Localized or itinerant: nature of strongly correlated f electron system and the quantum critical point-investigation based on the measurements of Fermi surface properties

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1. We have successfully developed a piston-cylinder type pressure cell which can measure the dHvA effect under pressures up to 3GPa2. It has been demonstrated from the measurements under pressure that the metamagnetic transition in CeRu2Si2 is closely related with the quantum critical point of this compound3. We have grown high quality single crystals of magnetic Uge_2 where superconductivity has been recently discovered under pressure. We have performed the de Haas-van Alpben (dHvA) effect measurements from the ambient pressure to the pressures beyond the quantum critical point(QCP). When the filed is applied parallel to the a-axis, the Fermi surface changes drastically across the quantum critical point(QCP) and the effective masses are enhanced around the QCP. When the field is applied parallel to the b-axis, two anomalies Bx(P) and Bm(P) are observed. Bm(P) corresponds to a metamagnetic transition and Bx(P) to the anomaly Tx. The electronic structure changes smoothly up to Bx(P) and changes drastically across Bx(P)4. We have perfomed the dHvA measurements in ZrZn_2 and Celn_3 under pressures up to their QCP's. In ZrZn_2 we have revealed the pressure dependences of the effective mass and frequencies of the α and β oscillations which correspond to the majority and minority spin bands, respectively. The effective masses do not change significantly with pressure. The frequency difference is found to remain finite even near the QCP. The dHvA frequency and effective of CeIn3 are found to increase with pressure5. We have revealed the pressure dependence of Fermi surface properties of PrPb_3