Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Magnetic impurities in gapless Fermi systems: perturbation theory   We consider a symmetric Anderson impurity model, with a soft-gap
hybridization vanishing at the Fermi level with a power law r > 0. Three facets
of the problem are examined. First the non-interacting limit, which despite its
simplicity contains much physics relevant to the U > 0 case: it exhibits both
strong coupling (SC) states (for r < 1) and local moment (LM) states (for r >
1), with characteristic signatures in both spectral properties and
thermodynamic functions. Second, we establish general conditions upon the
interaction self-energy for the occurence of a SC state for U > 0. This leads
to a pinning theorem, whereby the modified spectral function is pinned at the
Fermi level for any U where a SC state exists; it generalizes to arbitrary r
the familiar pinning condition for the normal r = 0 Anderson model. Finally, we
consider explicitly spectral functions at the simplest level: second order
perturbation theory in U, which we conclude is applicable for r < 1/2 and r > 1
but not for 1/2 < r < 1. Characteristic spectral features observed in numerical
renormalization group calculations are thereby recovered, for both SC and LM
phases; and for the SC state the modified spectral functions are found to
contain a generalized Abrikosov-Suhl resonance exhibiting a characteristic
low-energy Kondo scale with increasing interaction strength.

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The categories of this paper are: q-alg
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The categories of this paper are: plasm-ph
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The categories of this paper are: adap-org
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The categories of this paper are: econ
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The categories of this paper are: nucl-ex
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The categories of this paper are: physics
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The categories of this paper are: atom-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: chem-ph
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The categories of this paper are: stat
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The categories of this paper are: hep-ex
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The categories of this paper are: hep-th
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The categories of this paper are: patt-sol
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The categories of this paper are: math
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The categories of this paper are: hep-ph
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The categories of this paper are: mtrl-th