Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Collective excitations and quantum incompressibility in electron-hole
  bilayers   We apply quantum continuum mechanics to the calculation of the excitation
spectrum of a coupled electron-hole bilayer. The theory expresses excitation
energies in terms of ground-state intra- and inter-layer pair correlation
functions, which are available from Quantum Monte Carlo calculations. The final
formulas for the collective modes deduced from this approach coincide with the
formulas obtained in the "quasi-localized particle approximation" by Kalman et
al., and likewise, the theory predicts the existence of gapped excitations in
the charged channels, with the gap arising from electron-hole correlation. An
immediate consequence of the gap is that the static density-density response
function of the charged channel vanishes as $q^2$ for wave vector $q \to 0$,
rather than linearly in $q$, as commonly expected. In this sense, the system is
{\it incompressible}. This feature, which has no analogue in the classical
electron-hole plasma, is consistent with the existence of an excitonic ground
state and implies the existence of a discontinuity in the chemical potential of
electrons and holes when the numbers of electrons and holes are equal. It
should be experimentally observable by monitoring the densities of electrons
and holes in response to potentials that attempt to change these densities in
opposite directions.

33
The categories of this paper are: plasm-ph
6
The categories of this paper are: astro-ph
7
The categories of this paper are: hep-th
21
The categories of this paper are: adap-org
25
The categories of this paper are: patt-sol
15
The categories of this paper are: nucl-ex
18
The categories of this paper are: econ
22
The categories of this paper are: chao-dyn
16
The categories of this paper are: stat
29
The categories of this paper are: funct-an
26
The categories of this paper are: bayes-an
27
The categories of this paper are: comp-gas
1
The categories of this paper are: cs
30
The categories of this paper are: q-alg
12
The categories of this paper are: nucl-th
35
The categories of this paper are: mtrl-th