DE-HAAS-VAN-ALPHEN EFFECT IN THE VORTEX STATE OF NB3SN

de Haas-van Alphen (dHvA) oscillations corresponding to five extremal areas of the Fermi surface have been observed in the vortex state of N b3Sn, in magnetic fields as low as approximately 0.5B(c2). Both the dH vA frequencies and effective masses are unchanged from their values in the normal state, although the quantum oscillations experience a fiel d-dependent additional damping in the superconducting state. The resul ts show that the Landau quantization of quasiparticles persists in the mixed state of a clean type-II superconductor.