Strong field quenching of the quasiparticle effective mass in heavy fermion compound YbCo2Zn20

We found a metamagnetic like anomaly at Hm ≅ 5 kOe in a heavy fermion compound YbCo2Zn20 below the characteristic temperature Tχmax=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 γ≅8000 mJ/(K2·mol) and A=160 μΩ·cm/K2 in the electrical resistivity ρ=ρ0+AT2 at H=0 kOe are most likely due to the very low Kondo temperature. The √A value was, however, found to be strongly reduced from √A=12.6 (μΩ·cm/K2)1/2 at 0 kOe to 0.145 (μΩ·cm/K2)1/2 at 150 kOe. Therefore, we considered that the corresponding cyclotron effective mass mc*, which was determined from the temperature dependence of the de Haas-van Alphen (dHvA) amplitude, is also reduced with increasing magnetic field and is in fact not large, ranging from 2 to 9m0 at 117 kOe. From the field dependence of √A and mc*, we estimated the cyclotron effective mass at 0 kOe to be 100-500m0, revealing the largest cyclotron mass as far as we know. (author)