9,553 research outputs found
Growth without finance, finance without growth
The international comparative evidence on the nexus between finance and growth is ambiguous, owing to the many difficulties in isolating finance, separating its growth effect from that of the other factors. To overcome this problem, we study the effects of financial development on growth from 1960 to 2010 in one country – Italy. Thus we have the same political, legal and regulatory framework but also sharply differing development conditions between regions. After World War II Italy achieved an “economic miracle” similar to what China and India are now experiencing, followed by a lengthy phase of decline. Accordingly, we can distinguish the effect of financial development on growth from other potential causal factors while also considering regions with sharply different economic conditions. Our results show that from 1960 to 1980, when the Italian “economic miracle” was still under way, finance played no significant role in favouring the surge in economic growth, which most likely depended on internal consumption. Between 1980 and 2010, by contrast, the great expansion of Italian financial markets and institutions did have a positive effect on regional economic performance, but overall growth rates were nevertheless low. Although our empirical evidence supports the view that finance is more important for growth in less highly developed regions, it also shows that financial development has not helped to overcome the Italian economic divide
Classical limit of the quantum Zeno effect
The evolution of a quantum system subjected to infinitely many measurements
in a finite time interval is confined in a proper subspace of the Hilbert
space. This phenomenon is called "quantum Zeno effect": a particle under
intensive observation does not evolve. This effect is at variance with the
classical evolution, which obviously is not affected by any observations. By a
semiclassical analysis we will show that the quantum Zeno effect vanishes at
all orders, when the Planck constant tends to zero, and thus it is a purely
quantum phenomenon without classical analog, at the same level of tunneling.Comment: 10 pages, 2 figure
Dynamical properties across a quantum phase transition in the Lipkin-Meshkov-Glick model
It is of high interest, in the context of Adiabatic Quantum Computation, to
better understand the complex dynamics of a quantum system subject to a
time-dependent Hamiltonian, when driven across a quantum phase transition. We
present here such a study in the Lipkin-Meshkov-Glick (LMG) model with one
variable parameter. We first display numerical results on the dynamical
evolution across the LMG quantum phase transition, which clearly shows a
pronounced effect of the spectral avoided level crossings. We then derive a
phenomenological (classical) transition model, which already shows some
closeness to the numerical results. Finally, we show how a simplified quantum
transition model can be built which strongly improve the classical approach,
and shed light on the physical processes involved in the whole LMG quantum
evolution. From our results, we argue that the commonly used description in
term of Landau-Zener transitions is not appropriate for our model.Comment: 7 pages, 5 figures; corrected reference
Heat transfer mechanisms in bubbly Rayleigh-Benard convection
The heat transfer mechanism in Rayleigh-Benard convection in a liquid with a
mean temperature close to its boiling point is studied through numerical
simulations with point-like vapor bubbles, which are allowed to grow or shrink
through evaporation and condensation and which act back on the flow both
thermally and mechanically. It is shown that the effect of the bubbles is
strongly dependent on the ratio of the sensible heat to the latent heat as
embodied in the Jacob number Ja. For very small Ja the bubbles stabilize the
flow by absorbing heat in the warmer regions and releasing it in the colder
regions. With an increase in Ja, the added buoyancy due to the bubble growth
destabilizes the flow with respect to single-phase convection and considerably
increases the Nusselt number.Comment: 11 pages, 14 figure
Spatially embedded random networks
Many real-world networks analyzed in modern network theory have a natural spatial element; e.g., the Internet, social networks, neural networks, etc. Yet, aside from a comparatively small number of somewhat specialized and domain-specific studies, the spatial element is mostly ignored and, in particular, its relation to network structure disregarded. In this paper we introduce a model framework to analyze the mediation of network structure by spatial embedding; specifically, we model connectivity as dependent on the distance between network nodes. Our spatially embedded random networks construction is not primarily intended as an accurate model of any specific class of real-world networks, but rather to gain intuition for the effects of spatial embedding on network structure; nevertheless we are able to demonstrate, in a quite general setting, some constraints of spatial embedding on connectivity such as the effects of spatial symmetry, conditions for scale free degree distributions and the existence of small-world spatial networks. We also derive some standard structural statistics for spatially embedded networks and illustrate the application of our model framework with concrete examples
Influence of the Lower Hybrid Drift Instability on the onset of Magnetic Reconnection
Two-dimensional and three-dimensional kinetic simulation results reveal the
importance of the Lower-Hybrid Drift Instability LHDI to the onset of magnetic
reconnection. Both explicit and implicit kinetic simulations show that the LHDI
heats electrons anisotropically and increases the peak current density. Linear
theory predicts these modifications can increase the growth rate of the tearing
instability by almost two orders of magnitude and shift the fastest growing
modes to significantly shorter wavelengths. These predictions are confirmed by
nonlinear kinetic simulations in which the growth and coalescence of small
scale magnetic islands leads to a rapid onset of large scale reconnection
Centrality Measures in Spatial Networks of Urban Streets
We study centrality in urban street patterns of different world cities
represented as networks in geographical space. The results indicate that a
spatial analysis based on a set of four centrality indices allows an extended
visualization and characterization of the city structure. Planned and
self-organized cities clearly belong to two different universality classes. In
particular, self-organized cities exhibit scale-free properties similar to
those found in the degree distributions of non-spatial networks.Comment: 4 pages, 3 figure
Ambipolar Drift Heating in Turbulent Molecular Clouds
Although thermal pressure is unimportant dynamically in most molecular gas,
the temperature is an important diagnostic of dynamical processes and physical
conditions. This is the first of two papers on thermal equilibrium in molecular
clouds. We present calculations of frictional heating by ion-neutral (or
ambipolar) drift in three-dimensional simulations of turbulent, magnetized
molecular clouds.
We show that ambipolar drift heating is a strong function of position in a
turbulent cloud, and its average value can be significantly larger than the
average cosmic ray heating rate. The volume averaged heating rate per unit
volume due to ambipolar drift, H_AD ~ |JxB|^2 ~ B^4/L_B^2, is found to depend
on the rms Alfvenic Mach number, M_A, and on the average field strength, as
H_AD ~ M_A^2^4. This implies that the typical scale of variation of the
magnetic field, L_B, is inversely proportional to M_A, which we also
demonstrate.Comment: 37 pages, 9 figures include
Dissipative Structures in Supersonic Turbulence
We show that density-weighted moments of the dissipation rate, ,
averaged over a scale , in supersonic turbulence can be successfully
explained by the She and L\'ev\^eque model [Phys. Rev. Lett. {\bf 72}, 336
(1994)]. A general method is developed to measure the two parameters of the
model, and , based directly on their physical interpretations as
the scaling exponent of the dissipation rate in the most intermittent
structures () and the dimension of the structures (). We find that
the best-fit parameters ( and ) derived from the
scalings in a simulation of supersonic turbulence at Mach 6 agree
with their direct measurements, confirming the validity of the model in
supersonic turbulence.Comment: 4 pages, 3 figures, accepted by Phys. Rev. Let
Aging and Rejuvenation with Fractional Derivatives
We discuss a dynamic procedure that makes the fractional derivatives emerge
in the time asymptotic limit of non-Poisson processes. We find that two-state
fluctuations, with an inverse power-law distribution of waiting times, finite
first moment and divergent second moment, namely with the power index mu in the
interval 2<mu <3, yields a generalized master equation equivalent to the sum of
an ordinary Markov contribution and of a fractional derivative term. We show
that the order of the fractional derivative depends on the age of the process
under study. If the system is infinitely old, the order of the fractional
derivative, ord, is given by ord=3-mu . A brand new system is characterized by
the degree ord=mu -2. If the system is prepared at time -ta<0$ and the
observation begins at time t=0, we derive the following scenario. For times
0<t<<ta the system is satisfactorily described by the fractional derivative
with ord=3-mu . Upon time increase the system undergoes a rejuvenation process
that in the time limit t>>ta yields ord=mu -2. The intermediate time regime is
probably incompatible with a picture based on fractional derivatives, or, at
least, with a mono-order fractional derivative.Comment: 11 pages, 4 figure
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