Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Electron energy spectrum and the Berry phase in graphite bilayer   We emphasize that there exist four Dirac-type points in the electron-energy
spectrum of a graphite bilayer near the point K of its Brillouin zone. One of
the Dirac points is at the point K, and three Dirac points lie nearby. Each of
these three points generates the Berry phase $\pi$, while the Dirac point at K
gives the phase $-\pi$. It is these four points that determine the Berry phase
in the bilayer. If an electron orbit surrounds all these points, the Berry
phase is equal to $2\pi$.

37
The categories of this paper are: supr-con
5
The categories of this paper are: gr-qc
16
The categories of this paper are: stat
34
The categories of this paper are: chem-ph
10
The categories of this paper are: q-bio
24
The categories of this paper are: dg-ga
36
The categories of this paper are: cmp-lg
29
The categories of this paper are: funct-an
22
The categories of this paper are: chao-dyn
26
The categories of this paper are: bayes-an
23
The categories of this paper are: solv-int
25
The categories of this paper are: patt-sol
18
The categories of this paper are: econ
17
The categories of this paper are: q-fin
14
The categories of this paper are: math-ph
33
The categories of this paper are: plasm-ph