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

Abstract

We succeeded in growing high-quality single crystals of a valence fluctuating compound YbCu 2 Si 2 and a divalent compound YbCu 2 Ge 2 . The magnetic susceptibility of YbCu 2 Si 2 follows the Curie–Weiss law with Yb 3+ at high temperatures, but reveals a broad peak around 40 K for H ∥[100], which is due to the formation of a 4 f -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 4 f -itinerant LDA band model, and the 4 f -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 γ=150 mJ/(K 2 ·mol). These results indicate that the localized 4 f electrons at high temperatures become itinerant at low temperatures, forming a narrow renormalized conduction band. On the other hand, YbCu 2 Ge 2 is a divalent compound, indicating a Pauli paramagnetic susceptibility and a small γ value of 9–10 mJ/(K 2 ·mol). The corresponding 4 f bands in YbCu 2 Ge 2 are located below the Fermi energy, and do not contribute to the Fermi surfaces. The dHvA data are thus well explained by the Yb 2+ band model. These Fermi surfaces in YbCu 2 Si 2 and YbCu 2 Ge 2 are highly different from the Fermi surfaces of a non-4 f reference compound YCu 2 Si 2 and a 4 f -closed reference compound LuCu 2 Si 2 .