Characteristic heavy fermion properties in YbCu₂Si₂ and YbT₂Zn₂₀ (T: Co, Rh, Ir)

We studied the Fermi surface properties of YbCu₂Si₂ and YbCu₂Ge₂ with the tetragonal structure by measuring the de Haas-van Alphen (dHvA) oscillations, together with the energy band calculations. It was clarified that 4f electrons contribute to the Fermi surface of a valence fluctuating compound YbCu₂Si₂, but not to that of a divalent compound YbCu₂Ge₂. Namely, the Fermi surface of YbCu₂Si₂ contains the 4f-components considerably and the corresponding cyclotron effective mass m ^*_c of the main Fermi surface is large, m ^*_c = 30-40m₀ (m₀: rest mass of an electron). We also studied the heavy fermion properties of YbT ₂Zn₂₀ (T: Co, Rh, Ir) with the cubic caged structure. The metamagnetic behavior or an abrupt nonlinear increase of magnetization was observed at 6 kOe in YbCo₂Zn₂₀, 63 kOe in YbRh ₂Zn₂₀, and 97 kOe in YbIr₂Zn₂₀ for H ∥ «100», which was measured at 60 mK in YbCo₂Zn ₂₀ and 1.3 K in YbRh₂Zn₂₀ and YbIr ₂Zn₂₀. The metamagnetic field is found to be a good parameter to reach the quantum critical point under pressure for YbIr ₂Zn₂₀. By measuring the electrical resistivity under pressure and magnetic field, we clarified that the electronic state close to the quantum critical point is realized in YbIr₂Zn₂₀ at 5.2 GPa and also YbCo₂Zn₂₀ at ambient pressure. The corresponding heavy cyclotron effective mass of 100-500m0 at 0 kOe was estimated from the field dependence of the √A value in the low-temperature electrical resistivity ρ=ρ₀ + AT² under pressure and magnetic field and the cyclotron mass obtained from the dHvA experiment at ambient pressure and high magnetic fields.