Probing High-Temperature Superconductivity with Positive Muons

Muon-spin rotation (SR) is a unique tool to investigate internal magnetic fields in superconductors on a microscopic scale. In particular, the complex vortex structure in high-temperature superconductors can be explored with this technique. As an example, SR experiments on the vortex phase diagram of single-crystal Bi₂Sr₂CaCu₂O₈₊ are briefly described. A novel SR technique using low-energy muons allows the measurement of internal magnetic fields near the surface of a superconductor with a few nanometers' resolution. First results obtained with this technique on a thin YBa₂Cu₃O_7- film are presented.