1,908 research outputs found
Two particle correlations and orthogonality catastrophe in interacting Fermi systems
The wave function of two fermions, repulsively interacting in the presence of
a Fermi sea, is evaluated in detail. We consider large but finite systems in
order to obtain an unabiguous picture of the two-particle correlations. As
recently pointed out by Anderson, in two or lower dimensions the particles may
be correlated even when situated on the Fermi surface. The "partial exclusion
principle" for two particles with opposite spin on the same Fermi point is
discussed, and related to results from the T-matrix approximation. Particles on
different Fermi points are shown to be uncorrelated in dimensions d > 1. Using
the results for the two-particle correlations we find that the orthogonality
effect induced by adding an extra particle to a (tentative) two-dimensional
Fermi liquid is finite.Comment: 25 pages, LATEX, RWTH/ITP-C 10/9
Superconductivity in the attractive Hubbard model: functional renormalization group analysis
We present a functional renormalization group analysis of superconductivity
in the ground state of the attractive Hubbard model on a square lattice.
Spontaneous symmetry breaking is treated in a purely fermionic setting via
anomalous propagators and anomalous effective interactions. In addition to the
anomalous interactions arising already in the reduced BCS model, effective
interactions with three incoming legs and one outgoing leg (and vice versa)
occur. We accomplish their integration into the usual diagrammatic formalism by
introducing a Nambu matrix for the effective interactions. From a random-phase
approximation generalized through use of this matrix we conclude that the
impact of the 3+1 effective interactions is limited, especially considering the
effective interactions important for the determination of the order parameter.
The exact hierarchy of flow equations for one-particle irreducible vertex
functions is truncated on the two-particle level, with higher-order self-energy
corrections included in a scheme proposed by Katanin. Using a parametrization
of effective interactions by patches in momentum space, the flow equations can
be integrated numerically to the lowest scales without encountering
divergences. Momentum-shell as well as interaction-flow cutoff functions are
used, including a small external field or a large external field and a
counterterm, respectively. Both approaches produce momentum-resolved order
parameter values directly from the microscopic model. The size of the
superconducting gap is in reasonable agreement with expectations from other
studies.Comment: 22 pages, 16 figures, references added, some changes in the
introductio
Coding in the olfactory system : linking realistic and abstract models
© Metzner; licensee BioMed Central Ltd. 2014 This article is published under license to BioMed Central Ltd. This 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. The Creative Commons Public Domain Dedication waiver (http://​creativecommons.​org/​publicdomain/​zero/​1.​0/​) applies to the data made available in this article, unless otherwise stated[Poster presentation].In general, theoretical models for the function of the olfactory system can be divided into two broad categories. In one, the seemingly diffuse and unstructured pattern of neuronal connections within the olfactory cortex have been interpreted to suggest that it functions like a kind of an autoassociative neural network. An essential feature of autoassociative networks is their randomly structured connectivity and the special importance of the formation of connection weights in the process of its evolution. On the other hand, a very different functional structure has recently been proposed based on a detailed biologically realistic network model of the olfactory cortex. This work predicts that the olfactory cortex actually has a highly structured connectivity consisting of distinct different although highly spatially overlapping subnetworks, which are only sparsely connected to each otherPeer reviewe
UAG R-268
This study was supported by the Bureau of Land Management through
interagency agreement with the National Oceanic and Atmospheric Administration
under which a multi-year program responding to need of petroleum development
of the Alaskan continental shelf is managed by the Outer Continental Shelf
Environmental Assessment Program (OCSEAP) Office. Drawings were done by
Debbie Cocci a and Jim Burton. The idea of this project arose during a
conversation between George Edwardson, Jr., and one of the Principal Investigators.
Rachael Craig and Edna MacLean helped in initiating the project and Mrs. Molly
Pederson of the North Slope Borough Inupiaq Language Commission obtained
some of the narratives and interviews and provided translations. Teri
McClung assisted with transcription of interviews and editing. Finally,
the hospitality of the family of Kenneth Toovak during the translation
process is gratefully acknowledged.The objective of this program was to extend the data base on ice hazards
along the Beaufort Sea coast of Alaska backward in time by using the
knowledge and understanding of ice and weather conditions of the local
residents. Information for this pilot project was obtained through direct
interviews with residents, or from narratives supplied by them. The results
of these procedures were evaluated to provide a basis for improving similar
efforts in future. Observations of particular interest obtained from these
interviews and narratives include (1 ) a description of a major motion of the
landfast ice off Harrison Bay in late February, (2) a description of the
formation of ice push ridges and ride-up at Cape Halkett during break up,
(3) reports of whales traveling inshore of Cross Island during the fall
migration and of whales being taken by crews from the Prudhoe Bay area at
that time of year, and 4) descriptions of conditions in the nearshore area
during summer. In addition information of historical and cultural interest
was obtained.Supported by NOAA Contract 03-5-022-55, Task No. 6ABSTRACT -- ACKNOWLEDGEMENTS -- TABLE OF CONTENTS : INTRODUCTION ; PROCEDURES ; RESULTS ; DISCUSSION AND EVALUATION -- APPENDIX I -- APPENDIX II -- APPENDIX II
Spin lifetimes and strain-controlled spin precession of drifting electrons in zinc blende type semiconductors
We study the transport of spin polarized electrons in n-GaAs using spatially
resolved continuous wave Faraday rotation. From the measured steady state
distribution, we determine spin relaxation times under drift conditions and, in
the presence of strain, the induced spin splitting from the observed spin
precession. Controlled variation of strain along [110] allows us to deduce the
deformation potential causing this effect, while strain along [100] has no
effect. The electric field dependence of the spin lifetime is explained
quantitatively in terms of an increase of the electron temperature.Comment: 5 pages, 6 figure
Singular order parameter interaction at nematic quantum critical point in two dimensional electron systems
We analyze the infrared behavior of effective N-point interactions between
order parameter fluctuations for nematic and other quantum critical electron
systems with a scalar order parameter in two dimensions. The interactions
exhibit a singular momentum and energy dependence and thus cannot be
represented by local vertices. They diverge for all N greater or equal 4 in a
collinear static limit, where energy variables scale to zero faster than
momenta, and momenta become increasingly collinear. The degree of divergence is
not reduced by any cancellations and renders all N-point interactions marginal.
A truncation of the order parameter action at quartic or any other finite order
is therefore not justified. The same conclusion can be drawn for the effective
action describing fermions coupled to a U(1) gauge field in two dimensions.Comment: 18 pages, 1 figur
Competition of Fermi surface symmetry breaking and superconductivity
We analyze a mean-field model of electrons on a square lattice with two types
of interaction: forward scattering favoring a d-wave Pomeranchuk instability
and a BCS pairing interaction driving d-wave superconductivity. Tuning the
interaction parameters a rich variety of phase diagrams is obtained. If the BCS
interaction is not too strong, Fermi surface symmetry breaking is stabilized
around van Hove filling, and coexists with superconductivity at low
temperatures. For pure forward scattering Fermi surface symmetry breaking
occurs typically via a first order transition at low temperatures. The presence
of superconductivity reduces the first order character of this transition and,
if strong enough, can turn it into a continuous one. This gives rise to a
quantum critical point within the superconducting phase. The superconducting
gap tends to suppress Fermi surface symmetry breaking. For a relatively strong
BCS interaction, Fermi surface symmetry breaking can be limited to intermediate
temperatures, or can be suppressed completely by pairing.Comment: 14 pages, 10 figure
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