Nonlinear susceptibility in U0.9Th0.1Be13: Direct test of a quadrupolar Kondo ground state

We use nonlinear susceptibility measurements at different fixed temperatures chi(3)(T) as a direct experimental test for magnetism of the U(0.9)Th(0.1)Be(13) ground state, where thermodynamic and transport properties were observed recently to be consistent with the quadrupolar two-channel Kondo effect. At T < 6 K we found chi(3)(T) dependence, corresponding to the quadrupolar Kondo ground state. The experimental data may be understood as due to almost degenerate (for UBe(13)) and closely situated (for U(0.9)Th(0.1)Be(13)) Gamma(3) and Gamma(6) ground-state levels. In the dilute limit (U(0.07)Th(0.93)Be(13)) the ground state of the U-atom is found to be nonmagnetic quadrupolar.