We have studied a change of the Fermi surface in an antiferromagnet CeIn 3 via the de Haasvan Alphen experiment under pressure up to 3 GPa. With increasing pressure P, the Néel temperature T N ¼ 10 K decreases and becomes zero at a critical pressure P c ' 2:6 GPa. In the pressure region P > P c , we have observed a large main Fermi surface named a, which indicates that the electronic state of 4 f electron in CeIn 3 changes from localized to itinerant at P c , as observed in the similar antiferromagnets CeRh 2 Si 2 and CeRhIn 5 . The cyclotron effective mass m à c of this main Fermi surface is extremely enhanced around P c : m à c ' 60m 0 at 2.7 GPa for the magnetic field along the h100i direction.

We have carried out a de Hass-van Alphen (dHvA) experiment for the recently discovered CePt 3 Si, which is the first heavy fermion superconductor without inversion symmetry in the tetragonal crystal structure, together with a dHvA experiment for a non-4f reference compound LaPt 3 Si. As for LaPt 3 Si, several dHvA branches were observed. Among them, the two main dHvA branches with the dHvA frequency (the cyclotron effective mass) of 1.10 × 10 8 Oe (1.4 m 0 ) and 8.41 × 10 7 Oe (1.5 m 0 ) were found to be well explained from the FLAPW energy band calculations, corresponding to bands 64-and 63-multiply-connected hole Fermi surfaces, respectively. On the other hand, the dHvA frequencies of 10 7 Oe in CePt 3 Si are small in magnitude, although the corresponding cyclotron masses of 10-20 m 0 are extremely large.The recently discovered superconductor CePt 3 Si is unique, possessing a few characteristics [1]. Superconductivity is realized in the long-range antiferromagnetic ordering. The Néel temperature is 2.2 K, while the superconducting transition temperature is 0.75 K. Both transitions were observed clearly in the specific heat measurement. The electronic specific heat coefficient γ is large, 300-400 mJ K −2 mol −1 . In the previous cerium-based heavy fermion superconductors including a prototype superconductor CeCu 2 Si 2 and a quasi-two dimensional superconductor CeCoIn 5 with a large γ value of 1000 mJ K −2 mol −1 , superconductivity occurs in the non-magnetic or antiferromagnetically spin-fluctuating state [2][3][4]. Another example is CeIn 3 under pressure [4][5][6]. At ambient pressure, the Néel temperature in CeIn 3 is T N = 10 K 4 Author to whom any correspondence should be addressed.

We have carried out a de Haas-van Alphen experiment under pressure on a heavy-fermion superconductor, CeCoIn 5 . Large cyclotron masses of 15 m 0 (m 0 : the rest mass of an electron) in a nearly cylindrical Fermi surface called α i (i = 1, 2, 3) and 60 m 0 in a similar cylindrical Fermi surface called β 2 at ambient pressure are found to be strongly reduced to 7 and 40 m 0 , respectively, at 3 GPa. Correspondingly, the field-dependent cyclotron mass at ambient pressure becomes almost independent of the field at high pressures. These results indicate that CeCoIn 5 , which is in the vicinity of the quantum critical region at ambient pressure, is changed into a usual heavy-fermion state under high pressures of about 3 GPa.

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