Direct bulk-sensitive probe of 5f symmetry in URu₂Si₂

The second-order phase transition into a hidden order phase in  URu₂Si₂ goes along with an order parameter that is still a mystery, despite 30 years of research. However, it is understood that the symmetry of the order parameter must be related to the symmetry of the low-lying local electronic f-states. Here, we present results of a spectroscopic technique, namely core-level nonresonant inelastic X-ray scattering (NIXS). This method allows for the measurement of local high-multipole excitations and is bulk-sensitive. The observed anisotropy of the scattering function unambiguously shows that the 5f ground-state wave function is composed mainly of the Γ₁ with majority J_z = |4⟩ + |−4⟩ and/or Γ₂ singlet states. The incomplete dichroism indicates the possibility that quantum states of other irreducible representation are mixed into the ground state.