The transition metal chalcogenide Ta2NiSe5 undergoes a second-order phase transition at Tc = 328 K involving a small lattice distortion. Below Tc, a band gap at the center of its Brillouin zone ...increases up to about 0.35 eV. In this work, we study the electronic structure of Ta2NiSe5 in its low-temperature semiconducting phase, using resonant inelastic x-ray scattering (RIXS) at the Ni L3-edge. In addition to a weak fluorescence response, we observe a collection of intense Raman-like peaks that we attribute to electron-hole excitations. Using density functional theory calculations of its electronic band structure, we identify the main Raman-like peaks as interband transitions between valence and conduction bands. Further, by performing angle-dependent RIXS measurements, we uncover the dispersion of these electron-hole excitations that allows us to extract the low-energy boundary of the electron-hole continuum. From the dispersion of the valence band measured by angle-resolved photoemission spectroscopy, we derive the effective mass of the lowest unoccupied conduction band.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
We report the A -site spinel NiRh2O4 is the only known realization of a spin-1 diamond lattice magnet and is predicted to host unconventional magnetic phenomena driven by frustrated nearest and ...next-nearest neighbor exchange as well as orbital degeneracy. Previous works found no sign of magnetic order but found a gapped dispersive magnetic excitation indicating a possible valence bond magnetic ground state. However, the presence of many competing low energy degrees of freedom and limited empirical microscopic constraints complicates further analysis. Here we carry out resonant inelastic x-ray scattering (RIXS), x-ray absorption spectroscopy (XAS), and inelastic neutron scattering (INS) to characterize the local electronic structure and lattice dynamics of NiRh2O4. The RIXS data can be partly described by a single-ion model for tetrahedrally coordinated Ni2+ and indicates a tetragonal distortion Δt2 = 70 meV that splits the t2 orbitals into a high energy orbital singlet and lower energy orbital doublet. We identify features of the RIXS spectra that are consistent with a Rh-Ni two-site excitation indicating strong metal-metal hybridization mediated by oxygen in NiRh2O4. We also identify signatures of electron-phonon coupling through the appearance of phonon sidebands that dress crystal field excitations. These results establish the key energy scales relevant to the magnetism in NiRh2O4 and further demonstrate that covalency and lattice dynamics play essential roles in controlling the magnetic ground states of A -site spinels.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
The magnetic correlations within the cuprates have undergone intense scrutiny as part of efforts to understand high-temperature superconductivity. We explore the evolution of the magnetic ...correlations along the nodal direction of the Brillouin zone in La2–xSrxCuO4, spanning the doping phase diagram from the antiferromagnetic Mott insulator at x = 0 to the metallic phase at x = 0.26. Magnetic excitations along this direction are found to be systematically softened and broadened with doping, at a higher rate than the excitations along the antinodal direction. This phenomenology is discussed in terms of the nature of the magnetism in the doped cuprates. As a result, survival of the high-energy magnetic excitations, even in the overdoped regime, indicates that these excitations are marginal to pairing, while the influence of the low-energy excitations remains ambiguous.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
The discovery of superconductivity in square-planar low valence nickelates has ignited a vigorous debate regarding their essential electronic properties: Do these materials have appreciable oxygen ...charge-transfer character akin to the cuprates, or are they in a distinct Mott-Hubbard regime where oxygen plays a minimal role? Here, we resolve this question using O K-edge resonant inelastic x-ray scattering (RIXS) measurements of the low valence nickelate La_{4}Ni_{3}O_{8} and a prototypical cuprate La_{2-x}Sr_{x}CuO_{4} (x=0.35). As expected, the cuprate lies deep in the charge-transfer regime of the Zaanen-Sawatzky-Allen (ZSA) scheme. The nickelate, however, is not well described by either limit of the ZSA scheme and is found to be of mixed charge-transfer–Mott-Hubbard character with the Coulomb repulsion U of similar size to the charge-transfer energy Δ. Nevertheless, the transition-metal-oxygen hopping is larger in La_{4}Ni_{3}O_{8} than in La_{2-x}Sr_{x}CuO_{4}, leading to a significant superexchange interaction and an appreciable hole occupation of the ligand O orbitals in La_{4}Ni_{3}O_{8} despite its larger Δ. Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UL, UM, UPUK
The transition metal chalcogenide Ta2 NiSe5 undergoes a second-order phase transition at Tc = 328 K involving a small lattice distortion. Below Tc, a band gap at the center of its Brillouin zone ...increases up to about 0.35 eV. In this work, we study the electronic structure of Ta2 NiSe5 in its low-temperature semiconducting phase, using resonant inelastic x-ray scattering (RIXS) at the Ni L3 edge. In addition to a weak fluorescence response, we observe a collection of intense Raman-like peaks that we attribute to electron-hole excitations. Using density functional theory calculations of its electronic band structure, we identify the main Raman-like peaks as interband transitions between valence and conduction bands. By performing angle-dependent RIXS measurements, we uncover the dispersion of these electron-hole excitations that allows us to extract the low-energy boundary of the electron-hole continuum. From the dispersion of the valence band measured by angle-resolved photoemission spectroscopy, we derive the effective mass of the lowest unoccupied conduction band.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
PDF
