Basal-Plane Nonlinear Susceptibility: A Direct Probe of the Single-Ion Physics in URu2Si2

The microscopic nature of the hidden order state in URu2Si2 is dependent on the low-energy configurations of the uranium ions, and there is currently no consensus on whether it is predominantly 5f^2 or 5f^3. Here we show that measurement of the basal-plane nonlinear susceptibility can resolve this issue; its sign at low-temperatures is a distinguishing factor. We calculate the linear and nonlinear susceptibilities for specific 5f^2 and 5f^3 crystal-field schemes that are consistent with current experiment. Because of its dual magnetic and orbital character, a \Gamma_5 magnetic non-Kramers doublet ground-state of the U ion can be identified by $\chi_1^c(T) \propto \chi_3^\perp(T)$ where we have determined the constant of proportionality for URu2Si2.