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

We found a metamagnetic like anomaly at H_m ≃ 5 kOe in a heavy fermion compound YbCo₂Zn₂₀ below the characteristic temperature T_Xmax = 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 ≃ 8000mJ/(K²·mol) and A = 160μΩ·cm/K² in the electrical resistivity p = p₀ + AT² at H = 0kOe 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 0kOe to 0.145(μΩ·cm/K²)^1/2 at 150kOe. Therefore, we considered that the corresponding cyclotron effective mass m _c^*, 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 9m ₀ at 117kOe. From the field dependence of √A and m _c^*, we estimated the cyclotron effective mass at 0kOe to be 100{500m₀, revealing the largest cyclotron mass as far as we know.