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The second-order phase transition into a hidden order phase in URu$_2$Si$_2$ 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 5$f$ ground state wave function is composed mainly, but essentially not purely, of the $Gamma_1$ with majority $J_z$ =$|4rangle$ + $|-4rangle$ and/or $Gamma_2$ singlet states.
Since the 1985 discovery of the phase transition at $T_{rm HO}=17.5$ K in the heavy-fermion metal URu$_2$Si$_2$, neither symmetry change in the crystal structure nor magnetic ordering have been observed, which makes this hidden order enigmatic. Some
We have investigated the electronic states of the uranium monochalcogenide US, which is a typical ferromagnetic uranium compound, using soft x-ray photoemission spectroscopy (SX-PES). In early ultraviolet photoemission spectroscopy studies, two peak
An electronic nematic state spontaneously breaks a point-group symmetry of an underlying lattice. As a result, the nematic-isotropic transition accompanies a Fermi surface distortion. However, the anisotropic nature of the nematic state at a macrosco
The electronic structure of ThRu2Si2 was studied by angle-resolved photoelectron spectroscopy (ARPES) with incident photon energies of hn=655-745 eV. Detailed band structure and the three-dimensional shapes of Fermi surfaces were derived experimental
In heavy-fermion compounds, f electrons show both itinerant and localized behaviour depending on the external conditions, and the hybridization between localized f electrons and itinerant conduction bands gives rise to their exotic properties like he