Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Effects of a Parallel Magnetic Field on the Metal-Insulator Transition
  in a Dilute Two-Dimensional Electron System   The temperature dependence of conductivity $\sigma (T)$ of a two-dimensional
electron system in silicon has been studied in parallel magnetic fields B. At
B=0, the system displays a metal-insulator transition at a critical electron
density $n_c(0)$, and $d\sigma/dT >0$ in the metallic phase. At low fields
($B\lesssim 2$ T), $n_c$ increases as $n_c(B) - n_c(0) \propto B^{\beta}$
($\beta\sim 1$), and the zero-temperature conductivity scales as $\sigma
(n_s,B,T=0)/\sigma (n_s,0,0)=f(B^{\beta}/\delta_n)$ (where
$\delta_n=(n_s-n_c(0))/n_c(0)$, and $n_s$ is electron density) as expected for
a quantum phase transition. The metallic phase persists in fields of up to 18
T, consistent with the saturation of $n_c$ at high fields.

19
The categories of this paper are: eess
20
The categories of this paper are: acc-phys
2
The categories of this paper are: math
11
The categories of this paper are: hep-lat
5
The categories of this paper are: gr-qc
1
The categories of this paper are: cs
10
The categories of this paper are: q-bio
35
The categories of this paper are: mtrl-th
15
The categories of this paper are: nucl-ex
37
The categories of this paper are: supr-con
29
The categories of this paper are: funct-an
3
The categories of this paper are: physics
21
The categories of this paper are: adap-org
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The categories of this paper are: q-fin
13
The categories of this paper are: quant-ph
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The categories of this paper are: dg-ga