Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Spatial Mapping of Local Density Variations in Two-dimensional Electron
  Systems Using Scanning Photoluminescence   We have developed a scanning photoluminescence technique that can directly
map out the local two-dimensional electron density with a relative accuracy of
$\sim2.2\times10^8$ cm$^{-2}$. The validity of this approach is confirmed by
the observation of the expected density gradient in a high-quality GaAs quantum
well sample that was not rotated during the molecular beam epitaxy of its
spacer layer. In addition to this global variation in electron density, we
observe local density fluctuations across the sample. These random density
fluctuations are also seen in samples that were continuously rotated during
growth, and we attribute them to residual space charges at the
substrate-epitaxy interface. This is corroborated by the fact that the average
magnitude of density fluctuations is increased to $\sim9\times10^{9}$ cm$^{-2}$
from $\sim1.2\times10^9$ cm$^{-2}$ when the buffer layer between the substrate
and the quantum well is decreased by a factor of seven. Our data provide direct
evidence for local density inhomogeneities even in very high-quality
two-dimensional carrier systems.

15
The categories of this paper are: nucl-ex
3
The categories of this paper are: physics
20
The categories of this paper are: acc-phys
23
The categories of this paper are: solv-int
13
The categories of this paper are: quant-ph
30
The categories of this paper are: q-alg
37
The categories of this paper are: supr-con
0
The categories of this paper are: hep-ph
24
The categories of this paper are: dg-ga
33
The categories of this paper are: plasm-ph
31
The categories of this paper are: ao-sci
2
The categories of this paper are: math
25
The categories of this paper are: patt-sol
34
The categories of this paper are: chem-ph
8
The categories of this paper are: hep-ex
12
The categories of this paper are: nucl-th