Field-Induced Quadrupolar Ordered Phase for H\parallel\langle 111\rangle in Heavy-Fermion Compound YbCo2Zn20

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Magnetic susceptibility, high-field magnetization up to 500 kOe, and specific heat in a wide temperature range from 0.06 to 300 K were measured for single crystals of the cubic heavy-fermion compound YbCo2Zn20 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_{\text{mag}}/T below {\sim}1 K is approximately explained by the resonant level model for S = 1/2 with the Kondo temperature T_{\text{K}} = 1 K, and an extremely large C_{\text{mag}}/T\simeq 8 J/(K2\cdotmol) below about 0.2 K is explained by considering the magnetic entropy of the doublet ground state in the 4f crystalline electric field (CEF) scheme of an Yb3+ ion, which corresponds to an extremely large electronic specific heat coefficient. The field-induced ordered phase for H\parallel\langle 111\rangle, 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 \langle 111\rangle 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 \Gamma_{3}-type quadrupole moment O_{2}^{0}.
2011-11-15