4,133 research outputs found
Direct Observation of Large Amplitude Spin Excitations Localized in a Spin-Transfer Nanocontact
We report the direct observation of large amplitude spin-excitations
localized in a spin-transfer nanocontact using scanning transmission x-ray
microscopy. Experiments were conducted using a nanocontact to an ultrathin
ferromagnetic multilayer with perpendicular magnetic anisotropy. Element
resolved x-ray magnetic circular dichroism images show an abrupt onset of spin
excitations at a threshold current that are localized beneath the nanocontact,
with average spin precession cone angles of 25{\deg} at the contact center. The
results strongly suggest that we have observed a localized magnetic soliton.Comment: 5 pages, 3 figure
A Survey on Routing in Anonymous Communication Protocols
The Internet has undergone dramatic changes in the past 15 years, and now forms a global communication platform that billions of users rely on for their daily activities. While this transformation has brought tremendous benefits to society, it has also created new threats to online privacy, ranging from profiling of users for monetizing personal information to nearly omnipotent governmental surveillance. As a result, public interest in systems for anonymous communication has drastically increased. Several such systems have been proposed in the literature, each of which offers anonymity guarantees in different scenarios and under different assumptions, reflecting the plurality of approaches for how messages can be anonymously routed to their destination. Understanding this space of competing approaches with their different guarantees and assumptions is vital for users to understand the consequences of different design options. In this work, we survey previous research on designing, developing, and deploying systems for anonymous communication. To this end, we provide a taxonomy for clustering all prevalently considered approaches (including Mixnets, DC-nets, onion routing, and DHT-based protocols) with respect to their unique routing characteristics, deployability, and performance. This, in particular, encompasses the topological structure of the underlying network; the routing information that has to be made available to the initiator of the conversation; the underlying communication model; and performance-related indicators such as latency and communication layer. Our taxonomy and comparative assessment provide important insights about the differences between the existing classes of anonymous communication protocols, and it also helps to clarify the relationship between the routing characteristics of these protocols, and their performance and scalability
Long term monitoring of bright TeV Blazars with the MAGIC telescope
The MAGIC telescope has performed long term monitoring observations of the
bright TeV Blazars Mrk421, Mrk501 and 1ES1959+650. Up to 40 observations, 30 to
60 minutes each have been performed for each source evenly distributed over the
observable period of the year. The sensitivity of MAGIC is sufficient to
establish a flux level of 25% of the Crab flux for each measurement. These
observations are well suited to trigger multiwavelength ToO observations and
the overall collected data allow an unbiased study of the flaring statistics of
the observed AGNs.Comment: 4 pages, 4 figures, to appear in the proceedings of the 30th
International Cosmic Ray Conference, Merida, July 200
Hubbard band or oxygen vacancy states in the correlated electron metal SrVO?
We study the effect of oxygen vacancies on the electronic structure of the
model strongly correlated metal SrVO. By means of angle-resolved
photoemission (ARPES) synchrotron experiments, we investigate the systematic
effect of the UV dose on the measured spectra. We observe the onset of a
spurious dose-dependent prominent peak at an energy range were the lower
Hubbard band has been previously reported in this compound, raising questions
on its previous interpretation. By a careful analysis of the dose dependent
effects we succeed in disentangling the contributions coming from the oxygen
vacancy states and from the lower Hubbard band. We obtain the intrinsic ARPES
spectrum for the zero-vacancy limit, where a clear signal of a lower Hubbard
band remains. We support our study by means of state-of-the-art ab initio
calculations that include correlation effects and the presence of oxygen
vacancies. Our results underscore the relevance of potential spurious states
affecting ARPES experiments in correlated metals, which are associated to the
ubiquitous oxygen vacancies as extensively reported in the context of a
two-dimensional electron gas (2DEG) at the surface of insulating
transition metal oxides.Comment: Manuscript + Supplemental Material, 12 pages, 9 figure
Direct observation and imaging of a spin-wave soliton with like symmetry
The prediction and realization of magnetic excitations driven by electrical
currents via the spin transfer torque effect, enables novel magnetic
nano-devices where spin-waves can be used to process and store information. The
functional control of such devices relies on understanding the properties of
non-linear spin-wave excitations. It has been demonstrated that spin waves can
show both an itinerant character, but also appear as localized solitons. So
far, it was assumed that localized solitons have essentially cylindrical,
like symmetry. Using a newly developed high-sensitivity time-resolved
magnetic x-ray microscopy, we instead observe the emergence of a novel
localized soliton excitation with a nodal line, i.e. with like symmetry.
Micromagnetic simulations identify the physical mechanism that controls the
transition from to like solitons. Our results suggest a potential new
pathway to design artificial atoms with tunable dynamical states using
nanoscale magnetic devices
Asymptotic information leakage under one-try attacks
We study the asymptotic behaviour of (a) information leakage and (b) adversary’s error probability in information hiding systems modelled as noisy channels. Specifically, we assume the attacker can make a single guess after observing n independent executions of the system, throughout which the secret information is kept fixed. We show that the asymptotic behaviour of quantities (a) and (b) can be determined in a simple way from the channel matrix. Moreover, simple and tight bounds on them as functions of n show that the convergence is exponential. We also discuss feasible methods to evaluate the rate of convergence. Our results cover both the Bayesian case, where a prior probability distribution on the secrets is assumed known to the attacker, and the maximum-likelihood case, where the attacker does not know such distribution. In the Bayesian case, we identify the distributions that maximize the leakage. We consider both the min-entropy setting studied by Smith and the additive form recently proposed by Braun et al., and show the two forms do agree asymptotically. Next, we extend these results to a more sophisticated eavesdropping scenario, where the attacker can perform a (noisy) observation at each state of the computation and the systems are modelled as hidden Markov models
Rendimento e composição química de espécies em consórcio com milho safrinha e rendimento da soja em sucessão, em MS.
bitstream/item/66219/1/32015.pdfFERTBI
Choreographies with Secure Boxes and Compromised Principals
We equip choreography-level session descriptions with a simple abstraction of
a security infrastructure. Message components may be enclosed within (possibly
nested) "boxes" annotated with the intended source and destination of those
components. The boxes are to be implemented with cryptography. Strand spaces
provide a semantics for these choreographies, in which some roles may be played
by compromised principals. A skeleton is a partially ordered structure
containing local behaviors (strands) executed by regular (non-compromised)
principals. A skeleton is realized if it contains enough regular strands so
that it could actually occur, in combination with any possible activity of
compromised principals. It is delivery guaranteed (DG) realized if, in
addition, every message transmitted to a regular participant is also delivered.
We define a novel transition system on skeletons, in which the steps add
regular strands. These steps solve tests, i.e. parts of the skeleton that could
not occur without additional regular behavior. We prove three main results
about the transition system. First, each minimal DG realized skeleton is
reachable, using the transition system, from any skeleton it embeds. Second, if
no step is possible from a skeleton A, then A is DG realized. Finally, if a DG
realized B is accessible from A, then B is minimal. Thus, the transition system
provides a systematic way to construct the possible behaviors of the
choreography, in the presence of compromised principals
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