Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Evidence for nitrogen gas surface doping of the Bi$_2$Se$_3$ topological
  insulator   Using scanning tunneling spectroscopy we have studied the effects of nitrogen
gas exposure on the bismuth selenide density of states. We observe a shift in
the Dirac point which is qualitatively consistent with theoretical modeling of
nitrogen binding to selenium vacancies. In carefully controlled measurements,
Bi$_2$Se$_3$ crystals were initially cleaved in a helium gas environment and
then exposed to a 22 SCFH flow of ultra-high purity N$_2$ gas. We observe a
resulting change in the spectral curves, with the exposure effect saturating
after approximately 50 minutes, ultimately bringing the Dirac point about 50
meV closer to the Fermi level. These results are compared to density functional
theoretical calculations, which support a picture of $N_2$ molecules
physisorbing near Se vacancies and dissociating into individual N atoms which
then bind strongly to Se vacancies. In this interpretation, the binding of the
N atom to a Se vacancy site removes the surface defect state created by the
vacancy and changes the position of the Fermi energy with respect to the Dirac
point.

29
The categories of this paper are: funct-an
16
The categories of this paper are: stat
24
The categories of this paper are: dg-ga
20
The categories of this paper are: acc-phys
25
The categories of this paper are: patt-sol
19
The categories of this paper are: eess
18
The categories of this paper are: econ
31
The categories of this paper are: ao-sci
23
The categories of this paper are: solv-int
7
The categories of this paper are: hep-th
0
The categories of this paper are: hep-ph
6
The categories of this paper are: astro-ph
8
The categories of this paper are: hep-ex
17
The categories of this paper are: q-fin
35
The categories of this paper are: mtrl-th
27
The categories of this paper are: comp-gas