4,703 research outputs found
Performance Testing of Distributed Component Architectures
Performance characteristics, such as response time, throughput andscalability, are key quality attributes of distributed applications. Current practice,however, rarely applies systematic techniques to evaluate performance characteristics.We argue that evaluation of performance is particularly crucial in early developmentstages, when important architectural choices are made. At first glance, thiscontradicts the use of testing techniques, which are usually applied towards the endof a project. In this chapter, we assume that many distributed systems are builtwith middleware technologies, such as the Java 2 Enterprise Edition (J2EE) or theCommon Object Request Broker Architecture (CORBA). These provide servicesand facilities whose implementations are available when architectures are defined.We also note that it is the middleware functionality, such as transaction and persistenceservices, remote communication primitives and threading policy primitives,that dominates distributed system performance. Drawing on these observations, thischapter presents a novel approach to performance testing of distributed applications.We propose to derive application-specific test cases from architecture designs so thatthe performance of a distributed application can be tested based on the middlewaresoftware at early stages of a development process. We report empirical results thatsupport the viability of the approach
Electrical plasmon injection in double-layer graphene heterostructures
It is by now well established that high-quality graphene enables a
gate-tunable low-loss plasmonic platform for the efficient confinement,
enhancement, and manipulation of optical fields spanning a broad range of
frequencies, from the mid infrared to the Terahertz domain. While
all-electrical detection of graphene plasmons has been demonstrated, electrical
plasmon injection (EPI), which is crucial to operate nanoplasmonic devices
without the encumbrance of a far-field optical apparatus, remains elusive. In
this work, we present a theory of EPI in double-layer graphene, where a
vertical tunnel current excites acoustic and optical plasmon modes. We first
calculate the power delivered by the applied inter-layer voltage bias into
these collective modes. We then show that this system works also as a
spectrally-resolved molecular sensor.Comment: 10 pages, 6 figure
Photoemission spectra of massless Dirac fermions on the verge of exciton condensation
Angle-resolved photoemission spectroscopy (ARPES) is a powerful probe of
electron correlations in two-dimensional layered materials. In this Letter we
demonstrate that ARPES can be used to probe the onset of exciton condensation
in spatially-separated systems of electrons and holes created by gating
techniques in either double-layer graphene or topological-insulator thin films.Comment: 5 pages, 3 figure
Many-body orbital paramagnetism in doped graphene sheets
The orbital magnetic susceptibility (OMS) of a gas of noninteracting massless
Dirac fermions is zero when the Fermi energy is away from the Dirac point.
Making use of diagrammatic perturbation theory, we calculate exactly the OMS of
massless Dirac fermions to first order in the Coulomb interaction demonstrating
that it is finite and positive. Doped graphene sheets are thus unique systems
in which the OMS is completely controlled by many-body effects.Comment: 4 pages, 2 figures, submitte
Magnetic hallmarks of viscous electron flow in graphene
We propose a protocol to identify spatial hallmarks of viscous electron flow
in graphene and other two-dimensional viscous electron fluids. We predict that
the profile of the magnetic field generated by hydrodynamic electron currents
flowing in confined geometries displays unambiguous features linked to
whirlpools and backflow near current injectors. We also show that the same
profile sheds light on the nature of the boundary conditions describing
friction exerted on the electron fluid by the edges of the sample. Our
predictions are within reach of vector magnetometry based on nitrogen-vacancy
centers embedded in a diamond slab mounted onto a graphene layer.Comment: 5 pages, 6 figure
Electron density distribution and screening in rippled graphene sheets
Single-layer graphene sheets are typically characterized by long-wavelength
corrugations (ripples) which can be shown to be at the origin of rather strong
potentials with both scalar and vector components. We present an extensive
microscopic study, based on a self-consistent Kohn-Sham-Dirac
density-functional method, of the carrier density distribution in the presence
of these ripple-induced external fields. We find that spatial density
fluctuations are essentially controlled by the scalar component, especially in
nearly-neutral graphene sheets, and that in-plane atomic displacements are as
important as out-of-plane ones. The latter fact is at the origin of a
complicated spatial distribution of electron-hole puddles which has no evident
correlation with the out-of-plane topographic corrugations. In the range of
parameters we have explored, exchange and correlation contributions to the
Kohn-Sham potential seem to play a minor role.Comment: 13 pages, 13 figures, submitted. High-quality figures can be
requested to the author
Spin drag in an ultracold Fermi gas on the verge of a ferromagnetic instability
Recent experiments [Jo et al., Science 325, 1521 (2009)] have presented
evidence of ferromagnetic correlations in a two-component ultracold Fermi gas
with strong repulsive interactions. Motivated by these experiments we consider
spin drag, i.e., frictional drag due to scattering of particles with opposite
spin, in such systems. We show that when the ferromagnetic state is approached
from the normal side, the spin drag relaxation rate is strongly enhanced near
the critical point. We also determine the temperature dependence of the spin
diffusion constant. In a trapped gas the spin drag relaxation rate determines
the damping of the spin dipole mode, which therefore provides a precursor
signal of the ferromagnetic phase transition that may be used to experimentally
determine the proximity to the ferromagnetic phase.Comment: 4 pages, 3 fig
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