Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Angular dependence of the bulk nucleation field Hc2 of aligned MgB2
  crystallites   Studies on the new MgB2 superconductor, with a critical temperature Tc ~ 39
K, have evidenced its potential for applications although intense magnetic
relaxation effects limit the critical current density, Jc, at high magnetic
fields. This means that effective pinning centers must be added into the
material microstructure, in order to halt dissipative flux movements.
Concerning the basic microscopic mechanism to explain the superconductivity in
MgB2, several experimental and theoretical works have pointed to the relevance
of a phonon-mediated interaction, in the framework of the BCS theory. Questions
have been raised about the relevant phonon modes, and the gap and Fermi surface
anisotropies, in an effort to interpret spectroscopic and thermal data that
give values between 2.4 and 4.5 for the gap energy ratio. Preliminary results
on the anisotropy of Hc2 have shown a ratio, between the in-plane and
perpendicular directions, around 1.7 for aligned MgB2 crystallites and 1.8 for
epitaxial thin films. Here we show a study on the angular dependence of Hc2
pointing to a Fermi velocity anisotropy around 2.5. This anisotropy certainly
implies the use of texturization techniques to optimize Jc in MgB2 wires and
other polycrystalline components.

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The categories of this paper are: dg-ga
11
The categories of this paper are: hep-lat
10
The categories of this paper are: q-bio
37
The categories of this paper are: supr-con
2
The categories of this paper are: math
36
The categories of this paper are: cmp-lg
9
The categories of this paper are: nlin
6
The categories of this paper are: astro-ph
20
The categories of this paper are: acc-phys
3
The categories of this paper are: physics
18
The categories of this paper are: econ
22
The categories of this paper are: chao-dyn
8
The categories of this paper are: hep-ex
1
The categories of this paper are: cs
7
The categories of this paper are: hep-th
35
The categories of this paper are: mtrl-th