1,199 research outputs found
A weakly nonlinear analysis of the magnetorotational instability in a model channel flow
We show by means of a perturbative weakly nonlinear analysis that the
axisymmetric magnetorotational instability (MRI) of a viscous, resistive,
incompressible rotating shear flow in a thin channel gives rise to a real
Ginzburg-Landau equation for the disturbance amplitude. For small magnetic
Prandtl number (), the saturation amplitude is and the resulting momentum transport scales as , where is the {\em hydrodynamic} Reynolds number. Simplifying
assumptions, such as linear shear base flow, mathematically expedient boundary
conditions and continuous spectrum of the vertical linear modes, are used to
facilitate this analysis. The asymptotic results are shown to comply with
numerical calculations using a spectral code. They suggest that the transport
due to the nonlinearly developed MRI may be very small in experimental setups
with .Comment: Accepted to Physical Review Letters - Nov. 30, 2006. In final for
Effects of dissipation in an adiabatic quantum search algorithm
We consider the effect of two different environments on the performance of
the quantum adiabatic search algorithm, a thermal bath at finite temperature,
and a structured environment similar to the one encountered in systems coupled
to the electromagnetic field that exists within a photonic crystal. While for
all the parameter regimes explored here, the algorithm performance is worsened
by the contact with a thermal environment, the picture appears to be different
when considering a structured environment. In this case we show that, by tuning
the environment parameters to certain regimes, the algorithm performance can
actually be improved with respect to the closed system case. Additionally, the
relevance of considering the dissipation rates as complex quantities is
discussed in both cases. More particularly, we find that the imaginary part of
the rates can not be neglected with the usual argument that it simply amounts
to an energy shift, and in fact influences crucially the system dynamics.Comment: 18 pages, 9 figure
Mapping out the glassy landscape of a mesoscopic elastoplastic model
We develop a mesoscopic model to study the plastic behavior of an amorphous
material under cyclic loading. The model is depinning-like and driven by a
disordered thresholds dynamics which are coupled by long-range elastic
interactions. We propose a simple protocol of "glass preparation" which allows
us to mimic thermalisation at high temperature, as well as aging at vanishing
temperature. Various levels of glass stabilities (from brittle to ductile) can
be achieved by tuning the aging duration. The aged glasses are then immersed
into a quenched disorder landscape and serve as initial configurations for
various protocols of mechanical loading by shearing. The dependence of the
plastic behavior upon monotonous loading is recovered. The behavior under
cyclic loading is studied for different ages and system sizes. The size and age
dependence of the irreversibility transition is discussed. A thorough
characterization of the disorder-landscape is achieved through the analysis of
the transition graphs, which describe the plastic deformation pathways under
athermal quasi-static shear. In particular, the analysis of the stability
ranges of the strongly connected components of the transition graphs reveals
the emergence of a phase-separation like process associated with the aging of
the glass. Increasing the age and hence stability of the initial glass, results
in a gradual break-up of the landscape of dynamically accessible stable states
into three distinct regions: one region centered around the initially prepared
glass phase, and two additional regions, characterized by well-separated ranges
of positive and negative plastic strains, each of which is accessible only from
the initial glass phase by passing through the stress peak in the forward,
respectively, backward shearing directions.Comment: 20 pages, 12 figures, including supplemental materia
Quantum algorithm for the Boolean hidden shift problem
The hidden shift problem is a natural place to look for new separations
between classical and quantum models of computation. One advantage of this
problem is its flexibility, since it can be defined for a whole range of
functions and a whole range of underlying groups. In a way, this distinguishes
it from the hidden subgroup problem where more stringent requirements about the
existence of a periodic subgroup have to be made. And yet, the hidden shift
problem proves to be rich enough to capture interesting features of problems of
algebraic, geometric, and combinatorial flavor. We present a quantum algorithm
to identify the hidden shift for any Boolean function. Using Fourier analysis
for Boolean functions we relate the time and query complexity of the algorithm
to an intrinsic property of the function, namely its minimum influence. We show
that for randomly chosen functions the time complexity of the algorithm is
polynomial. Based on this we show an average case exponential separation
between classical and quantum time complexity. A perhaps interesting aspect of
this work is that, while the extremal case of the Boolean hidden shift problem
over so-called bent functions can be reduced to a hidden subgroup problem over
an abelian group, the more general case studied here does not seem to allow
such a reduction.Comment: 10 pages, 1 figur
The Effective Temperature in Elasto-Plasticity of Amorphous Solids
An effective temperature which differs from the bath
temperature is believed to play an essential role in the theory of
elasto-plasticity of amorphous solids. The definition of a measurable in the literature on sheared solids suffers however from being connected
to a fluctuation-dissipation theorem which is correct only in equilibrium. Here
we introduce a natural definition of based on measurable
structural features without recourse to any questionable assumption. The value
of is connected, using theory and scaling concepts, to the flow
stress and the mean energy that characterize the elasto-plastic flow.Comment: 4 pages, 5 figure
Improved Error-Scaling for Adiabatic Quantum State Transfer
We present a technique that dramatically improves the accuracy of adiabatic
state transfer for a broad class of realistic Hamiltonians. For some systems,
the total error scaling can be quadratically reduced at a fixed maximum
transfer rate. These improvements rely only on the judicious choice of the
total evolution time. Our technique is error-robust, and hence applicable to
existing experiments utilizing adiabatic passage. We give two examples as
proofs-of-principle, showing quadratic error reductions for an adiabatic search
algorithm and a tunable two-qubit quantum logic gate.Comment: 10 Pages, 4 figures. Comments are welcome. Version substantially
revised to generalize results to cases where several derivatives of the
Hamiltonian are zero on the boundar
Magnetically Torqued Thin Accretion Disks
We compute the properties of a geometrically thin, steady accretion disk
surrounding a central rotating, magnetized star. The magnetosphere is assumed
to entrain the disk over a wide range of radii. The model is simplified in that
we adopt two (alternate) ad hoc, but plausible, expressions for the azimuthal
component of the magnetic field as a function of radial distance. We find a
solution for the angular velocity profile tending to corotation close to the
central star, and smoothly matching a Keplerian curve at a radius where the
viscous stress vanishes. The value of this ''transition'' radius is nearly the
same for both of our adopted B-field models. We then solve analytically for the
torques on the central star and for the disk luminosity due to gravity and
magnetic torques. When expressed in a dimensionless form, the resulting
quantities depend on one parameter alone, the ratio of the transition radius to
the corotation radius. For rapid rotators, the accretion disk may be powered
mostly by spin-down of the central star. These results are independent of the
viscosity prescription in the disk. We also solve for the disk structure for
the special case of an optically thick alpha disk. Our results are applicable
to a range of astrophysical systems including accreting neutron stars,
intermediate polar cataclysmic variables, and T Tauri systems.Comment: 9 sharper figs, updated reference
Atomic Disorder in Fossil Tooth and Bone Mineral: An FTIR Study Using the Grinding Curve Method
Bone and tooth mineral generally undergo diagenetic changes. These changes in the carbonate hydroxyapatite structure and composition can affect the signals embedded in the mineral phase, such as migration behavior, age of the specimen and the reconstruction of past environments. Mineral preservation state can be assessed using infrared spectroscopy which provides information on crystal disorder at the atomic level and mineral composition. Here we present a new approach to evaluate carbonate hydroxyapatite atomic disorder using infrared spectroscopy and the standard KBr sample mounting method. We show that by repeated grinding of the sample and then plotting the infrared splitting factor against the width of the major phosphate absorption peak after each grinding, grinding curves with well defined trend lines can be obtained. The offsets between curves reflect differences in atomic disorder. We show that grinding curve offsets can be used to evaluate the state of preservation of bone, dentine and enamel mineral.Les os et les dents subissent gĂ©nĂ©ralement des changements diagĂ©nĂ©tiques. Ces changements de la structure et de la composition de lâhydroxyapatite carbonatĂ©e peuvent affecter les signaux inclus dans la phase minĂ©rale, tels que les comportements de migration, lâĂąge des spĂ©cimens ou la reconstitution des environnements passĂ©s. LâĂ©tat de prĂ©servation de la phase minĂ©rale peut ĂȘtre dĂ©terminĂ© par la spectromĂ©trie infrarouge qui fournit des informations sur le dĂ©sordre structural et sur la composition du minĂ©ral. Dans cette Ă©tude, nous prĂ©sentons une nouvelle approche permettant dâĂ©valuer le dĂ©sordre cristallin en utilisant la spectromĂ©trie infrarouge. Nous montrons ici quâun broyage rĂ©pĂ©tĂ© de lâĂ©chantillon et la reprĂ©sentation graphique des valeurs de splitting factor et de la largeur Ă mi-hauteur de la principale bande dâabsorption des phosphates aprĂšs chaque broyage, peut permettre de dĂ©finir des courbes de broyage avec des tendances bien dĂ©finies. Un dĂ©calage entre ces courbes reflĂšte des diffĂ©rences de dĂ©sorganisation Ă lâĂ©chelle atomique
Factors shaping young tourists' intentions to travel to Greece, Israel and Portugal: Universal or idisyncratic perceptions of young Greeks, Israelis and Portugese?
Tourism constitutes an important branch of economy, especially in countries experiencing economic crisis and harsh socio-political events. The current study compares the intention of young people from three different countries â Greece, Israel and Portugal - to travel to two selected destinations. For each country of origin the other two countries were the destination of interest.
This research examined the factors affecting young tourists' intention to travel to different destinations assuming that those determinants depend on the country of origin as well as on the destination country.
The sample comprised of 648 students: Israelis (277), Portuguese (200), Greeks (171). The research questionnaire was distributed in Israeli, Greek and Portuguese academic institutions in 2014.
The results indicate that over half of the Israelis intend to visit Greece or Portugal. The majority of the Portuguese anticipate visiting Greece, while only 32.5% expect to visit Israel. Among the Greeks nearly 50% intend to visit Portugal and do not intend to visit Israel.
The study showed that the main factors affecting the decisions of young Portuguese to travel to Israel were: the image of Israel, the perceived risk of traveling to a country with political unrest and the number of trips abroad. The main factors affecting the decisions of young Greeks were the perceived risk of traveling to a country with political unrest and Israel's image. Israelis' decision to travel to Greece was affected by their image of Greece; the Portuguese also took into account the perceived risk regarding economic situation. For Israelis, considering Portugal as a traveling destination was related to the destination's image. The Greeks were affected by the number of Facebook friends and the Innovation, Entrepreneurship and Digital Ecosystems perceived economic risk. This research suggests that the socio-economic, political context of the country of origin and of the tourist destination plays an important role in young tourists' travelling-related decision-making.info:eu-repo/semantics/publishedVersio
Analyzing the effect of socio-political context on tourism: perceptions of young tourists in Greece, Portugal and Israel
International crises that affect tourism, such as terror attacks, political unrest, and economic crises have become
more frequent, and their influence has become broader. The influence of such extreme events depends on their salience in the tourists' awareness. Hence, it is important to understand the mechanisms underlying tourists' selection of travel destinations, especially their perceptions of crisis-related events and the impact of the sociopolitical and economic context in their countries of origin. The current study examined how the socio-political and economic context in the home countries of potential young tourists affected their selection of travel destinations. The objective was to elucidate how the salience of various crises (economic and political) in the tourists' perceptions, due to their experiences at home, color their construal of destinations affected by similar hazards and influence their travel intentions. The study focused on student tourists from Israel, Greece, and
Portugal. Today about a fifth of international tourism is based on young people, especially students. These countries were chosen since Greece and Portugal are in the midst of economic crises. In addition, Greece and Portugal have experienced political instability, while Israel has security-related problems (including terrorist incidents). In 2013, a total of 648 students, responded to a questionnaire that included questions concerning attitudes and risk perceptions regarding travel to destinations with various risk hazards as well as socio-demographic details. The results indicate that over half of the Israelis intend to visit Greece or Portugal. The majority of the Portuguese intend to visit Greece, while less than a third of them intend to visit Israel.
About half of the Greeks intend to visit Portugal, and most of them do not intend to visit Israel. The results indicate that greater perceived importance of economic crises mitigates the intention to travel to destinations with economic crises for tourists from origin countries that are also marked by economic crises, such as Greece and Portugal. However, for tourists from Israel, a country with a relatively stable economy, issues related to the economy barely affect their intention to travel to the other two countries. The findings also suggest that Greeks and Portuguese who are highly concerned about political unrest are unlikely to select Israel as a tourist destination. In addition, strong apprehension regarding terrorism impedes the intention to travel to destinations marked by terrorist incidents, such as Israel. The current research contributes to the existing literature by highlighting the impact of travelers' personal previous experience with crisis on their risk perceptions and in turn on their intentions to travel to countries with similar risks. Therefore, in a world where such incidents are on the rise, understanding
tourists' risk perceptions and behavior and the factors influencing their destination-related decisions are crucial for countries that wish to increase the numbers of incoming tourists.info:eu-repo/semantics/publishedVersio
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