9,000 research outputs found
Magnetic properties of the Anderson model: a local moment approach
We develop a local moment approach to static properties of the symmetric
Anderson model in the presence of a magnetic field, focussing in particular on
the strong coupling Kondo regime. The approach is innately simple and
physically transparent; but is found to give good agreement, for essentially
all field strengths, with exact results for the Wilson ratio, impurity
magnetization, spin susceptibility and related properties.Comment: 7 pages, 3 postscript figues. Latex 2e using the epl.cls Europhysics
Letters macro packag
The Collapse of International Trade During the 2008-2009 Crisis: In Search of the Smoking Gun
One of the most striking aspects of the recent recession is the collapse in international trade. This paper uses disaggregated quarterly and monthly data on U.S. imports and exports to shed light on the anatomy of this collapse. We find that the recent reduction in trade relative to overall economic activity is far larger than in previous downturns. Information on quantities and prices of both domestic absorption and imports reveals a more than 50% shortfall in imports, relative to what would be predicted by a simple import demand relationship. In a sample of imports and exports disaggregated at the 6-digit NAICS level, we find that sectors used as intermediate inputs experienced significantly higher percentage reductions in both imports and exports. We also find support for compositional effects: sectors with larger reductions in domestic output had larger drops in trade. By contrast, we find no support for the hypothesis that trade credit played a role in the recent trade collapse.2008-2009 Crisis, International Trade
Finite temperature dynamics of the Anderson model
The recently introduced local moment approach (LMA) is extended to encompass
single-particle dynamics and transport properties of the Anderson impurity
model at finite-temperature, T. While applicable to arbitrary interaction
strengths, primary emphasis is given to the strongly correlated Kondo regime
(characterized by the T=0 Kondo scale ). In particular the
resultant universal scaling behaviour of the single-particle spectrum
D(\omega; T) \equiv F(\frac{\w}{\omega_{\rm K}}; \frac{T}{\omega_{\rm K}})
within the LMA is obtained in closed form; leading to an analytical description
of the thermal destruction of the Kondo resonance on all energy scales.
Transport properties follow directly from a knowledge of . The -dependence of the resulting resistivity , which is
found to agree rather well with numerical renormalization group calculations,
is shown to be asymptotically exact at high temperatures; to concur well with
the Hamann approximation for the s-d model down to ,
and to cross over smoothly to the Fermi liquid form in the low-temperature limit. The underlying
approach, while naturally approximate, is moreover applicable to a broad range
of quantum impurity and related models
Single-particle dynamics of the Anderson model: a two-self-energy description within the numerical renormalization group approach
Single-particle dynamics of the Anderson impurity model are studied using
both the numerical renormalization group (NRG) method and the local moment
approach (LMA). It is shown that a 'two-self-energy' description of dynamics
inherent to the LMA, as well as a conventional 'single-self-energy'
description, arise within NRG; each yielding correctly the same local
single-particle spectrum. Explicit NRG results are obtained for the broken
symmetry spectral constituents arising in a two-self-energy description, and
the total spectrum. These are also compared to analytical results obtained from
the LMA as implemented in practice. Very good agreement between the two is
found, essentially on all relevant energy scales from the high-energy Hubbard
satellites to the low-energy Kondo resonance.Comment: 12 pages, 6 figure
A spin-dependent local moment approach to the Anderson impurity model
We present an extension of the local moment approach to the Anderson impurity
model with spin-dependent hybridization. By employing the two-self-energy
description, as originally proposed by Logan and co-workers, we applied the
symmetry restoration condition for the case with spin-dependent hybridization.
Self-consistent ground states were determined through variational minimization
of the ground state energy. The results obtained with our spin-dependent local
moment approach applied to a quantum dot system coupled to ferromagnetic leads
are in good agreement with those obtained from previous work using numerical
renormalization group calculations
The Role of Financial Factors in the Trade Collapse: A Skeptic's View
This paper explores the role of financial factors in the 2008-9 collapse of U.S. imports
and exports. Using highly disaggregated international trade data, we examine whether the
cross-sectoral variation in how much imports or exports fell during this episode can be
explained by financial variables. To do this, we employ a wide variety of possible
indicators, such as standard measures of trade credit and external finance dependence,
proxies for shipping lags at the sector level, and shares of intra-firm trade in each sector.
Overall, there is very little evidence that financial factors played a role in the collapse of
U.S. trade.2008-2009 Crisis, International Trade, Financial Factors
Classification and reduction of pilot error
Human error is a primary or contributing factor in about two-thirds of commercial aviation accidents worldwide. With the ultimate goal of reducing pilot error accidents, this contract effort is aimed at understanding the factors underlying error events and reducing the probability of certain types of errors by modifying underlying factors such as flight deck design and procedures. A review of the literature relevant to error classification was conducted. Classification includes categorizing types of errors, the information processing mechanisms and factors underlying them, and identifying factor-mechanism-error relationships. The classification scheme developed by Jens Rasmussen was adopted because it provided a comprehensive yet basic error classification shell or structure that could easily accommodate addition of details on domain-specific factors. For these purposes, factors specific to the aviation environment were incorporated. Hypotheses concerning the relationship of a small number of underlying factors, information processing mechanisms, and error types types identified in the classification scheme were formulated. ASRS data were reviewed and a simulation experiment was performed to evaluate and quantify the hypotheses
Spectral scaling and quantum critical behaviour in the pseudogap Anderson model
The pseudogap Anderson impurity model provides a classic example of an
essentially local quantum phase transition. Here we study its single-particle
dynamics in the vicinity of the symmetric quantum critical point (QCP)
separating generalized Fermi liquid and local moment phases, via the local
moment approach. Both phases are shown to be characterized by a low-energy
scale that vanishes at the QCP; and the universal scaling spectra, on all
energy scales, are obtained analytically. The spectrum precisely at the QCP is
also obtained; its form showing clearly the non-Fermi liquid, interacting
nature of the fixed point.Comment: 7 pages, 2 figure
BCS - BEC crossover at T=0: A Dynamical Mean Field Theory Approach
We study the T=0 crossover from the BCS superconductivity to Bose-Einstein
condensation in the attractive Hubbard Model within dynamical mean field
theory(DMFT) in order to examine the validity of Hartree-Fock-Bogoliubov (HFB)
mean field theory, usually used to describe this crossover, and to explore
physics beyond it. Quantum fluctuations are incorporated using iterated
perturbation theory as the DMFT impurity solver. We find that these
fluctuations lead to large quantitative effects in the intermediate coupling
regime leading to a reduction of both the superconducting order parameter and
the energy gap relative to the HFB results. A qualitative change is found in
the single-electron spectral function, which now shows incoherent spectral
weight for energies larger than three times the gap, in addition to the usual
Bogoliubov quasiparticle peaks.Comment: 11 pages,12 figures, Published versio
Anderson impurity in a semiconductor
We consider an Anderson impurity model in which the locally correlated
orbital is coupled to a host with a gapped density of states. Single-particle
dynamics are studied, within a perturbative framework that includes both
explicit second-order perturbation theory and self-consistent perturbation
theory to all orders in the interaction. Away from particle-hole symmetry the
system is shown to be a generalized Fermi liquid (GFL) in the sense of being
perturbatively connectable to the non-interacting limit; and the exact Friedel
sum rule for the GFL phase is obtained. We show by contrast that the
particle-hole symmetric point of the model is not perturbatively connected to
the non-interacting limit, and as such is a non-Fermi liquid for all non-zero
gaps. Our conclusions are in agreement with NRG studies of the problem.Comment: 7 pages, 4 figure
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