de Haas–van Alphen Effect and Fermi Surface Properties in High-Quality Single Crystals YbCu<SUB>2</SUB>Si<SUB>2</SUB> and YbCu<SUB>2</SUB>Ge<SUB>2</SUB>

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  • de Haas-van Alphen effect and Fermi surface properties in high-quality single crystals YbCu2Si2 and YbCu2Ge2

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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||[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 m0, which is consistent with the electronic specific heat coefficient γ=150 mJ/(K2·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 γ value of 9–10 mJ/(K2·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.

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