Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Pattern Formation in Exciton System near Quantum Degeneracy   We discuss models of the modulational instability in a cold exciton system in
coupled quantum wells. One mechanism involves exciton formation in a
photoexcited electron-hole system in the presence of stimulated binding
processes which build up near exciton degeneracy. It is shown that such
processes may give rise to Turing instability leading to a spatially modulated
state. The structure and symmetry of resulting patterns depend on
dimensionality and symmetry. In the spatially uniform 2d electron-hole system,
the instability leads to a triangular lattice pattern while, at an
electron-hole interface, a periodic 1d pattern develops. Wavelength selection
mechanism is analyzed, revealing that the transition is abrupt (type I) for the
uniform 2d system, and continuous (type II) for the electron-hole interface.
Another mechanism that could possibly drive the instability involves long-range
attraction of the excitons. We illustrate how such an interaction can result
from current induced by exciton recombination ('plasmon wind'), derive
stability criterion, and discuss likelihood of such a scenario.

32
The categories of this paper are: atom-ph
16
The categories of this paper are: stat
15
The categories of this paper are: nucl-ex
35
The categories of this paper are: mtrl-th
8
The categories of this paper are: hep-ex
30
The categories of this paper are: q-alg
26
The categories of this paper are: bayes-an
0
The categories of this paper are: hep-ph
27
The categories of this paper are: comp-gas
13
The categories of this paper are: quant-ph
5
The categories of this paper are: gr-qc
19
The categories of this paper are: eess
33
The categories of this paper are: plasm-ph
22
The categories of this paper are: chao-dyn
9
The categories of this paper are: nlin
24
The categories of this paper are: dg-ga