3,552 research outputs found
Heat Current Characteristics in Nanojunctions with Superconducting Leads
As a fundamental requisite for thermotronics, controlling heat flow has been
a longstanding quest in solid state physics. Recently, there has been a lot of
interest in nanoscale hybrid systems as possible candidates for thermal
devices. In this context, we study the heat current in the simplest hybrid
device of a two level system weakly coupled to two heat baths. We use the
reduced density matrix approach together with a simple Born-Markov
approximation to calculate the heat current in the steady state. We consider
different kinds of reservoirs and show that the nature of the reservoir plays a
very important role in determining the thermal characteristics of the device.
In particular, we investigate the effectiveness of a conventional
superconductor as a reservoir with regard to manipulating the heat current. In
the emergent temperature characteristics, we find that superconductivity in the
reservoirs leads to enhanced thermal currents and that the superconducting
phase transition is clearly visible in the heat current. We observe negative
differential thermal conductance and a pronounced rectification of the heat
current, making this a good building block for a quantum thermal diode.Comment: 10 pages, 6 figures, submitted to Physical Review
From old wars to new wars and global terrorism
Even before 9/11 there were claims that the nature of war had changed
fundamentally. The 9/11 attacks created an urgent need to understand
contemporary wars and their relationship to older conventional and terrorist
wars, both of which exhibit remarkable regularities. The frequency-intensity
distribution of fatalities in "old wars", 1816-1980, is a power-law with
exponent 1.80. Global terrorist attacks, 1968-present, also follow a power-law
with exponent 1.71 for G7 countries and 2.5 for non-G7 countries. Here we
analyze two ongoing, high-profile wars on opposite sides of the globe -
Colombia and Iraq. Our analysis uses our own unique dataset for killings and
injuries in Colombia, plus publicly available data for civilians killed in
Iraq. We show strong evidence for power-law behavior within each war. Despite
substantial differences in contexts and data coverage, the power-law
coefficients for both wars are tending toward 2.5, which is a value
characteristic of non-G7 terrorism as opposed to old wars. We propose a
plausible yet analytically-solvable model of modern insurgent warfare, which
can explain these observations.Comment: For more information, please contact [email protected] or
[email protected]
Landsliding and its multiscale influence on mountainscapes
Landsliding is a complex process that modifies mountainscapes worldwide. Its severe and sometimes long-lasting negative effects contrast with the less-documented positive effects on ecosystems, raising numerous questions about the dual role of landsliding, the feedbacks between biotic and geomorphic processes, and, ultimately, the ecological and evolutionary responses of organisms. We present a conceptual model in which feedbacks between biotic and geomorphic processes, landslides, and ecosystem attributes are hypothesized to drive the dynamics of mountain ecosystems at multiple scales. This model is used to integrate and synthesize a rich, but fragmented, body of literature generated in different disciplines, and to highlight the need for profitable collaborations between biologists and geoscientists. Such efforts should help identify attributes that contribute to the resilience of mountain ecosystems, and also should help in conservation, restoration, and hazard assessment. Given the sensitivity of mountains to land-use and global climate change, these endeavors are both relevant and timel
Negative-energy perturbations in cylindrical equilibria with a radial electric field
The impact of an equilibrium radial electric field on negative-energy
perturbations (NEPs) (which are potentially dangerous because they can lead to
either linear or nonlinear explosive instabilities) in cylindrical equilibria
of magnetically confined plasmas is investigated within the framework of
Maxwell-drift kinetic theory. It turns out that for wave vectors with a
non-vanishing component parallel to the magnetic field the conditions for the
existence of NEPs in equilibria with E=0 [G. N. Throumoulopoulos and D.
Pfirsch, Phys. Rev. E 53, 2767 (1996)] remain valid, while the condition for
the existence of perpendicular NEPs, which are found to be the most important
perturbations, is modified. For ( is the
electrostatic potential) and ( is
the total plasma pressure), a case which is of operational interest in magnetic
confinement systems, the existence of perpendicular NEPs depends on ,
where is the charge of the particle species . In this case the
electric field can reduce the NEPs activity in the edge region of tokamaklike
and stellaratorlike equilibria with identical parabolic pressure profiles, the
reduction of electron NEPs being more pronounced than that of ion NEPs.Comment: 30 pages, late
Synchronization in large directed networks of coupled phase oscillators
We extend recent theoretical approximations describing the transition to
synchronization in large undirected networks of coupled phase oscillators to
the case of directed networks. We also consider extensions to networks with
mixed positive/negative coupling strengths. We compare our theory with
numerical simulations and find good agreement
Approximating the largest eigenvalue of network adjacency matrices
The largest eigenvalue of the adjacency matrix of a network plays an
important role in several network processes (e.g., synchronization of
oscillators, percolation on directed networks, linear stability of equilibria
of network coupled systems, etc.). In this paper we develop approximations to
the largest eigenvalue of adjacency matrices and discuss the relationships
between these approximations. Numerical experiments on simulated networks are
used to test our results.Comment: 7 pages, 4 figure
Multi-tier Network Performance Analysis using a Shotgun Cellular System
This paper studies the carrier-to-interference ratio (CIR) and
carrier-to-interference-plus-noise ratio (CINR) performance at the mobile
station (MS) within a multi-tier network composed of M tiers of wireless
networks, with each tier modeled as the homogeneous n-dimensional (n-D, n=1,2,
and 3) shotgun cellular system, where the base station (BS) distribution is
given by the homogeneous Poisson point process in n-D. The CIR and CINR at the
MS in a single tier network are thoroughly analyzed to simplify the analysis of
the multi-tier network. For the multi-tier network with given system
parameters, the following are the main results of this paper: (1)
semi-analytical expressions for the tail probabilities of CIR and CINR; (2) a
closed form expression for the tail probability of CIR in the range
[1,Infinity); (3) a closed form expression for the tail probability of an
approximation to CIR in the entire range [0,Infinity); (4) a lookup table based
approach for obtaining the tail probability of CINR, and (5) the study of the
effect of shadow fading and BSs with ideal sectorized antennas on the CIR and
CINR. Based on these results, it is shown that, in a practical cellular system,
the installation of additional wireless networks (microcells, picocells and
femtocells) with low power BSs over the already existing macrocell network will
always improve the CINR performance at the MS.Comment: 6 pages, 3 figures, accepted at IEEE Globecom 201
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