Quantum oscillations have been investigated far into the mixed state of the type II superconductor ZH-NbSei. The de Haas-van Alphen effect is found to persist to fields as low as -0 . 3 B a and to t e m p e r a m of -0.003Tc. A selfamistent band shuclure calcularion.performed for ?H-NbSez in the normal non-charge-density-wave state using a full potentid linearized augmented p h e wave (FLAPW) method, accounts well for the experimental angular dependence of the quantum oscillations in term of a small Ral hole Fermi surface around the r point. This feaNre derives mainly from chalcogen p bands and has a massenhancement &tor much smaller than expected from the specific heat QuanNm oscillations in the presence of the vortex lattice are observed to experience an additional damping, from which the order parameter is atimated to be A(0) = 0.6 f 0.1 meV.

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