Direct bulk sensitive probe of 5f symmetry in URu2Si2

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The second-order phase transition into a hidden order phase in URu₂Si₂ goes along with an order parameter which 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 novel spectroscopy, namely core-level non-resonant inelastic x-ray scattering (NIXS). This method allows for the measurement of local high-multipole excitations and it is bulk sensitive. The observed anisotropy of the scattering function unambiguously shows that the 5f ground state wave function is composed mainly, but essentially not purely, of the Γ₁ with majority Jz =|4 + |-4 and/or Γ₂ singlet states.