Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Conductivity of two-dimensional narrow gap semiconductors subjected to
  strong Coulomb disorder   In the ideal disorder-free situation, a two-dimensional band gap insulator
has an activation energy for conductivity equal to half the band gap $\Delta$.
But transport experiments usually exhibit a much smaller activation energy at
low temperature, and the relation between this activation energy and $\Delta$
is unclear. Here we consider the temperature-dependent conductivity of a
two-dimensional insulator on a substrate containing Coulomb impurities, with
random potential amplitude $\Gamma \gg \Delta$. We show that the conductivity
generically exhibits three regimes of conductivity, and only the highest
temperature regime exhibits an activation energy that reflects the band gap. At
lower temperatures, the conduction proceeds through activated hopping or
Efros-Shklovskii variable-range hopping between electron and hole puddles
created by the disorder. We show that the activation energy and characteristic
temperature associated with these processes steeply collapse near a critical
impurity concentration. Larger concentrations lead to an exponentially small
activation energy and exponentially long localization length, which in
mesoscopic samples can appear as a disorder-induced insulator-to-metal
transition. We also arrive at a similar steep disorder driven insulator-metal
transition in thin films of three-dimensional topological insulators with large
dielectric constant, for which Coulomb impurities inside the film create a
large disorder potential due to confinement of their electric field inside the
film.

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The categories of this paper are: chem-ph
10
The categories of this paper are: q-bio
33
The categories of this paper are: plasm-ph
2
The categories of this paper are: math
0
The categories of this paper are: hep-ph
23
The categories of this paper are: solv-int
14
The categories of this paper are: math-ph
5
The categories of this paper are: gr-qc
16
The categories of this paper are: stat
12
The categories of this paper are: nucl-th
1
The categories of this paper are: cs
32
The categories of this paper are: atom-ph
21
The categories of this paper are: adap-org
27
The categories of this paper are: comp-gas
31
The categories of this paper are: ao-sci
19
The categories of this paper are: eess