Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Rigid vortices in MgB2   Magnetic relaxation of high-pressure synthesized MgB$_2$ bulks with different
thickness is investigated. It is found that the superconducting dia-magnetic
moment depends on time in a logarithmic way; the flux-creep activation energy
decreases linearly with the current density (as expected by Kim-Anderson
model); and the activation energy increases linearly with the thickness of
sample when it is thinner than about 1 mm. These features suggest that the
vortices in the MgB$_2$ are rather rigid, and the pinning and creep can be well
described by Kim-Anderson model.

8
The categories of this paper are: hep-ex
35
The categories of this paper are: mtrl-th
37
The categories of this paper are: supr-con
5
The categories of this paper are: gr-qc
36
The categories of this paper are: cmp-lg
14
The categories of this paper are: math-ph
11
The categories of this paper are: hep-lat
3
The categories of this paper are: physics
9
The categories of this paper are: nlin
33
The categories of this paper are: plasm-ph
2
The categories of this paper are: math
22
The categories of this paper are: chao-dyn
10
The categories of this paper are: q-bio
6
The categories of this paper are: astro-ph
32
The categories of this paper are: atom-ph
17
The categories of this paper are: q-fin