Penetration depth and Hc2 anisotropy of MgB 2 by [formula omitted]B NMR

Abstract

11 B nuclear magnetic resonance (NMR) measurements have been performed to investigate vortex states of polycrystalline MgB 2. 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 ∼4.7. We also determine the temperature-dependent penetration depth from the linewidth of the broad peak.