Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Long-living excited states of a 2D diamagnetic exciton   Hydrogenic excited states of a 2D exciton are degenerate. In the presence of
a weak magnetic field, the $S$-states with a zero momentum of the center of
mass get coupled to the $P$-states with finite momentum of the center of mass.
This field-induced coupling leads to a strong modification of the dispersion
branches of the exciton spectrum. Namely, the lower branch acquires a shape of
a "mexican hat" with a minimum at a finite momentum. At certain magnetic field,
exciton branches exhibit a linear crossing, similarly to the spectrum of a 2D
electron in the presence of spin-orbit coupling. While spin is not involved,
degenerate $S$ and $P$ states play the role of the spin projections. Lifting of
degeneracy due to diamagnetic shifts and deviation of electron-hole attraction
from purely Coulomb suppresses the linear crossing.

22
The categories of this paper are: chao-dyn
33
The categories of this paper are: plasm-ph
23
The categories of this paper are: solv-int
12
The categories of this paper are: nucl-th
0
The categories of this paper are: hep-ph
8
The categories of this paper are: hep-ex
29
The categories of this paper are: funct-an
13
The categories of this paper are: quant-ph
26
The categories of this paper are: bayes-an
14
The categories of this paper are: math-ph
2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
28
The categories of this paper are: alg-geom
35
The categories of this paper are: mtrl-th
20
The categories of this paper are: acc-phys
37
The categories of this paper are: supr-con