7,266 research outputs found
InAs/InP single quantum wire formation and emission at 1.5 microns
Isolated InAs/InP self-assembled quantum wires have been grown using in situ
accumulated stress measurements to adjust the optimal InAs thickness. Atomic
force microscopy imaging shows highly asymmetric nanostructures with average
length exceeding more than ten times their width. High resolution optical
investigation of as-grown samples reveals strong photoluminescence from
individual quantum wires at 1.5 microns. Additional sharp features are related
to monolayer fluctuations of the two dimensional InAs layer present during the
early stages of the quantum wire self-assembling process.Comment: 4 pages and 3 figures submitted to Applied Physics Letter
Instability of scale-free networks under node-breaking avalanches
The instability introduced in a large scale-free network by the triggering of
node-breaking avalanches is analyzed using the fiber-bundle model as conceptual
framework. We found, by measuring the size of the giant component, the
avalanche size distribution and other quantities, the existence of an abrupt
transition. This test of strength for complex networks like Internet is more
stringent than others recently considered like the random removal of nodes,
analyzed within the framework of percolation theory. Finally, we discuss the
possible implications of our results and their relevance in forecasting
cascading failures in scale-free networks.Comment: 4 pages, 4 figures, final version to be published in Europhys. Let
Thresholds for epidemic spreading in networks
We study the threshold of epidemic models in quenched networks with degree
distribution given by a power-law. For the susceptible-infected-susceptible
(SIS) model the activity threshold lambda_c vanishes in the large size limit on
any network whose maximum degree k_max diverges with the system size, at odds
with heterogeneous mean-field (HMF) theory. The vanishing of the threshold has
not to do with the scale-free nature of the connectivity pattern and is instead
originated by the largest hub in the system being active for any spreading rate
lambda>1/sqrt{k_max} and playing the role of a self-sustained source that
spreads the infection to the rest of the system. The
susceptible-infected-removed (SIR) model displays instead agreement with HMF
theory and a finite threshold for scale-rich networks. We conjecture that on
quenched scale-rich networks the threshold of generic epidemic models is
vanishing or finite depending on the presence or absence of a steady state.Comment: 5 pages, 4 figure
Exciton Gas Compression and Metallic Condensation in a Single Semiconductor Quantum Wire
We study the metal-insulator transition in individual self-assembled quantum
wires and report optical evidences of metallic liquid condensation at low
temperatures. Firstly, we observe that the temperature and power dependence of
the single nanowire photoluminescence follow the evolution expected for an
electron-hole liquid in one dimension. Secondly, we find novel spectral
features that suggest that in this situation the expanding liquid condensate
compresses the exciton gas in real space. Finally, we estimate the critical
density and critical temperature of the phase transition diagram at
cm and K, respectively.Comment: 4 pages, 5 figure
Critical load and congestion instabilities in scale-free networks
We study the tolerance to congestion failures in communication networks with
scale-free topology. The traffic load carried by each damaged element in the
network must be partly or totally redistributed among the remaining elements.
Overloaded elements might fail on their turn, triggering the occurrence of
failure cascades able to isolate large parts of the network. We find a critical
traffic load above which the probability of massive traffic congestions
destroying the network communication capabilities is finite.Comment: 4 pages, 3 figure
Mechanical properties of wood-derived silicon carbide aluminum-alloy composites as a function of temperature
The mechanical behavior [i.e., stiffness, strength, and toughness (K_(IC))] of SiC Al–Si–Mg metal–ceramic composites (50:50 by volume) was studied at temperatures ranging from 25 to 500 °C. The SiC phase was derived from wood precursors, which resulted in an interconnected anisotropic ceramic that constrained the pressure melt-infiltrated aluminum alloy. The composites were made using SiC derived from two woods (sapele and beech) and were studied in three orthogonal orientations. The mechanical properties and corresponding deformation micromechanisms were different in the longitudinal (LO) and transverse directions, but the influence of the precursor wood was small. The LO behavior was controlled by the rigid SiC preform and the load transfer from the metal to the ceramic. Moduli in this orientation were lower than the Halpin–Tsai predictions due to the nonlinear and nonparallel nature of the Al-filled pores. The LO K_(IC) agreed with the Ashby model for the K_(IC) contribution of ductile inclusions in a brittle ceramic
Spreading of sexually transmitted diseases in heterosexual populations
The spread of sexually transmitted diseases (e.g. Chlamydia, Syphilis,
Gonorrhea, HIV) across populations is a major concern for scientists and health
agencies. In this context, both data collection on sexual contact networks and
the modeling of disease spreading, are intensively contributing to the search
for effective immunization policies. Here, the spreading of sexually
transmitted diseases on bipartite scale-free graphs, representing heterosexual
contact networks, is considered. We analytically derive the expression for the
epidemic threshold and its dependence with the system size in finite
populations. We show that the epidemic outbreak in bipartite populations, with
number of sexual partners distributed as in empirical observations from
national sex surveys, takes place for larger spreading rates than for the case
in which the bipartite nature of the network is not taken into account.
Numerical simulations confirm the validity of the theoretical results. Our
findings indicate that the restriction to crossed infections between the two
classes of individuals (males and females) has to be taken into account in the
design of efficient immunization strategies for sexually transmitted diseases.Comment: 7 pages, 3 figures and 2 table
Generation of uncorrelated random scale-free networks
Uncorrelated random scale-free networks are useful null models to check the
accuracy an the analytical solutions of dynamical processes defined on complex
networks. We propose and analyze a model capable to generate random
uncorrelated scale-free networks with no multiple and self-connections. The
model is based on the classical configuration model, with an additional
restriction on the maximum possible degree of the vertices. We check
numerically that the proposed model indeed generates scale-free networks with
no two and three vertex correlations, as measured by the average degree of the
nearest neighbors and the clustering coefficient of the vertices of degree ,
respectively
Spacelike surfaces with free boundary in the Lorentz-Minkowski space
We investigate a variational problem in the Lorentz-Minkowski space \l^3
whose critical points are spacelike surfaces with constant mean curvature and
making constant contact angle with a given support surface along its common
boundary. We show that if the support surface is a pseudosphere, then the
surface is a planar disc or a hyperbolic cap. We also study the problem of
spacelike hypersurfaces with free boundary in the higher dimensional
Lorentz-Minkowski space \l^{n+1}.Comment: 16 pages. Accepted in Classical and Quantum Gravit
Charge control in laterally coupled double quantum dots
We investigate the electronic and optical properties of InAs double quantum
dots grown on GaAs (001) and laterally aligned along the [110] crystal
direction. The emission spectrum has been investigated as a function of a
lateral electric field applied along the quantum dot pair mutual axis. The
number of confined electrons can be controlled with the external bias leading
to sharp energy shifts which we use to identify the emission from neutral and
charged exciton complexes. Quantum tunnelling of these electrons is proposed to
explain the reversed ordering of the trion emission lines as compared to that
of excitons in our system.Comment: 4 pages, 4 figures submitted to PRB Rapid Com
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