Heavy fermions in YbAl3 studied by the de Haas-van Alphen effect

The carried out the de Haas-van Alphen (dHvA) experiment in YbAl3 with the cubic crystal structure. One of the six dHvA branches, named beta with a dH vA frequency of 4.5 x 10(7) Oe, is nearly spherical in topology and has a l arge cyclotron mass of 23.9 m(0). The other branches were observed only in small angular regions centered at symmetrical directions. Their cyclotron m asses are in the range from 13.5 to 17.4 m(0). The topology of the Fermi su rface cannot be explained by the conventional band model because the fully occupied 4f-levels in YbAl3 are below the Fermi level. A small alteration s o as to shift upwards the 4f levels is necessary to fit to the experimental dHvA data. These give a direct evidence that the 4f-electrons form heavy f ermions at low temperatures, which is most likely related to the many-body Kondo effect.