de Haas-van Alphen Studies of YbAs above and below Néel Temperature

We have studied the Fermi surface and the cyclotron mass of a semimetallic heavy fermion compound YbAs both in the antiferromagnetic state and the paramagnetic state. We have found the Fermi surface of three ellipsoidal electron surfaces and one spherical hole surface, and that the shape of these Fermi surfaces is the same in both states. In addition, we have found that the temperature dependence of the dHvA oscillatory amplitudes change anomalously at the Neel temperature. This anomaly is ascribed to the change of the cyclotron mass and the Dingle temperature at the Neel temperature. To explain the large electronic specific heat coefficient, the extremely heavy hole surface with about 10 3 m 0 should exist.