Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Stable nontrivial Z2 topology in ultrathin Bi (111) films: a
  first-principles study   Recently, there have been intense efforts in searching for new topological
insulator (TI) materials. Based on first-principles calculations, we find that
all the ultrathin Bi (111) films are characterized by a nontrivial Z2 number
independent of the film thickness, without the odd-even oscillation of
topological triviality as commonly perceived. The stable nontrivial Z2 topology
is retained by the concurrent band gap inversions at multiple
time-reversal-invariant k-points and associated with the intermediate
inter-bilayer coupling of the multi-bilayer Bi film. Our calculations further
indicate that the presence of metallic surface states in thick Bi(111) films
can be effectively removed by surface adsorption.

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The categories of this paper are: hep-th
1
The categories of this paper are: cs
28
The categories of this paper are: alg-geom
11
The categories of this paper are: hep-lat
18
The categories of this paper are: econ
5
The categories of this paper are: gr-qc
9
The categories of this paper are: nlin
32
The categories of this paper are: atom-ph
22
The categories of this paper are: chao-dyn
34
The categories of this paper are: chem-ph
26
The categories of this paper are: bayes-an
13
The categories of this paper are: quant-ph
16
The categories of this paper are: stat
24
The categories of this paper are: dg-ga
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The categories of this paper are: acc-phys
14
The categories of this paper are: math-ph