de Haas-van Alphen Effect and Fermi Surface Properties in High-Quality Single Crystals YbCu2Si2 and YbCu2Ge2

We succeeded in growing high-quality single crystals of a valence fluctuating compound YbCu2Si2 and a divalent compound YbCu2Ge2. The magnetic susceptibility of YbCu2Si2 follows the Curie-Weiss law with Yb3+ at high temperatures, but reveals a broad peak around 40 K for H parallel to [100], which is due to the formation of a 4f-itinerant heavy fermion state at lower temperatures. This is also reflected in the temperature dependence of Hall coefficient, thermoelectric power and thermal expansion. The corresponding de Haas-van Alphen (dHvA) branches are approximately explained by the 4f-itinerant LDA band model, and the 4f-itinerant LDA+U model is found to be much applicable to the dHvA data. The cyclotron effective masses of main Fermi surfaces are relatively large, being 30-40 m(0), which is consistent with the electronic specific heat coefficient gamma = 150 mJ/(K-2.mol). These results indicate that the localized 4f electrons at high temperatures become itinerant at low temperatures, forming a narrow renormalized conduction band. On the other hand, YbCu2Ge2 is a divalent compound, indicating a Pauli paramagnetic susceptibility and a small gamma value of 9-10 mJ/(K-2.mol). The corresponding 4f bands in YbCu2Ge2 are located below the Fermi energy, and do not contribute to the Fermi surfaces. The dHvA data are thus well explained by the Yb2+ band model. These Fermi surfaces in YbCu2Si2 and YbCu2Ge2 are highly different from the Fermi surfaces of a non-4f reference compound YCu2Si2 and a 4f-closed reference compound LuCu2Si2.