29,670 research outputs found
Kondo resonance narrowing in d- and f-electron systems
By developing a simple scaling theory for the effect of Hund's interactions
on the Kondo effect, we show how an exponential narrowing of the Kondo
resonance develops in magnetic ions with large Hund's interaction. Our theory
predicts an exponential reduction of the Kondo temperature with spin S of the
Hund's coupled moment, a little-known effect first observed in d-electron
alloys in the 1960's, and more recently encountered in numerical calculations
on multi-band Hubbard models with Hund's interactions. We discuss the
consequences of Kondo resonance narrowing for the Mott transition in d-band
materials, particularly iron pnictides, and the narrow ESR linewidth recently
observed in ferromagnetically correlated f-electron materials.Comment: 4 pages, 3 figure
Two-fluid behavior of the Kondo lattice in the 1/N slave boson approach
It has been recently shown by Nakatsuji, Pines, and Fisk [S. Nakatsuji, D.
Pines, and Z. Fisk, Phys. Rev. Lett. 92, 016401 (2004)] from the
phenomenological analysis of experiments in Ce1-xLaxCoIn5 and CeIrIn5 that
thermodynamic and transport properties of Kondo lattices below coherence
temperature can be very successfully described in terms of a two-fluid model,
with Kondo impurity and heavy electron Fermi liquid contributions. We analyze
thermodynamic properties of Kondo lattices using 1/N slave boson treatment of
the periodic Anderson model and show that these two contributions indeed arise
below the coherence temperature. We find that the Kondo impurity contribution
to thermodynamics corresponds to thermal excitations into the flat portion of
the energy spectrum.Comment: 7 pages, 2 figure
Atomic Model of Susy Hubbard Operators
We apply the recently proposed susy Hubbard operators to an atomic model. In
the limiting case of free spins, we derive exact results for the entropy which
are compared with a mean field + gaussian corrections description. We show how
these results can be extended to the case of charge fluctuations and calculate
exact results for the partition function, free energy and heat capacity of an
atomic model for some simple examples. Wavefunctions of possible states are
listed. We compare the accuracy of large N expansions of the susy spin
operators with those obtained using `Schwinger bosons' and `Abrikosov
pseudo-fermions'. For the atomic model, we compare results of slave boson,
slave fermion, and susy Hubbard operator approximations in the physically
interesting but uncontrolled limiting case of N->2. For a mixed representation
of spins we estimate the accuracy of large N expansions of the atomic model. In
the single box limit, we find that the lowest energy saddle-point solution
reduces to simply either slave bosons or slave fermions, while for higher boxes
this is not the case. The highest energy saddle-point solution has the
interesting feature that it admits a small region of a mixed representation,
which bears a superficial resemblance to that seen experimentally close to an
antiferromagnetic quantum critical point.Comment: 17 pages + 7 pages Appendices, 14 figures. Substantial revision
An Analysis of the Impact of European Union and United States Dairy Policies on EU-U.S. Trade in Milk Protein Concentrate
During 1996-2000, U.S. imports of milk protein concentrate (MPC) increased rapidly. At the same time, Commodity Credit Corporation (CCC) stocks of non-fat dry milk (NFDM) went from nothing to more than 500 million pounds. Consequently, U.S. milk producers attributed low milk prices and dairy farmer income during this period to the increased imports of MPC. U.S. milk producers were especially concerned with MPC imports for two reasons. First, MPC between 40 and 90 percent protein had been classified in subheading 0404.90.10 of the Harmonized Tariff Schedule of the United States (HTS). Thus, MPC was not subject to the tariff-rate quotas applied to many other dairy products. Second, MPC produced in the European Union (EU) and exported to the United States was eligible for production and export subsidies. Along with the high U.S. internal milk protein prices maintained by the Dairy Price Support Program, and volatile world prices of NFDM, these policies created economic rents for trade in MPC between the European Union and the United States. To test the relationship between these policies and U.S. imports of MPC, these economic rents, which were not directly observable, were estimated by combing a set of identifiable variables: (1) the CCC purchase price, (2) the EU export refund, (3) EU casein production aid, and (4) the world price of NFDM as expressed by the Western Europe export price. A vector autoregression model was then estimated using monthly U.S. imports of MPC and the estimate of economic rents. This estimation showed that nearly 40 percent of the variability in U.S. MPC imports was attributable to the estimate of economic rents. These results demonstrate that U.S. and EU policies can not be analyzed in isolation when evaluating the impact of dairy policies on U.S. MPC imports.Agricultural and Food Policy, International Relations/Trade,
Heavy-fermion metals with hybridization nodes: Unconventional Fermi liquids and competing phases
Microscopic models for heavy-fermion materials often assume a local, i.e.,
momentum-independent, hybridization between the conduction band and the
local-moment f electrons. Motivated by recent experiments, we consider
situations where this neglect of momentum dependence is inappropriate, namely
when the hybridization function has nodes in momentum space. We explore the
thermodynamic and optical properties of the highly anisotropic heavy Fermi
liquid, resulting from Kondo screening in a higher angular-momentum channel.
The dichotomy in momentum space has interesting consequences: While e.g. the
low-temperature specific heat is dominated by heavy quasiparticles, the
electrical conductivity at intermediate temperatures is carried by unhybridized
light electrons. We then discuss aspects of the competition between Kondo
effect and ordering phenomena induced by inter-moment exchange: We propose that
the strong momentum-space anisotropy plays a vital role in selecting competing
phases. Explicit results are obtained for the interplay of unconventional
hybridization with unconventional, magnetically mediated, superconductivity,
utilizing variants of large-N mean-field theory. We make connections to recent
experiments on CeCoIn5 and other heavy-fermion materials.Comment: 16 pages, 8 figs, (v2) remark on Wiedemann-Franz added, small
changes, final version as publishe
Spontaneous interlayer coherence in bilayer Kondo systems
Bilayer Kondo systems present interesting models to illustrate the
competition between the Kondo effect and intermoment exchange. Such bilayers
can exhibit two sharply distinct Fermi liquid phases which are distinguished by
whether or not the local moments participate in the Fermi sea. We study these
phases and the evolution from one to the other upon changing Kondo coupling. We
argue that an ordered state with spontaneous interlayer phase coherence
generically intervenes between the two Fermi liquids. Such a condensate phase
breaks a U(1) symmetry and is bounded by a finite-temperature
Kosterlitz-Thouless transition. Based on general arguments and mean-field
calculations we investigate the phase diagram and associated quantum phase
transitions.Comment: 4 pages, 3 figs, (v2) misprints in eqs corrected, final version as
publishe
Jet Modification in a Brick of QGP Matter
We have implemented the LPM effect into a microscopic transport model with
partonic degrees of freedom by following the algorithm of Zapp & Wiedemann. The
Landau-Pomeranchuk-Migdal (LPM) effect is a quantum interference process that
modifies the emission of radiation in the presence of a dense medium. In QCD
this results in a quadratic length dependence for radiative energy loss. This
is an important effect for the modification of jets by their passage through
the QGP.
We verify the leading parton energy loss in the model against the leading
order Baier-Dokshitzer-Mueller-Peigne-Schiff-Zakharov (BDMPS-Z) result.
We apply our model to the recent observations of the modification of di-jets
at the LHC.Comment: Presented at Panic 1
Corrosion-protective coatings from electrically conducting polymers
In a joint effort between NASA Kennedy and LANL, electrically conductive polymer coatings were developed as corrosion protective coatings for metal surfaces. At NASA Kennedy, the launch environment consist of marine, severe solar, and intermittent high acid and/or elevated temperature conditions. Electrically conductive polymer coatings were developed which impart corrosion resistance to mild steel when exposed to saline and acidic environments. Such coatings also seem to promote corrosion resistance in areas of mild steel where scratches exist in the protective coating. Such coatings appear promising for many commercial applications
Local Moments in an Interacting Environment
We discuss how local moment physics is modified by the presence of
interactions in the conduction sea. Interactions in the conduction sea are
shown to open up new symmetry channels for the exchange of spin with the
localized moment. We illustrate this conclusion in the strong-coupling limit by
carrying out a Schrieffer Wolff transformation for a local moment in an
interacting electron sea, and show that these corrections become very severe in
the approach to a Mott transition. As an example, we show how the Zhang Rice
reduction of a two-band model is modified by these new effects.Comment: Latex file with two postscript figures. Revised version, with more
fully detailed calculation
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