8,007,123 research outputs found
Comment on "Do Earthquakes Exhibit Self-Organized Criticality?"
It is shown that earthquakes do not know how large they will become, at least
from the information collected at seismic catalogs. In other words, the
magnitude is independent on previous magnitudes as well as on the waiting time
between previous earthquakes. In contrast, the time to the next event does
depend on the magnitude. Also it is argued that SOC systems do not necessarily
shown a Poisson-type behavior in time, and SOC does not exclude the possibility
of some degree of prediction.Comment: Tentative comment to Yang, Du, Ma, PRL 92, 228501 (2004
Drift-diffusion model for single layer transition metal dichalcogenide field-effect transistors
A physics-based model for the surface potential and drain current for
monolayer transition metal dichalcogenide (TMD) field-effect transistor (FET)
is presented. Taking into account the 2D density-of-states of the atomic layer
thick TMD and its impact on the quantum capacitance, a model for the surface
potential is presented. Next, considering a drift-diffusion mechanism for the
carrier transport along the monolayer TMD, an explicit expression for the drain
current has been derived. The model has been benchmarked with a measured
prototype transistor. Based on the proposed model, the device design window
targeting low-power applications is discussed.Comment: 10 pages, 3 figure
Universal Earthquake-Occurrence Jumps, Correlations with Time, and Anomalous Diffusion
Spatiotemporal properties of seismicity are investigated for a worldwide (WW)
catalog and for Southern California in the stationary case (SC), showing a
nearly universal scaling behavior. Distributions of distances between
consecutive earthquakes (jumps) are magnitude independent and show two
power-law regimes, separated by jump values about 200 km (WW) and 15 km (SC).
Distributions of waiting times conditioned to the value of jumps show that both
variables are correlated in general, but turn out to be independent when only
short or long jumps are considered. Finally, diffusion profiles reflect the
shape of the jump distribution.Comment: Short pape
Economic Small-World Behavior in Weighted Networks
The small-world phenomenon has been already the subject of a huge variety of
papers, showing its appeareance in a variety of systems. However, some big
holes still remain to be filled, as the commonly adopted mathematical
formulation suffers from a variety of limitations, that make it unsuitable to
provide a general tool of analysis for real networks, and not just for
mathematical (topological) abstractions. In this paper we show where the major
problems arise, and how there is therefore the need for a new reformulation of
the small-world concept. Together with an analysis of the variables involved,
we then propose a new theory of small-world networks based on two leading
concepts: efficiency and cost. Efficiency measures how well information
propagates over the network, and cost measures how expensive it is to build a
network. The combination of these factors leads us to introduce the concept of
{\em economic small worlds}, that formalizes the idea of networks that are
"cheap" to build, and nevertheless efficient in propagating information, both
at global and local scale. This new concept is shown to overcome all the
limitations proper of the so-far commonly adopted formulation, and to provide
an adequate tool to quantitatively analyze the behaviour of complex networks in
the real world. Various complex systems are analyzed, ranging from the realm of
neural networks, to social sciences, to communication and transportation
networks. In each case, economic small worlds are found. Moreover, using the
economic small-world framework, the construction principles of these networks
can be quantitatively analyzed and compared, giving good insights on how
efficiency and economy principles combine up to shape all these systems.Comment: 17 pages, 10 figures, 4 table
Eigenmodes and thermodynamics of a Coulomb chain in a harmonic potential
The density of ions trapped in a harmonic potential in one dimension is not
uniform. Consequently the eigenmodes are not phonons. We calculate the long
wavelength modes in the continuum limit, and evaluate the density of states in
the short wavelength limit for chains of ions. Remarkably, the results
that are found analytically in the thermodynamic limit provide a good estimate
of the spectrum of excitations of small chains down to few tens of ions. The
spectra are used to compute the thermodynamic functions of the chain.
Deviations from extensivity of the thermodynamic quantities are found. An
analytic expression for the critical transverse frequency determining the
stability of a linear chain is derived.Comment: 4 pages, 4 figure
Two-photon and EIT-assisted Doppler cooling in a three-level cascade system
Laser cooling is theoretically investigated in a cascade three-level scheme,
where the excited state of a laser-driven transition is coupled by a second
laser to a top, more stable level, as for alkali-earth atoms. The second laser
action modifies the atomic scattering cross section and produces temperatures
lower than those reached by Doppler cooling on the lower transition. When
multiphoton processes due to the second laser are relevant, an electromagnetic
induced transparency modifies the absorption of the first laser, and the final
temperature is controlled by the second laser parameters. When the intermediate
state is only virtually excited, the dynamics is dominated by the two-photon
process and the final temperature is determined by the spontaneous decay rate
of the top state.Comment: 5 pages, 3 figures. Revised version, accepted for publication in
Phys. Rev A (Rapid Comm.
A physical model of quantum cascade lasers: Application to GaAs, GaN and SiGe devices
The philosophy behind this work has been to build a predictive bottom up physical model of quantum cascade lasers (QCLs) for use as a design tool, to interpret experimental results and hence improve understanding of the physical processes occurring inside working devices and as a simulator for developing new material systems. The standard model uses the envelope function and effective mass approximations to solve two complete periods of the QCL under an applied bias. Other models, such as k·p and empirical pseudopotential, have been employed in p-type systems where the more complex band structure requires it. The resulting wave functions are then used to evaluate all relevant carrier-phonon, carrier-carrier and alloy scattering rates from each quantised state to all others within the same and the neighbouring period. This information is then used to construct a rate equation for the equilibrium carrier density in each subband and this set of coupled rate equations are solved self-consistently to obtain the carrier density in each eigenstate. The latter is a fundamental description of the device and can be used to calculate the current density and gain as a function of the applied bias and temperature, which in turn yields the threshold current and expected temperature dependence of the device characteristics. A recent extension which includes a further iteration of an energy balance equation also yields the average electron (or hole) temperature over the subbands. This paper will review the method and describe its application to mid-infrared and terahertz, GaAs, GaN, SiGe cascade laser designs
Bulgarian sport policy 1945-1989: A strategic relation perspective
The 2008 Beijing Olympic Games have stimulated discussions about the success of different sport systems and the Chinese model in particular. Revisiting explanations of sport in the former communist countries of Eastern Europe during the Cold War seems timely, as the current Chinese model of sport was largely designed after the Soviet example established in this period. This paper examines Bulgarian sport policy between 1945 and 1989. It employs a Strategic Relation approach (Jessop, 1990) to analyse sport policy making as a strategic relation closely linked to the dominant state project of building a new stateness. It goes beyond ideological interpretations and argues that the state represents a strategic terrain where these relations have to be established in struggles, the outcomes of which are always uncertain. Furthermore, past and present struggles and their outcomes create various socio-political environments that presuppose the forms of state selectivity and intervention in sport. The process of constructing sport policy was influenced by two main categories of strategic relations: intra-state, including political, organisational and personal relations between the Party, state apparatus and various sport and non-sport organisations and their managers, and transnational, concerning ideological, political, economic and organisational relations with both communist and western countries and international sport organisations
Multi-domain ferroelectricity as a limiting factor for voltage amplification in ferroelectric field-effect transistors
We revise the possibility of having an amplified surface potential in
ferroelectric field-effect transistors pointed out by [S. Salahuddin and S.
Datta, Nano Lett. 8, 405 (2008)]. We show that the negative-capacitance regime
that allows for such an amplification is actually bounded by the appearance of
multi-domain ferroelectricity. This imposes a severe limit to the maximum
step-up of the surface potential obtainable in the device. We indicate new
device design rules taking into account this scenario.Comment: 4 pages, 3 figure
Physical model of quantum-well infrared photodetectors
A fully quantum mechanical model for electron transport in quantum well infrared photodetectors is
presented, based on a self-consistent solution of the coupled rate equations. The important macroscopic
parameters like current density, responsivity and capture probability can be estimated directly from this
first principles calculation. The applicability of the model was tested by comparison with experimental
measurements from a GaAs/AlGaAs device, and good agreement was found. The model is general and can
be applied to any other material system or QWIP design
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