Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Diagrammatic expansion for positive density-response spectra:
  Application to the electron gas   In a recent paper [Phys. Rev. B 90, 115134 (2014)] we put forward a
diagrammatic expansion for the self-energy which guarantees the positivity of
the spectral function. In this work we extend the theory to the density
response function. We write the generic diagram for the density-response
spectrum as the sum of partitions. In a partition the original diagram is
evaluated using time-ordered Green's functions (GF) on the left-half of the
diagram, antitime-ordered GF on the right-half of the diagram and lesser or
greater GF gluing the two halves. As there exist more than one way to cut a
diagram in two halves, to every diagram corresponds more than one partition. We
recognize that the most convenient diagrammatic objects for constructing a
theory of positive spectra are the half-diagrams. Diagrammatic approximations
obtained by summing the squares of half-diagrams do indeed correspond to a
combination of partitions which, by construction, yield a positive spectrum. We
develop the theory using bare GF and subsequently extend it to dressed GF. We
further prove a connection between the positivity of the spectral function and
the analytic properties of the polarizability. The general theory is
illustrated with several examples and then applied to solve the long-standing
problem of including vertex corrections without altering the positivity of the
spectrum. In fact already the first-order vertex diagram, relevant to the study
of gradient expansion, Friedel oscillations, etc., leads to spectra which are
negative in certain frequency domain. We find that the simplest approximation
to cure this deficiency is given by the sum of the zero-th order bubble
diagram, the first-order vertex diagram and a partition of the second-order
ladder diagram. We evaluate this approximation in the 3D homogeneous electron
gas and show the positivity of the spectrum for all frequencies and densities.

26
The categories of this paper are: bayes-an
37
The categories of this paper are: supr-con
15
The categories of this paper are: nucl-ex
8
The categories of this paper are: hep-ex
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The categories of this paper are: alg-geom
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The categories of this paper are: solv-int
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The categories of this paper are: adap-org
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The categories of this paper are: quant-ph
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The categories of this paper are: hep-lat
5
The categories of this paper are: gr-qc
29
The categories of this paper are: funct-an
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The categories of this paper are: cs
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The categories of this paper are: dg-ga
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The categories of this paper are: cmp-lg
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The categories of this paper are: astro-ph
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The categories of this paper are: patt-sol