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Register a new userObservation of a dispersive charge mode in hole-doped cuprates using resonant inelastic x-ray scattering at the oxygen K edge
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We investigate electronic excitations in La2-x(Br,Sr)xCuO4 using resonant inelastic x-ray scattering (RIXS) at the oxygen K edge. RIXS spectra of the hole-doped cuprates show clear momentum dependence below 1 eV. The spectral weight exhibits positive dispersion and shifts to higher energy with increasing hole concentration. Theoretical calculation of the dynamical charge structure factor on oxygen orbitals in a three-band Hubbard model is consistent with the experimental observation of the momentum and doping dependence, and therefore the dispersive mode is ascribed to intraband charge excitations which have been observed in electron-doped cuprates.
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We present calculations for resonant inelastic x-ray scattering (RIXS) in edge-shared copper oxide systems, such as CuGeO$_{3}$ and Li$_{2}$CuO$_{2}$, appropriate for hard x-ray scattering where the photoexcited electron lies above oxygen 2p and copper 3d orbital energies. We perform exact diagonalizations of the multi-band Hubbard and determine the energies, orbital character and resonance profiles of excitations which can be probed via RIXS. We find excellent agreement with recent results on Li$_{2}$CuO$_{2}$ and CuGeO$_{3}$ in the 2-7 eV photon energy loss range.
Resonant inelastic X-ray scattering (RIXS) experiments performed at the oxygen-$K$ edge on the iridate perovskites {SIOS} and {SION} reveal a sequence of well-defined dispersive modes over the energy range up to $sim 0.8$ eV. The momentum dependence of these modes and their variation with the experimental geometry allows us to assign each of them to specific collective magnetic and/or electronic excitation processes, including single and bi-magnons, and spin-orbit and electron-hole excitons. We thus demonstrated that dispersive magnetic and electronic excitations are observable at the O-$K$ edge in the presence of the strong spin-orbit coupling in the $5d$ shell of iridium and strong hybridization between Ir $5d$ and O $2p$ orbitals, which confirm and expand theoretical expectations. More generally, our results establish the utility of O-$K$ edge RIXS for studying the collective excitations in a range of $5d$ materials that are attracting increasing attention due to their novel magnetic and electronic properties. Especially, the strong RIXS response at O-$K$ edge opens up the opportunity for investigating collective excitations in thin films and heterostructures fabricated from these materials.
We report a comprehensive Cu $K$-edge RIXS investigation of $rm La_{2-x}Sr_xCuO_4$ (LSCO) for 0$leq$x$leq$0.35, stripe-ordered $rm La_{1.875}Ba_{0.125}CuO_4$ (LBCO), and $rm La_{2}Cu_{0.96}Ni_{0.04}O_4$ (LCNO) crystals. The RIXS spectra measured at three high-symmetry momentum transfer (textbf{q}) positions are compared as a function of doping and for the different dopants. The spectra in the energy range 1-6 eV can be described with three broad peaks, which evolve systematically with increased doping. The most systematic trend was observed for textbf{q}=($pi$, 0) corresponding to the zone boundary. As hole doping increased, the spectral weight transfer from high energies to low energies is nearly linear with emph{x} at this textbf{q}. We interpret the peaks as interband transitions in the context of existing band models for this system, assigning them to Zhang-Rice band$rightarrow$upper Hubbard band, lower-lying band$rightarrow$upper Hubbard band, and lower-lying band$rightarrow$Zhang-Rice band transitions. The spectrum of stripe-ordered LBCO was also measured, and found to be identical to the correspondingly doped LSCO, except for a relative enhancement of the near-infrared peak intensity around 1.5-1.7 eV. The temperature dependence of this near-infrared peak in LBCO was more pronounced than for other parts of the spectrum, continuously decreasing in intensity as the temperature was raised from 25 K to 300 K. Finally, we find that 4% Ni substitution in the Cu site has a similar effect on the spectra as does Sr substitution in the La site.
We have performed resonant inelastic x-ray scattering (RIXS) near the Cu-K edge on cuprate superconductors La(2-x)Sr(x)CuO(4), La(2-x)Ba(x)CuO(4), La(2-x)Sr(x)Cu(1-y)Fe(y)O(4) and Bi(1.76)Pb(0.35)Sr(1.89)CuO(6+d), covering underdoped to heavily overdoped regime and focusing on charge excitations inside the charge-transfer gap. RIXS measurements of the 214 systems with Ei = 8.993 keV have revealed that the RIXS intensity at 1 eV energy transfer has a minimum at (0,0) and maxima at (0.4pi, 0) and $(0, 0.4pi) for all doping points regardless of the stripe ordered state, suggesting that the corresponding structure is not directly related to stripe order. Measurements with Ei = 9.003 keV on metallic La(1.7)Sr(0.3)CuO(4) and Bi(1.76)Pb(0.35)Sr(1.89)CuO(6+d) exhibit a dispersive intra-band excitation below 4 eV, similar to that observed in the electron-doped Nd(1.85)Ce(0.15)CuO(4). This is the first observation of a dispersive intra-band excitation in a hole doped system, evidencing that both electron and hole doped systems have a similar dynamical charge correlation function.
We performed a resonant inelastic X-ray scattering (RIXS) study of La$_{2-x}$Sr$_{x}$NiO$_{4+{delta}}$ (LSNO) at the oxygen $K$ edge to investigate the nature of the doped holes with regard to charge excitations. Charge excitations of the hole-doped nickelates are found to be almost independent of momentum transfer, indicating that the doped holes are strongly localized in character. Additionally, conspicuous changes in energy position are in temperature dependence. These characters are observed in stark contrast to those of the high-$T_{c}$ cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO), where delocalized doped holes form charge excitations with sizable momentum dependence in the CuO$_2$ plane. This distinct nature of charge excitations of doped holes is consistent with the metallicity of the materials and could be caused by strong electron-phonon coupling and weak quantum spin fluctuation in the nickelates.
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