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Abstract: Resonant inelastic x-ray scattering (RIXS) is a powerful probe of elementary excitations in solids. It is now widely applied to study magnetic excitations. However, its complex cross section means that RIXS has been more difficult to interpret than inelastic neutron scattering (INS). Here we report ∼ 37 meV resolution RIXS measurements of the magnetic excitations in La 2 CuO 4 , the antiferromagnetic parent of one system of high-temperature superconductors. At high energies ( ∼ 2 eV), the RIXS spectra show angular-dependent d d orbital excitations in agreement with previous RIXS studies but show new structure. They are interpreted with single-site multiplet calculations. At low energies ( ≲ 0.3 eV), we model the wave-vector-dependent single magnon RIXS intensity as the product of the calculated single-ion spin-flip RIXS cross section and the dynamical structure factor S ( Q , ω ) of the spin-wave excitations. When S ( Q , ω ) is extracted from our data, the wave-vector-dependence of the single-magnon pole intensity shows a similar variation to that observed by INS. Our results confirm that suitably corrected RIXS data can yield the genuine wave-vector and energy dependence of S ( Q , ω ) for a cuprate antiferromagnet. In addition to spin waves, our data show structured multimagnon excitations with dispersing peaks in the intensity at energies higher than the single-magnon excitations.

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Abstract: We utilized high-energy-resolution resonant inelastic x-ray scattering (RIXS) at both the Ta and Ni L 3 edges to map out element-specific particle-hole excitations in Ta 2 Ni Se 5 across the phase transition. Our results reveal a momentum-dependent gaplike feature in the low-energy spectrum, which agrees well with the band gap in element-specific joint density of states calculations based on ab initio estimates of the electronic structure in both the low-temperature monoclinic and high-temperature orthorhombic structures. Below T c , the RIXS energy-momentum map shows a minimal gap at the Brillouin zone center ( ∼ 0.16 eV ) , confirming that Ta 2 Ni Se 5 possesses a direct band gap in its low-temperature ground state. However, inside the gap, no signature of anticipated collective modes with an energy scale comparable to the gap size can be identified. Upon increasing the temperature to above T c , whereas the gap at the zone center closes, the RIXS map at finite momenta still possesses the gross features of the low-temperature map, suggesting a substantial mixing between the Ta and Ni orbits in the conduction and valence bands, which does not change substantially across the phase transition. Our experimental observations and comparison to the theoretical calculations lend further support to the phase transition and the corresponding gap opening in Ta 2 Ni Se 5 being largely structural by nature, with a possible minor contribution from the putative exciton condensate.

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Abstract: We report a comprehensive study on the magnetic ground state of La 1.5 Ca 0.5 Co O 4 combining single crystal neutron diffraction and resonant magnetic x-ray scattering at the Co L 2 , 3 edges. Three single-crystal samples obtained from the same boule were investigated exhibiting magnetic phase transitions from a high-temperature paramagnetic phase to an antiferromagnetic phase at T N ≈ 52 K . Single crystal neutron diffraction reveals that the crystal structure at room temperature shows an orthorhombic A -centered lattice but with a and b axes almost equal in length. The structural phase transition (charge-ordering-like) from the parent tetragonal cell takes place above 523 K into the space group A 2 m m where two nonequivalent compressed and expanded Co O 6 octahedra are ordered showing a checkerboard pattern in the a b plane. The charge segregation between the nonequivalent Co sites is about 0.4(1) electrons. Resonant magnetic x-ray reflections indexed as ( 1 / 4 , 1 / 4 , 0 ) t , ( 1 / 4 , 1 / 4 , 1 ) t , and ( 1 / 4 , 1 / 4 , 1 / 2 ) t in the parent tetragonal cell were observed at low temperature at the Co L 2 , 3 -edge energy range. The resonant spectral shape, with a noticeable absence of any resonant enhancement at the Co L 2 edge, indicates that only Co 2 + -like ions participate in the magnetic ordering. The polarization analysis discloses that the orientation of Co magnetic moments is the same for the three magnetic orders and they are long-range ordered along the diagonal in the a b plane of the parent tetragonal cell with a slight tilt in the c axis. Despite the onset temperatures for the three resonant magnetic reflections being the same, ≈ 55 K , different thermal behavior is observed between ( 1 / 4 , 1 / 4 , 1 / 2 ) t and ( 1 / 4 , 1 / 4 , L ) t ( L = integer ) reflections whose intensities maximize at different temperatures, suggesting the coexistence of two magnetic arrangements. Moreover, the intensity of the ( 1 / 4 , 1 / 4 , 1 / 2 ) t magnetic reflection is at least ten times larger than that of the ( 1 / 4 , 1 / 4 , L ) t ( L = integer ) ones. On the other hand\, neutron diffraction measurements only detect a single type of antiferromagnetic ordering following the propagation vector k = ( 1 / 4 , 1 / 4 , 1 / 2 ) t that involves half of the Co atoms in the unit cell. We conclude that the bulk magnetic order in La 1.5 Ca 0.5 Co O 4 corresponds then to this propagation vector k = ( 1 / 4 , 1 / 4 , 1 / 2 ) t while ( 1 / 4 , 1 / 4 , 0 ) t and ( 1 / 4 , 1 / 4 , 1 ) t magnetic reflections correspond to a minority magnetic phase that must be due to changes in the oxygen stoichiometry near the surface.

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Abstract: In the novel stoichiometric iron-based material RbEuFe 4 As 4 , superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states. Using angle-resolved photoemission spectroscopy, we reveal a complex three-dimensional electronic structure and compare it with density functional theory calculations. Multiple superconducting gaps were measured on various sheets of the Fermi surface. High-resolution resonant photoemission spectroscopy reveals magnetic order of the Eu 4f states deep into the superconducting phase. Both the absolute values and the anisotropy of the superconducting gaps are remarkably similar to the sibling compound without Eu, indicating that Eu magnetism does not affect the pairing of electrons. A complete decoupling between Fe- and Eu-derived states was established from their evolution with temperature, thus unambiguously demonstrating that superconducting and a long-range magnetic orders exist independently from each other. The established electronic structure of RbEuFe 4 As 4 opens opportunities for the future studies of the highly unorthodox electron pairing and phase competition in this family of iron-based superconductors with doping.

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Abstract: We investigate the electronic structure of the 2 H and 3 R polytypes of Nb S 2 . The Fermi surfaces measured by angle-resolved photoemission spectroscopy show a remarkable difference in size, reflecting a significantly increased band filling in 3 R − Nb 1 + x S 2 compared to 2 H − Nb S 2 , which we attribute to the presence of additional interstitial Nb, which act as electron donors. Thus, we find that the stoichiometry, rather than the stacking arrangement, is the most important factor in the difference in electronic and physical properties of the two phases. Our high resolution data on the 2 H phase shows kinks in the spectral function that are fingerprints of the electron-phonon coupling. However, the strength of the coupling is found to be much larger for the the sections of bands with Nb 4 d x 2 − y 2 , x y character than for the Nb 4 d 3 z 2 − r 2 . Our results provide an experimental framework for interpreting the two-gap superconductivity and latent charge density wave in 2 H − Nb S 2 .