3,629 research outputs found
Equilibration in long-range quantum spin systems from a BBGKY perspective
The time evolution of -spin reduced density operators is studied for a
class of Heisenberg-type quantum spin models with long-range interactions. In
the framework of the quantum Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY)
hierarchy, we introduce an unconventional representation, different from the
usual cluster expansion, which casts the hierarchy into the form of a
second-order recursion. This structure suggests a scaling of the expansion
coefficients and the corresponding time scales in powers of with the
system size , implying a separation of time scales in the large system
limit. For special parameter values and initial conditions, we can show
analytically that closing the BBGKY hierarchy by neglecting -spin
correlations does never lead to equilibration, but gives rise to quasi-periodic
time evolution with at most independent frequencies. Moreover, for the
same special parameter values and in the large- limit, we solve the complete
recursion relation (the full BBGKY hierarchy), observing a superexponential
decay to equilibrium in rescaled time .Comment: 3 figure
A simple topological model with continuous phase transition
In the area of topological and geometric treatment of phase transitions and
symmetry breaking in Hamiltonian systems, in a recent paper some general
sufficient conditions for these phenomena in -symmetric systems
(i.e. invariant under reflection of coordinates) have been found out. In this
paper we present a simple topological model satisfying the above conditions
hoping to enlighten the mechanism which causes this phenomenon in more general
physical models. The symmetry breaking is testified by a continuous
magnetization with a nonanalytic point in correspondence of a critical
temperature which divides the broken symmetry phase from the unbroken one. A
particularity with respect to the common pictures of a phase transition is that
the nonanalyticity of the magnetization is not accompanied by a nonanalytic
behavior of the free energy.Comment: 17 pages, 7 figure
Analysing the relationship between ectomycorrhizal infection and forest decline using marginal models
This statistical survey originates from the problem of discovering which relationship exists between root ectomycorrhizal infection and health status of forest plants. The sampling scheme takes observations from roots that come from sectors around the tree resulting in a hierarchical association structure of the observations. Marginal regression models are used to analyze the mean effect of the ectomycorrhizal state on a response variable proxy for the health degree of the plants
HAIL: An Algorithm for the Hardware Accelerated Identification of Languages, Master\u27s Thesis, May 2006
This thesis examines in detail the Hardware-Accelerated Identification of Languages (HAIL) project. The goal of HAIL is to provide an accurate means to identify the language and encoding used in streaming content, such as documents passed over a high-speed network. HAIL has been implemented on the Field-programmable Port eXtender (FPX), an open hardware platform developed at Washington University in St. Louis. HAIL can accurately identify the primary languages and encodings used in text at rates much higher than what can be achieved by software algorithms running on microprocessors
Electronic phase diagrams of carriers in self-assembled InAs/GaAs quantum dots: violation of Hund's rule and the Aufbau principle for holes
We study the orbital and spin configurations of up to six electrons or holes
charged into self-assembled InAs/GaAs quantum dots via single-particle
pseudopotential and many-particle configuration interaction method. We find
that while the charging of {\it electrons} follows both Hund's rule and the
Aufbau principle, the charging of {\it holes} follows a non-trivial charging
pattern which violates both the Aufbau principle and Hund's rule, and is robust
against the details of the quantum dot size. The predicted hole charging
sequence offers a new interpretation of recent charging experiments
Transport properties of annealed CdSe nanocrystal solids
Transport properties of artificial solids composed of colloidal CdSe
nanocrystals (NCs) are studied from 6 K to 250 K, before and after annealing.
Annealing results in greatly enhanced dark and photocurrent in NC solids, while
transmission electron microscopy (TEM) micrographs show that the inter-dot
separation decreases. The increased current can be attributed to the
enhancement of inter-dot tunneling caused by the decreased separation between
NCs and by chemical changes in their organic cap. In addition, the absorption
spectra of annealed solids are slightly red-shifted and broadened. These
red-shifts may result from the change of the dielectric environment around the
NCs. Our measurements also indicate that Coulomb interactions between charges
on neighboring NCs play an important role in the tunneling current.Comment: 24 pages,4 figures, 1 tabl
Imaging the charge transport in arrays of CdSe nanocrystals
A novel method to image charge is used to measure the diffusion coefficient
of electrons in films of CdSe nanocrystals at room temperature. This method
makes possible the study of charge transport in films exhibiting high
resistances or very small diffusion coefficients.Comment: 4 pages, 4 jpg figure
Neutron activation analysis traces copper artifacts to geographical point of origin
Impurities remaining in the metallic copper are identified and quantified by spectrographic and neutron activation analysis. Determination of the type of ore used for the copper artifact places the geographic point of origin of the artifact
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