Field-induced quadrupolar ordered phase for H ∥ 〈111〉 in heavy-fermion compound YbCo₂Zn₂₀

Magnetic susceptibility, high-field magnetization up to 500 kOe, and specific heat in a wide temperature range from 0.06 to 300K were measured for single crystals of the cubic heavy-fermion compound YbCo₂Zn ₂₀ in order to elucidate the electronic states of the compound at low temperatures. A strong increase in the magnetic specific heat in the form of C_mag=T below ∼1K is approximately explained by the resonant level model for S = 1=2 with the Kondo temperature T_K = 1 K, and an extremely large C_mag=T ≃ 8 J/(K²·mol) below about 0.2K is explained by considering the magnetic entropy of the doublet ground state in the 4f crystalline electric field (CEF) scheme of an Yb ³⁺ ion, which corresponds to an extremely large electronic specific heat coefficient. The field-induced ordered phase for H k h111i, which has recently been found by low-temperature magnetization measurements, was precisely studied by electrical resistivity and specific heat measurements, and was observed in a limited angular range around the h111i direction. On the basis of CEF analyses, the level crossing of the two lowest CEF states is essentially important to understand the field-induced ordered phase, which can be reduced to a field-induced antiferro-quadrupolar ordering based on the Γ₃-type quadrupole moment O₂⁰