Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Observation of vortex structure in MgB$_2$ single crystals by Bitter
  decoration technique   We report the observation of superconducting vortices in pure and lightly Al
doped MgB$_2$ single crystals. Low field experiments allow for the estimation
of the London penetration depth, lambda ~ 1900 $\AA$ for T$\sim$6 K.
Experiments in higher fields (e.g. 200 Oe) clearly show a triangular vortex
lattice in both real space (13 $\mu$m by 13 $\mu$m Bitter decoration image of
over 1000 vortices) and reciprocal space.

27
The categories of this paper are: comp-gas
30
The categories of this paper are: q-alg
34
The categories of this paper are: chem-ph
8
The categories of this paper are: hep-ex
18
The categories of this paper are: econ
6
The categories of this paper are: astro-ph
15
The categories of this paper are: nucl-ex
0
The categories of this paper are: hep-ph
33
The categories of this paper are: plasm-ph
1
The categories of this paper are: cs
20
The categories of this paper are: acc-phys
5
The categories of this paper are: gr-qc
35
The categories of this paper are: mtrl-th
32
The categories of this paper are: atom-ph
2
The categories of this paper are: math
14
The categories of this paper are: math-ph