Point-contact spectroscopy on URu2Si2

Tunnel and point-contact experiments have been made in a URu2Si2 single crystal along the c axis. The experiments were performed changing temperature and contact size in a low-temperature scanning tunneling microscope. A resonance develops at the Fermi level at T similar to 60 K. This resonance Splits and becomes asymmetric when the 17.5 K phase transition is crossed. These results are consistent with the existence of Kondo like bound states of the U4+ ionic configurations and the conduction electrons. Below the transition, these configurations are split by the development of quadrupolar ordering. The peak separation can be interpreted as a direct measurement of the order parameter. Measurements on a polycrystalline UAu2Si2 sample are also reported, with a comparative study of the behavior of both materials.