Looking at Vortices with NMR

W. P. Halperin

Department of Physics and Astronomy,
Northwestern University, Evanston, Illinois 60208

Magnetic resonance has been extensively used to probe vortex structures in superconductors. At Northwestern University we have used ¹⁷O NMR as a probe of vortices in cuprates: YBCO and BSCCO. From the NMR spectrum we can identify the vortex liquid-to-solid transitions and determine the vortex melting phase diagram. At the National High Magnetic Field Laboratory we extended NMR to magnetic fields up to 42T to study the electronic excitations in the vortex core. The advantage of high fields is to increase the fraction of probe nuclei in the vortex core. New results from our NMR studies in BSCCO suggest that the vortex cores carry a net charge and might be responsible for a reconstruction of vortex structures owing to a repulsive interaction between vortex pancakes on adjacent planes.