Surface impedance investigations of superconducting niobium

Measurements of the surface resistance derivative dR/dH in dependence on magnetic field strength H are presented as observed at pure superconducting niobium single crystals. At crystals not annealed after preparation procedure the measured impedance spectra do not represent the phase transitions appearing at critical fields indicating thus the existence of surface layers with properties different from the bulk of pure niobium. At annealed crystals the critical magnetic field strengths are marked as characteristic discontinuities in the spectra. The spectra observed at recently annealed crystals being in Meissner state can be interpreted by means of magnetic surface states. (author)