Penetration depth anisotropy in MgB2 single crystals and powders

We present small-angle neutron scattering (SANS) measurements of the vortex lattice in single crystal and powder samples of MgB2-a two band superconductor. SANS is a powerful method allowing us to measure directly the distribution of vortex separations and hence the anisotropy of the penetration depth, gamma(lambda). Single crystal results show the expected rise of gamma(lambda), with applied field as the weaker, almost isotropic, pi-band is suppressed. Results from the powder samples do not show as significant a rise in gamma(lambda) with field. We believe this originates from 'averaged' properties of the penetration depth experienced by vortices in each multi-crystallite powder grain. (c) 2005 Published by Elsevier Ltd.