20,241 research outputs found
Extended Variational Cluster Approximation
The variational cluster approximation (VCA) proposed by M. Potthoff {\it et
al.} [Phys. Rev. Lett. {\bf 91}, 206402 (2003)] is extended to electron or spin
systems with nonlocal interactions. By introducing more than one source field
in the action and employing the Legendre transformation, we derive a
generalized self-energy functional with stationary properties. Applying this
functional to a proper reference system, we construct the extended VCA (EVCA).
In the limit of continuous degrees of freedom for the reference system, EVCA
can recover the cluster extension of the extended dynamical mean-field theory
(EDMFT). For a system with correlated hopping, the EVCA recovers the cluster
extension of the dynamical mean-field theory for correlated hopping. Using a
discrete reference system composed of decoupled three-site single impurities,
we test the theory for the extended Hubbard model. Quantitatively good results
as compared with EDMFT are obtained. We also propose VCA (EVCA) based on
clusters with periodic boundary conditions. It has the (extended) dynamical
cluster approximation as the continuous limit. A number of related issues are
discussed.Comment: 23 pages, 5 figures, statements about DCA corrected; published
versio
Towards Distributed Convoy Pattern Mining
Mining movement data to reveal interesting behavioral patterns has gained
attention in recent years. One such pattern is the convoy pattern which
consists of at least m objects moving together for at least k consecutive time
instants where m and k are user-defined parameters. Existing algorithms for
detecting convoy patterns, however do not scale to real-life dataset sizes.
Therefore a distributed algorithm for convoy mining is inevitable. In this
paper, we discuss the problem of convoy mining and analyze different data
partitioning strategies to pave the way for a generic distributed convoy
pattern mining algorithm.Comment: SIGSPATIAL'15 November 03-06, 2015, Bellevue, WA, US
Optical Counterpart of the Ultraluminous X-ray Source IC 342 X-1
We present Chandra and HST observations of the ultraluminous X-ray source
(ULX) IC 342 X-1. The Chandra and HST images are aligned using two X-ray
emitting foreground stars. The astrometry corrected position for X-1 is R.A. =
03h45m55.61s, Decl. = +68d04m55.3s (J2000) with an error circle of 0.2". One
extended optical source is found in the error circle, which could be the
optical counterpart of X-1. The source shows an extended feature in HST images
at long wavelengths, which is likely to be a superposition of two point
sources, although it is possible that the dimmer one could be a jet. Both
sources are much redder than typical for ULX optical counterparts. The brighter
one has an absolute magnitude M_V = -5.2 +/- 0.2 and (B-V)_0 = 0.66 +/- 0.13
and the dimmer star is not detected in B and has (B-V)_0 > 2.1. Their colors
are consistent with an F8 to G0 Ib supergiant or a carbon star, respectively.
However, it is likely that part or most of the optical emission may be due to
X-rays reprocessed by the companion star or the accretion disk. The stellar
neighborhood of IC 342 X-1 lacks O stars and has a minimum age of ~10 Myr. This
excludes the possibility that the surrounding nebula is powered by an energetic
explosion of a single massive star that formed a black hole. We suggest that
the nebula is most likely powered by an outflow from the X-ray source.Comment: 10 pages, 8 figures, accepted for publication in Ap
Aquaporin-4-dependent K(+) and water transport modeled in brain extracellular space following neuroexcitation.
Potassium (K(+)) ions released into brain extracellular space (ECS) during neuroexcitation are efficiently taken up by astrocytes. Deletion of astrocyte water channel aquaporin-4 (AQP4) in mice alters neuroexcitation by reducing ECS [K(+)] accumulation and slowing K(+) reuptake. These effects could involve AQP4-dependent: (a) K(+) permeability, (b) resting ECS volume, (c) ECS contraction during K(+) reuptake, and (d) diffusion-limited water/K(+) transport coupling. To investigate the role of these mechanisms, we compared experimental data to predictions of a model of K(+) and water uptake into astrocytes after neuronal release of K(+) into the ECS. The model computed the kinetics of ECS [K(+)] and volume, with input parameters including initial ECS volume, astrocyte K(+) conductance and water permeability, and diffusion in astrocyte cytoplasm. Numerical methods were developed to compute transport and diffusion for a nonstationary astrocyte-ECS interface. The modeling showed that mechanisms b-d, together, can predict experimentally observed impairment in K(+) reuptake from the ECS in AQP4 deficiency, as well as altered K(+) accumulation in the ECS after neuroexcitation, provided that astrocyte water permeability is sufficiently reduced in AQP4 deficiency and that solute diffusion in astrocyte cytoplasm is sufficiently low. The modeling thus provides a potential explanation for AQP4-dependent K(+)/water coupling in the ECS without requiring AQP4-dependent astrocyte K(+) permeability. Our model links the physical and ion/water transport properties of brain cells with the dynamics of neuroexcitation, and supports the conclusion that reduced AQP4-dependent water transport is responsible for defective neuroexcitation in AQP4 deficiency
Fresnel transmission coefficients for thermal phonons at solid interfaces
Interfaces play an essential role in phonon-mediated heat conduction in
solids, impacting applications ranging from thermoelectric waste heat recovery
to heat dissipation in electronics. From a microscopic perspective, interfacial
phonon transport is described by transmission and reflection coefficients,
analogous to the well-known Fresnel coefficients for light. However, these
coefficients have never been directly measured, and thermal transport processes
at interfaces remain poorly understood despite considerable effort. Here, we
report the first measurements of the Fresnel transmission coefficients for
thermal phonons at a metal-semiconductor interface using ab-initio phonon
transport modeling and a thermal characterization technique, time-domain
thermoreflectance. Our measurements show that interfaces act as thermal phonon
filters that transmit primarily low frequency phonons, leading to these phonons
being the dominant energy carriers across the interface despite the larger
density of states of high frequency phonons. Our work realizes the
long-standing goal of directly measuring thermal phonon transmission
coefficients and demonstrates a general route to study microscopic processes
governing interfacial heat conduction
Mesoscopic conductance fluctuations in a coupled quantum dot system
We study the transport properties of an Aharonov-Bohm ring containing two
quantum dots. One of the dots has well-separated resonant levels, while the
other is chaotic and is treated by random matrix theory. We find that the
conductance through the ring is significantly affected by mesoscopic
fluctuations. The Breit-Wigner resonant peak is changed to an antiresonance by
increasing the ratio of the level broadening to the mean level spacing of the
random dot. The asymmetric Fano form turns into a symmetric one and the
resonant peak can be controlled by magnetic flux. The conductance distribution
function clearly shows the influence of strong fluctuations.Comment: 4 pages, 4 figures; revised for publicatio
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