A new ternary family RT2X20 exhibits diverse ground states including quadrupolar order,
heavy-fermion, and superconductivity, which allows us systematic studies on various exotic
phenomena under the identical structure. Among them, all the Yb members YbT2Zn20 are
known to exhibit heavy-fermion ground state. In particular, YbCo2Zn20 has an enormously
large Sommerfeld coefficient 8 J/molK2 at ambient pressure, and that can be modied by
relatively small external elds. Whereas pressure induces antiferromagnetic order, magnetic
eld yields metamagnetic transitions at around 0.5 T, and new ordered state in the narrow
region around B∥〈111〉 above 6 T [2,3]. In order to get insights into electronic states and its
magnetic eld response, neutron scattering experiments were carried out.
Inelastic neutron scattering experiments on poly-crystalline sample reveal intense low-energy
excitations which consists of quasielastic response and low crystal-eld excitation below 1 meV.
Upon applying eld, quasielastic response was rapidly suppressed while crystal-eld level was
lifted. The result suggests degenerated energy scale between Kondo and crystal-eld interac-
tions in YbCo2Zn20. For B∥〈111〉 in the eld-induced ordered phase, we have succeeded to
observe magnetic reections with q=(1/2 1/2 1/2). The induced antiferromagnetic peak ac-
companies characteristic diffuse scattering, and showing complex eld dependence. Nature of
the eld-induced ordered state will be discussed.
 M. S. Torikachvili et al., Proc. Natl. Acad. Sci. 104, 9960 (2007).
 Y. Saiga et al., J. Phys.: Conf. Ser. 150, 042168 (2009).
 M. Ohya et al., J. Phys. Soc. Jpn. 79, 083601 (2010).