Novel electronic states of heavy fermion compound YbCo₂Zn ₂₀

We studied the heavy fermion compound YbCo₂Zn₂₀ with an electronic specific heat coefficient γ ≥ 8000 mJ/(K ²·mol) by measuring the de Haas-van Alphen (dHvA) oscillation, Hall effect, magnetic susceptibility, and magnetization at ambient pressure, as well as the electrical resistivity in magnetic fields of up to 320 kOe and at pressures of up to 5 GPa. The detected Fermi surfaces are small in volume, reflecting the small Brillouin zone based on the large cubic lattice constant a = 14.005 A. The cyclotron effective masses, which were determined from the dHvA experiment, are found to be markedly reduced in magnetic fields. In other words, the detected cyclotron masses of 2.2-8.9m₀ (m₀: the rest mass of an electron) at Hav = 117 kOe are enhanced to 100-500m₀ at 0 kOe. By applying pressure, the heavy fermion state disappears at P_c ' 1.8 GPa and orders antiferromagnetically for P > P_c. The field-induced antiferroquadrupolar phase, which is observed only for H k h111i in the magnetic field range from HQ = 60 kOe to H₀ Q = 210 kOe, is found to shift to lower magnetic fields and merge with theantiferromagnetic phase at 4.5 GPa.