DEHAAS-VANALPHEN MEASUREMENTS IN TYPE-II SUPERCONDUCTORS

Measurements of the de Haas-van Alphen effect in YBCO carried out by Fowler et al. using fields over 100 T and temperatures between 2.4 and 4.0 K are reviewed. Samples were oriented so that the c or (0 0 1) axis lay along the magnetic field direction. Three distinct external cross-sectional orbital frequencies at 0.53, 0.78 and 3.51 kT were found. But in this work no transition through B(c2) was observed in this regime. Tentatively, it has been hypothesized that B(c2) may be higher than 100 T at 2.4 K in Los Alamos samples. If this is the case then the dHvA effect seen in these experiments may involve quantal oscillations, periodic in 1/B, in fields below B(c2). Such an effect was first observed by Graebner and Robbins in NbSe2 in 1976. Additional experiments illustrating a dHvA effect below B(c2) in V3Si will be presented using a theoretical analysis of experiments of Lowndes and Arko. The theoretical situation of the dHvA effect in superconductors below B(c2) is reviewed.