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Register a new userElectronic structure of URu$_2$Si$_2$ in paramagnetic phase: Three-dimensional angle resolved photoelectron spectroscopy study
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The three-dimensional (3D) electronic structure of the hidden order compound URu$_2$Si$_2$ in a paramagnetic phase was revealed using a 3D angle-resolved photoelectron spectroscopy where the electronic structure of the entire Brillouin zone is obtained by scanning both incident photon energy and detection angles of photoelectrons. The quasi-particle bands with enhanced contribution from the $mathrm{U}~5f$ state were observed near $E_mathrm{F}$, formed by the hybridization with the $mathrm{Ru}~4d$ states. The energy dispersion of the quasi-particle band is significantly depend on $k_z$, indicating that they inherently have a 3D nature. The band-structure calculation qualitatively explain the characteristic features of the band structure and Fermi surface although the electron correlation effect strongly renormalizes the quasi-particle bands. The 3D and strongly-correlated nature of the quasi-particle bands in URu$_2$Si$_2$ is an essential ingredient for modeling its hidden-order transition.
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The application of pressure as well as the successive substitution of Ru with Fe in the hidden order (HO) compound URu$_2$Si$_2$ leads to the formation of the large moment antiferromagnetic phase (LMAFM). Here we have investigated the substitution series URu$_{2-x}$Fe$_x$Si$_2$ with $x$ = 0.2 and 0.3 with non-resonant inelastic x-ray scattering (NIXS) and 4$f$ core-level photoelectron spectroscopy with hard x-rays (HAXPES). NIXS shows that the substitution of Fe has no impact on the symmetry of the ground-state wave function. In HAXPES we find no shift of spectral weight that would be indicative for a change of the 5$f$-electron count. Consequently, changes in the exchange interaction $cal{J}$ due to substitution must be minor so that the conjecture of chemical pressure seems unlikely. An alternative scenario is discussed, namely the formation of long range magnetic order due the substitution induced local enhancement of the magnetization in the vicinity of the $f$-electron ions while the overall electronic structure remains unchanged.
The electronic structure of the unconventional superconductor UTe$_2$ was studied by resonant photoelectron spectroscopy (RPES) and angle-resolved photoelectron spectroscopy (ARPES) with soft X-ray synchrotron radiation. The partial $mathrm{U}~5f$ density of states of UTe$_2$ were imaged by the $mathrm{U}~4d$--$5f$ RPES and it was found that the $mathrm{U}~5f$ state has an itinerant character, but there exists an incoherent peak due to the strong electron correlation effects. Furthermore, an anomalous admixture of the $mathrm{U}~5f$ states into the $mathrm{Te}~5p$ bands was observed at a higher binding energy, which cannot be explained by band structure calculations. On the other hand, the band structure of UTe$_2$ was obtained by ARPES and its overall band structure were mostly explained by band structure calculations. These results suggest that the $mathrm{U}~5f$ states of UTe$_2$ have itinerant but strongly-correlated nature with enhanced hybridization with the $mathrm{Te}~5p$ states.
The electronic structures of the ferromagnetic superconductors $mathrm{UGe}_2$ and $mathrm{UCoGe}$ in the paramagnetic phase were studied by angle-resolved photoelectron spectroscopy using soft X-rays ($h u =400-500$). The quasi-particle bands with large contributions from $mathrm{U}~5f$ states were observed in the vicinity of $E_mathrm{F}$, suggesting that the $mathrm{U}~5f$ electrons of these compounds have an itinerant character. Their overall band structures were explained by the band-structure calculations treating all the $mathrm{U}~5f$ electrons as being itinerant. Meanwhile, the states in the vicinity of $E_mathrm{F}$ show considerable deviations from the results of band-structure calculations, suggesting that the shapes of Fermi surface of these compounds are qualitatively different from the calculations, possibly caused by electron correlation effect in the complicated band structures of the low-symmetry crystals. Strong hybridization between $mathrm{U}~5f$ and $mathrm{Co}~3d$ states in $mathrm{UCoGe}$ were found by the $mathrm{Co}~2p-3d$ resonant photoemission experiment, suggesting that $mathrm{Co}~3d$ states have finite contributions to the magnetic, transport, and superconducting properties.
We report a neutron scattering study of the magnetic excitation spectrum in each of the three temperature and pressure driven phases of URu$_2$Si$_2$. We find qualitatively similar excitations throughout the (H0L) scattering plane in the hidden order and large moment phases, with no changes in the $hbaromega$-widths of the excitations at the $Sigma$ = (1.407,0,0) and $Z$ = (1,0,0) points, within our experimental resolution. There is, however, an increase in the gap at the $Sigma$ point from 4.2(2) meV to 5.5(3) meV, consistent with other indicators of enhanced antiferromagnetism under pressure.
A second-order phase transition is associated with emergence of an order parameter and a spontaneous symmetry breaking. For the heavy fermion superconductor URu$_2$Si$_2$, the symmetry of the order parameter associated with its ordered phase below 17.5 K has remained ambiguous despite 30 years of research, and hence is called hidden order (HO). Here we use polarization resolved Raman spectroscopy to specify the symmetry of the low energy excitations above and below the HO transition. These excitations involve transitions between interacting heavy uranium 5f orbitals, responsible for the broken symmetry in the HO phase. From the symmetry analysis of the collective mode, we determine that the HO parameter breaks local vertical and diagonal reflection symmetries at the uranium sites, resulting in crystal field states with distinct chiral properties, which order to a commensurate chirality density wave ground state.
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