Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Gap formation in helical edge states with magnetic impurities   Helical edge states appear at the surface of two dimensional topological
insulators and are characterized by spin up traveling in one direction and the
spin down traveling in the opposite direction. Such states are protected by
time reversal symmetry and no backscattering due to scalar impurities can
occur. However, magnetic impurities break time reversal symmetry and lead to
backscattering. Often their presence is unintentional, but in some cases they
are introduced into the sample to open up gaps in the spectrum. We investigate
the influence of random impurities on helical edge states, specifically how the
gap behaves in the realistic case of impurities having both a magnetic and a
scalar component. It turns out that for a fixed magnetic contribution the gap
closes when either the scalar component, or Fermi velocity is increased. We
compare diagrammatic techniques in the self-consistent Born approximation to
numerical calculations which yields good agreement. For experimentally relevant
parameters we find that even moderate scalar components can be quite
detrimental for the gap formation.

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The categories of this paper are: supr-con
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The categories of this paper are: solv-int
35
The categories of this paper are: mtrl-th
12
The categories of this paper are: nucl-th
8
The categories of this paper are: hep-ex
31
The categories of this paper are: ao-sci
36
The categories of this paper are: cmp-lg
6
The categories of this paper are: astro-ph
10
The categories of this paper are: q-bio
16
The categories of this paper are: stat
11
The categories of this paper are: hep-lat
13
The categories of this paper are: quant-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: chao-dyn
5
The categories of this paper are: gr-qc
2
The categories of this paper are: math