The electronic structure of the vortex cores in copper oxide high temperature superconductors (HTS) is a long-standing puzzle. Using scanning tunneling spectroscopy at an unprecedented low magnetic field for any HTS, we succeeded in measuring the intrinsic vortex core structure in heavily overdoped Bi2Sr2CaCu2O8+d. We find the characteristic electronic signature predicted for a vortex core in a d-wave superconductor by Wang and MacDonald in 1995. Interestingly, this electronic signature is only seen in isolated vortices at low fields; at fields above 1 Tesla, we measure the same unusual core features observed until now in optimally and underdoped crystals, including the checkerboard and subgap states. While our experimental data show that isolated vortex cores are fully compatible with a standard d-wave vortex core, the more unusal core signature prevailing at higher magnetic fields is yet to be understood and demands further studies.
Collaboration with Genda Gu, Brookahven National lboratory.