1. Department of Physics, University of California, San Diego, 9500 Gilman Drive, La Jolla, Caifornia 92093-0319, Departamento de Física, Facultad de Bioquímica y Ciencias Biológicas, Universidad Nacional del Litoral, and INTEC (CONICET-UNL), Güemes 3450, 3000 Santa Fe, Argentina, and Center for Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310