NMR study of [formula omitted]: Antiferromagnetic vortex cores or Bragg glass?

Abstract

We studied the vortex state of an oriented polycrystalline high- T c superconductor Tl 2 Ba 2 CuO 6 + δ ( T c ∼ 85 K ) by Tl 205 NMR spin-echo technique. Although more than 10 years have passed since the sample was synthesized, no aging effects were found in the low-field magnetic susceptibility and the zero-field Cu 63 , 65 nuclear quadrupole resonance spectrum. From Tl 205 NMR spectra at 2 T, we observed vortex melting, vortex partial melting, imperfect vortex lattices, and the absence of static antiferromagnetic ordering at vortex cores.