We have studied the Fermi surface properties of CeRu2Si2 below and above the metamagnetic transition (MT) under high pressures. The frequencies of the dHvA oscillations change considerably with pressure both below and above the MT. The pressure coefficient of the frequencies are larger than those of normal metals by more than ten times. The sign and value of the pressure coefficient depend on the dHvA frequency or the orbit. Below the MT the effective masses decrease with increasing pressure, while that above the MT is found to slightly increase. The dHvA oscillations can be traced through the MT at high pressures and it is found that the Fermi surface changes from that of the itinerant f electron to another Fermi surface which is similar to that of the localized f electron in a field range much narrower than that of the MT width in the magnetization. An analysis based on the pressure effect of the Fermi surface indicates that the Fermi surface remains unchanged when the filed approaches the MT field (Hm), if the volume is constant. If we extrapolate the experimental Hm vs. pressure relation to the negative pressure side, the volume at Hm = 0 coincides with the volume at the quantum critical point suggested by the studies of CeRu2Ge2 system under high pressure or CeRu2(Ge1-xSix)2 system. The experimental observations are discussed to suggest that the MT of CeRu2Si2 is a phenomenon closely related with the quantum critical point.
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