Abstract
We present the nonlinear susceptibility as a direct test of the quadrupolar
Kondo scenario for heavy fermion behavior, and apply it to the case of cubic
crystal-field symmetry. Within a single-ion model we compute the nonlinear
susceptibility resulting from low-lying \Gamma_3 (5f^2 and Kramers (5f^3)
doublets. We find that nonlinear susceptibility measurements on single-crystal
UBe_{13} are inconsistent with a quadrupolar 5f^2 ground-state of the uranium
ion; the experimental data indicate that the low-lying magnetic excitations of
UBe_{13} are predominantly dipolar in character.