Penetration Depth Anisotropy in MgB2 Powder Measured by
Small Angle Neutron Scattering
R. cubitt,1, C. D Dewhurst1, M. R. Eskildsen2, S. J. Levett1, J. Jun4, S. M. Kazakov4, J.
Karpinski4
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1
2
Institut Laue Langevin, 6 rue Jules Horowitz, 38042 Grenoble, France
Department of Physics, University of Notre Dame, Indiana 46556, USA
3
Solid State Physics Laboratory, ETH, CH-8093 Zurich, Switzerland
4
Ames Laboratory, Ames, Iowa 50011, USA
Small-angle neutron scattering (SANS) measurements of the vortex lattice in powder samples
of MgB2 – a two band superconductor will be presented. SANS is a powerful method
allowing us to measure directly the distribution of vortex separations and hence the
anisotropy of the penetration depth, . This is the first time that scattering from the flux
lattice in a powder sample has been measured and the success is principally due to the
relatively short value of  allowing the signal to exceed the strong background scattering
from the powder grains. If there was no anisotropy then a ring of intensity of width
determined by the instrument resolution is expected. Excess broadening is assumed to be due
to a penetration depth anisotropy inducing a spread in vortex plane separations.
A model
has been developed to simulate this effect leading to a precise measurement of assuming
there are no other sources of lattice spacing spread. Single crystal results [1] show the
expected rise of  with applied field as the weaker, almost isotropic, -band is suppressed.
Measurements of  from the powder samples [2] agree at low fields with the single crystal
data but do not show a significant rise in  with field (fig. 1). It is possible this originates
from ‘averaged’ properties of the penetration depth anisotropy experienced by vortices in
multi-crystalline grains.
Figure 1
The ring of scattering from the vortex lattice at 2K and
0.7T. The ring shows broadening consistent with a of 1.6(1) to be
compared with 2.7(3) from the single crystal data.
[1] R. Cubitt et. al., Phys. Rev. Lett., 90 157002 (2003)
[2] R. Cubitt et. al., Phys. Rev. Lett., 91 47002 (2003)