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Using inelastic X-ray scattering beyond the dipole limit and hard X-ray photoelectron spectroscopy we establish the dual nature of the U 5f electrons in UM2Si2 (M = Pd, Ni, Ru, Fe), regardless of their degree of delocalization. We have observed that the compounds have in common a local atomic-like state that is well described by the U 5f(2) configuration with the Gamma((1))(1) and Gamma(2) quasi-doublet symmetry. The amount of the U 5f(3) configuration, however, varies considerably across the UM2Si2 series, indicating an increase of U 5f itineracy in going from M = Pd to Ni to Ru and to the Fe compound. The identified electronic states explain the formation of the very large ordered magnetic moments in UPd2Si2 and UNi2Si2, the availability of orbital degrees of freedom needed for the hidden order in URu2Si2 to occur, as well as the appearance of Pauli paramagnetism in UFe2Si2. A unified and systematic picture of the UM2Si2 compounds may now be drawn, thereby providing suggestions for additional experiments to induce hidden order and/or superconductivity in U compounds with the tetragonal body-centered ThCr2Si2 structure.

Using inelastic X-ray scattering beyond the dipole limit and hard X-ray photoelectron spectroscopy we establish the dual nature of the U 5f electrons in UM2Si2 (M = Pd, Ni, Ru, Fe), regardless of their degree of delocalization. We have observed that the compounds have in common a local atomic-like state that is well described by the U 5f(2) configuration with the Gamma((1))(1) and Gamma(2) quasi-doublet symmetry. The amount of the U 5f(3) configuration, however, varies considerably across the UM2Si2 series, indicating an increase of U 5f itineracy in going from M = Pd to Ni to Ru and to the Fe compound. The identified electronic states explain the formation of the very large ordered magnetic moments in UPd2Si2 and UNi2Si2, the availability of orbital degrees of freedom needed for the hidden order in URu2Si2 to occur, as well as the appearance of Pauli paramagnetism in UFe2Si2. A unified and systematic picture of the UM2Si2 compounds may now be drawn, thereby providing suggestions for additional experiments to induce hidden order and/or superconductivity in U compounds with the tetragonal body-centered ThCr2Si2 structure.

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 Gamma(1) with majority J(z) = |4 > + |4 > and/or Gamma(2) singlet states. The incomplete dichroism indicates the possibility that quantum states of other irreducible representation are mixed into the ground state.

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 Gamma(1) with majority J(z) = |4 > + |4 > and/or Gamma(2) singlet states. The incomplete dichroism indicates the possibility that quantum states of other irreducible representation are mixed into the ground state.