Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Tightly bound excitons in two-dimensional semiconductors with a flat
  valence band   This theoretical paper offers an explicit expression for the binding energy
of excitons in a two-dimensional semiconductor with a flat valence band. The
formula has been derived quasiclassically assuming that the exciton is tightly
bound; i.e., its ground-state radius is determined by the intrinsic
polarizability of the semiconductor rather than by the dielectric properties of
the environment. The model is relevant to a few two-dimensional semiconductors
discovered recently, including distorted 1T-TiSe$_2$, with a supposedly
unstable electronic ground state. The valence band flatness reduces the exciton
binding energy, which also may have an effect on the phase transition to an
excitonic insulator.

27
The categories of this paper are: comp-gas
9
The categories of this paper are: nlin
26
The categories of this paper are: bayes-an
35
The categories of this paper are: mtrl-th
3
The categories of this paper are: physics
37
The categories of this paper are: supr-con
2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
32
The categories of this paper are: atom-ph
19
The categories of this paper are: eess
16
The categories of this paper are: stat
10
The categories of this paper are: q-bio
6
The categories of this paper are: astro-ph
24
The categories of this paper are: dg-ga
23
The categories of this paper are: solv-int
20
The categories of this paper are: acc-phys