matrices and its dual
We provide convergent hierarchies for the cone C of copositive matrices and
its dual, the cone of completely positive matrices. In both cases the
corresponding hierarchy consists of nested spectrahedra and provide outer
(resp. inner) approximations for C (resp. for its dual), thus complementing
previous inner (resp. outer) approximations for C (for the dual). In
particular, both inner and outer approximations have a very simple
interpretation. Finally, extension to K-copositivity and K-complete positivity
for a closed convex cone K, is straightforward.Comment: 8
Study of the Negative Magneto-Resistance of Single Proton-Implanted Lithium-Doped ZnO Microwires
The magneto-transport properties of single proton-implanted ZnO and of
Li(7\%)-doped ZnO microwires have been studied. The as-grown microwires were
highly insulating and not magnetic. After proton implantation the Li(7\%) doped
ZnO microwires showed a non monotonous behavior of the negative
magneto-resistance (MR) at temperature above 150 K. This is in contrast to the
monotonous NMR observed below 50 K for proton-implanted ZnO. The observed
difference in the transport properties of the wires is related to the amount of
stable Zn vacancies created at the near surface region by the proton
implantation and Li doping. The magnetic field dependence of the resistance
might be explained by the formation of a magnetic/non magnetic heterostructure
in the wire after proton implantation.Comment: 6 pages with 5 figure
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