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We found a metamagnetic like anomaly at H m ' 5 kOe in a heavy fermion compound YbCo 2 Zn 20 below the characteristic temperature T max ¼ 0:32 K where the ac-susceptibility shows a broad peak, suggesting that an electronic state with a very low Kondo temperature is realized. Interestingly, the metamagnetic like behavior was observed as two peaks at 4.0 and 7.5 kOe at 95 mK in the magnetic field dependence of the electronic specific heat C=T. The extremely large values of the electronic specific heat coefficient ' 8000 mJ/(K 2 Ámol) and A ¼ 160 mÁcm/K 2 in the electrical resistivity Most of the cerium or uranium compounds order antiferromagnetically on the basis of the RKKY interaction. Some cerium or uranium compounds such as CeCu 6 , CeRu 2 Si 2 and UPt 3 , however, exhibit no long-range magnetic ordering.1) The magnetic susceptibility of these compounds increases with decreasing temperature, following the Curie-Weiss law at high temperatures, and shows a maximum at a characteristic temperature T max . Below T max , the susceptibility becomes almost temperature-independent, and the f-electron system is changed into a new electronic state, called the heavy fermion state. The almost localized f-electrons at high temperatures become itinerant at temperatures below T max via the many-body Kondo effect. The crossover from ''localized'' to ''itinerant'' occurs at a characteristic temperature T max , which approximately corresponds to the Kondo temperature T K .The conduction electrons in the heavy fermion state are the interacting electrons or quasiparticles corresponding to the Landau Fermi liquid. An enhanced Pauli susceptibility ðT ! 0Þ ¼ 0 correlates with a large electronic specific heat coefficient C=TðT ! 0Þ ¼ and a large coefficient ffiffiffi A p in the electrical resistivity ¼ 0 þ AT 2 , where 0 is the residual resistivity. Note that the value is roughly estimated as 10 4 =T K [mJ/(K 2 Ámol)] or R ln 2=T K . One of the characteristic properties in the heavy fermion compounds is the metamagnetic behavior or an abrupt nonlinear increase of magnetization at the magnetic field H m at temperatures lower than T max . The metamagnetic behavior appears at H m ¼ 77 kOe in CeRu 2 Si 2 2) and H m ¼ 200 kOe in UPt 3 , 3) for example. Very recently, we found the metamagnetic behavior at H m ' 100 kOe in YbIr 2 Zn 20 with the cubic crystal structure, 4) which is expected to occur even in Yb-based compounds on the basis of the hole-electron analogy between 4f 13 -Yb 3þ and 4f 1 -Ce 3þ electronic configuration. YbIr 2 Zn 20 is a new Yb-based heavy fermion compound with ¼ 540 mJ/(K 2 Ámol), 5) and in fact, the conduction electrons with large cyclotron effective masses of 10 { 30m 0 (m 0 : rest mass of an electron) are detected in the de Haas-van Alphen (dHvA) experiment.4) The corresponding magnetic susceptibility follows the Curie-Weiss law of Yb 3þ at high temperatures, but shows a broad peak at T max ¼ 7:4 K.We continued our experimental study for a similar compound of YbCo 2 Zn 20 . The following characteristic features w...

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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 YbCo 2 Zn 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 $1 K is approximately explained by the resonant level model for S ¼ 1=2 with the Kondo temperature T K ¼ 1 K, and an extremely large C mag =T ' 8 J/(K 2 Ámol) 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 Yb 3þ 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 À 3 -type quadrupole moment O 0 2 .