6,670 research outputs found
Control of the finite size corrections in exact diagonalization studies
We study the possibility of controlling the finite size corrections in exact
diagonalization studies quantitatively. We consider the one- and two
dimensional Hubbard model. We show that the finite-size corrections can be be
reduced systematically by a grand-canonical integration over boundary
conditions. We find, in general, an improvement of one order of magnitude with
respect to studies with periodic boundary conditions only. We present results
for ground-state properties of the 2D Hubbard model and an evaluation of the
specific heat for the 1D and 2D Hubbard model.Comment: Phys. Rev. B (Brief Report), in pres
A Monte Carlo Method for Fermion Systems Coupled with Classical Degrees of Freedom
A new Monte Carlo method is proposed for fermion systems interacting with
classical degrees of freedom. To obtain a weight for each Monte Carlo sample
with a fixed configuration of classical variables, the moment expansion of the
density of states by Chebyshev polynomials is applied instead of the direct
diagonalization of the fermion Hamiltonian. This reduces a cpu time to scale as
compared to for the
diagonalization in the conventional technique; is the dimension
of the Hamiltonian. Another advantage of this method is that parallel
computation with high efficiency is possible. These significantly save total
cpu times of Monte Carlo calculations because the calculation of a Monte Carlo
weight is the bottleneck part. The method is applied to the double-exchange
model as an example. The benchmark results show that it is possible to make a
systematic investigation using a system-size scaling even in three dimensions
within a realistic cpu timescale.Comment: 6 pages including 4 figure
Spin-excitation spectra and resistance minima in amorphous ferromagnetic alloys
Resistance minima have been found in recent years to occur in amorphous ferromagnetic alloys below the magnetic ordering temperature. Although a well-developed theory exists for resistance minima in very dilute alloys, the meaning of the phenomena has remained in question for alloys in which the neglect of spin-spin interactions is not justifiable. In this paper it is shown that the observation of resistance minima implies that these alloys have a finite density of near zero frequency excitations. Specifically, the theory of inverse transport coefficients, reformulated in terms of linear response, is used to derive a general expression for the resistivity due to the conduction-electron-spin interaction. Expanding perturbatively, the nth-order contribution is determined by an nth-order spin correlation function. To third order it is shown that the coefficient of the lnk T term responsible for the resistance anomaly according to the accepted Kondo theory receives contributions in the low-temperature limit only from those parts of the spin correlation functions which have frequencies less than k TK /ℏ where TK is the Kondo temperature
More than open space! the case for green infrastructure teaching in planning curricula
Since the mid-1990s, the concept of Green Infrastructure (GI) has been gaining traction in fields such as ecology and forestry, (landscape) architecture, environmental and hydrological engineering, public health as well as urban and regional planning. Definitions and aims ascribed to GI vary. Yet, agreement broadly exists on GI’s ability to contribute to sustainability by means of supporting, for example, biodiversity, human and animal health, and storm water management as well as mitigating urban heat island effects. Given an acknowledged role of planners in delivering sustainable cities and towns, professional bodies have highlighted the need for spatial planners to understand and implement GI. This raises questions of what sort of GI knowledge planners may require and moreover by whom and how GI knowledge and competencies may be conveyed? Examining knowledge and skills needs vis-à-vis GI education opportunities indicates a provision reliant primarily on continued professional education and limited ad hoc opportunities in Higher Education. The resulting knowledge base appears fragmented with limited theoretical foundations leading the authors to argue that a systematic inclusion of green infrastructure knowledges in initial planning education is needed to promote and aid effective GI implementation
Calculation of Densities of States and Spectral Functions by Chebyshev Recursion and Maximum Entropy
We present an efficient algorithm for calculating spectral properties of
large sparse Hamiltonian matrices such as densities of states and spectral
functions. The combination of Chebyshev recursion and maximum entropy achieves
high energy resolution without significant roundoff error, machine precision or
numerical instability limitations. If controlled statistical or systematic
errors are acceptable, cpu and memory requirements scale linearly in the number
of states. The inference of spectral properties from moments is much better
conditioned for Chebyshev moments than for power moments. We adapt concepts
from the kernel polynomial approximation, a linear Chebyshev approximation with
optimized Gibbs damping, to control the accuracy of Fourier integrals of
positive non-analytic functions. We compare the performance of kernel
polynomial and maximum entropy algorithms for an electronic structure example.Comment: 8 pages RevTex, 3 postscript figure
Disaggregating Lone-actor Grievance-fuelled Violence: Comparing Lone-actor Terrorists and Mass Murderers
Research suggests that lone-actor terrorists and mass murderers may be better conceptualized as lone-actor grievance-fueled violence (LAGFV) offenders, rather than as distinct types. The present study sought to examine the extent to which these offenders could (or could not) be disaggregated along dimensions relevant to the threat assessment of both. Drawing on a Risk Analysis Framework (RAF), the offending process was theorized as interactions among propensity, situation, preparatory, leakage and network indicators. We analyzed a dataset of 183 U.S. offenders, including sixty-eight lone-actor terrorists and 115 solo mass murderers. Cluster analysis identified profiles within each of the components: propensity (stable, criminal, unstable), situation (low stress, high stress (social), high stress (interpersonal), preparatory (fixated, novel aggression, equipped, clandestine, predatory, preparatory), leakage (high leakage, low leakage), and network (lone, associated, connected). Bi-variate analysis examined the extent to which the profiles classified offenders previously labeled as lone-actor terrorists or mass murderers. The results suggest that while significant differences may exist at the periphery of these dimensions, offenders previously classified as lone-actor terrorists or mass murderers occupy a noteworthy shared space. Moreover, no profile classifies a single “type” of offender exclusively. Lastly, we propose a dynamic, interactional model of LAGFV and discuss the implications of these findings for the threat assessment and management of LAGFV offenders
Analysis of colorectal cancers in British Bangladeshi identifies early onset, frequent mucinous histotype and a high prevalence of RBFOX1 deletion
PMCID: PMC3544714This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited
Chebyshev approach to quantum systems coupled to a bath
We propose a new concept for the dynamics of a quantum bath, the Chebyshev
space, and a new method based on this concept, the Chebyshev space method. The
Chebyshev space is an abstract vector space that exactly represents the
fermionic or bosonic bath degrees of freedom, without a discretization of the
bath density of states. Relying on Chebyshev expansions the Chebyshev space
representation of a bath has very favorable properties with respect to
extremely precise and efficient calculations of groundstate properties, static
and dynamical correlations, and time-evolution for a great variety of quantum
systems. The aim of the present work is to introduce the Chebyshev space in
detail and to demonstrate the capabilities of the Chebyshev space method.
Although the central idea is derived in full generality the focus is on model
systems coupled to fermionic baths. In particular we address quantum impurity
problems, such as an impurity in a host or a bosonic impurity with a static
barrier, and the motion of a wave packet on a chain coupled to leads. For the
bosonic impurity, the phase transition from a delocalized electron to a
localized polaron in arbitrary dimension is detected. For the wave packet on a
chain, we show how the Chebyshev space method implements different boundary
conditions, including transparent boundary conditions replacing infinite leads.
Furthermore the self-consistent solution of the Holstein model in infinite
dimension is calculated. With the examples we demonstrate how highly accurate
results for system energies, correlation and spectral functions, and
time-dependence of observables are obtained with modest computational effort.Comment: 18 pages, 13 figures, to appear in Phys. Rev.
Fast algorithm for calculating two-photon absorption spectra
We report a numerical calculation of the two-photon absorption coefficient of
electrons in a binding potential using the real-time real-space higher-order
difference method. By introducing random vector averaging for the intermediate
state, the task of evaluating the two-dimensional time integral is reduced to
calculating two one-dimensional integrals. This allows the reduction of the
computation load down to the same order as that for the linear response
function. The relative advantage of the method compared to the straightforward
multi-dimensional time integration is greater for the calculation of non-linear
response functions of higher order at higher energy resolution.Comment: 4 pages, 2 figures. It will be published in Phys. Rev. E on 1, March,
199
Description of recent large- neutron inclusive scattering data from liquid He
We report dynamical calculations for large- structure functions of liquid
He at =1.6 and 2.3 K and compare those with recent MARI data. We extend
those calculations far beyond the experimental range q\le 29\Ain in order to
study the approach of the response to its asymptotic limit for a system with
interactions having a strong short-range repulsion. We find only small
deviations from theoretical behavior, valid for smooth . We repeat an
extraction by Glyde et al of cumulant coefficients from data. We argue that
fits determine the single atom momentum distribution, but express doubt as to
the extraction of meaningful Final State Interaction parameters.Comment: 37 pages, 13 postscript fig
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