Penetration depth and H-c2 anisotropy of MgB2 by B-11 NMR

Abstract

B-11 nuclear magnetic resonance (NMR) measurements have been performed to investigate vortex states of polycrystalline MgB2. The spectrum below T-c shows a composite structure with a narrow symmetric peak at an unshifted position and a broad asymmetric peak with a large diamagnetic shift. We identify that the broad peak originates from the vortex state whereas the narrow peak comes from crystallites in the normal state. This behavior can be understood as the NMR powder spectrum with significant anisotropy of an upper critical field H-c2. From temperature dependence of the broad fraction, we find the anisotropy of H-c2 is similar to4.7. We also determine the temperature-dependent penetration depth from the linewidth of the broad peak. (C) 2003 Elsevier B.V. All rights reserved.