Unraveling the nature of charge excitations in La₂CuO₄ with momentum-resolved Cu K-edge resonant inelastic x-ray scattering

Imaging the atomic-scale electronic states induced by a pair of hole dopants in Ca2CuO2Cl2 Mott insulator

We use scanning tunneling microscopy to visualize the atomic-scale electronic states induced by a pair of hole dopants in Ca₂CuO₂Cl₂ parent Mott insulator of cuprates. We find that when the two dopants approach each other, the transfer of spectral weight from high energy Hubbard band to low energy in-gap state creates a broad peak and nearly V-shaped gap around the Fermi level. The peak position shows a sudden drop at distance around 4 a₀ and then remains almost constant. The in-gap states exhibit peculiar spatial distributions depending on the configuration of the two dopants relative to the underlying Cu lattice. These results shed important new lights on the evolution of low energy electronic states when a few holes are doped into parent cuprates.

Exploring itinerant states in divalent hexaborides using rare-earth L edge resonant inelastic x-ray scattering

We present a study of resonant inelastic x-ray scattering (RIXS) spectra collected at the rare-earth L edges of divalent hexaborides YbB₆ and EuB₆. In both systems, RIXS-active features are observed at two distinct resonances separated by [Formula: see text] eV in incident energy, with angle-dependence suggestive of distinct photon scattering processes. RIXS spectra collected at the divalent absorption peak resemble the unoccupied 5d density of states calculated using density functional theory. We discuss possible origins of this correspondence including a scenario which changes the 4f  valence. In addition, anomalous resonant scattering is observed at higher incident energy, where no corresponding absorption feature is present. Our results demonstrate the potential for L-edge RIXS to assess the itinerant-state properties of f -electron materials.

Nodeless High-T_{c} Superconductivity in the Highly Overdoped CuO_{2} Monolayer

We study the electronic structure and superconductivity in a CuO_{2} monolayer grown recently on the d-wave cuprate superconductor Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}. Density functional theory calculations indicate a significant charge transfer across the interface such that the CuO_{2} monolayer is heavily overdoped into the hole-rich regime yet inaccessible in bulk cuprates. We show that both the Cu d_{x^{2}-y^{2}} and d_{3z^{2}-r^{2}} orbitals become important and the Fermi surface contains one electron and one hole pocket associated with the two orbitals, respectively. Constructing a minimal correlated two-orbital model for the e_{g} complex, we show that the spin-orbital exchange interactions produce a nodeless superconductor with extended s-wave pairing symmetry and a pairing energy gap comparable to the bulk d-wave gap, in agreement with recent experiments. The findings point to a direction of realizing new high-T_{c} superconductors in ozone grown transition-metal-oxide monolayer heterostructures.

Computing Resonant Inelastic X-Ray Scattering Spectra Using The Density Matrix Renormalization Group Method

We present a method for computing the resonant inelastic x-ray scattering (RIXS) spectra in one-dimensional systems using the density matrix renormalization group (DMRG) method. By using DMRG to address this problem, we shift the computational bottleneck from the memory requirements associated with exact diagonalization (ED) calculations to the computational time associated with the DMRG algorithm. This approach is then used to obtain RIXS spectra on cluster sizes well beyond state-of-the-art ED techniques. Using this new procedure, we compute the low-energy magnetic excitations observed in Cu L-edge RIXS for the challenging corner shared CuO4 chains, both for large multi-orbital clusters and downfolded t-J chains. We are able to directly compare results obtained from both models defined in clusters with identical momentum resolution. In the strong coupling limit, we find that the downfolded t-J model captures the main features of the magnetic excitations probed by RIXS only after a uniform scaling of the spectra is made.

Electron-lattice interactions strongly renormalize the charge-transfer energy in the spin-chain cuprate Li2CuO2

Strongly correlated insulators are broadly divided into two classes: Mott-Hubbard insulators, where the insulating gap is driven by the Coulomb repulsion U on the transition-metal cation, and charge-transfer insulators, where the gap is driven by the charge-transfer energy Δ between the cation and the ligand anions. The relative magnitudes of U and Δ determine which class a material belongs to, and subsequently the nature of its low-energy excitations. These energy scales are typically understood through the local chemistry of the active ions. Here we show that the situation is more complex in the low-dimensional charge-transfer insulator Li2CuO2, where Δ has a large non-electronic component. Combining resonant inelastic X-ray scattering with detailed modelling, we determine how the elementary lattice, charge, spin and orbital excitations are entangled in this material. This results in a large lattice-driven renormalization of Δ, which significantly reshapes the fundamental electronic properties of Li2CuO2.

A multiplexed high-resolution imaging spectrometer for resonant inelastic soft X-ray scattering spectroscopy

The optical design of a two-dimensional imaging soft X-ray spectrometer is described. A monochromator will produce a dispersed spectrum in a narrow vertical illuminated stripe (∼2 µm wide by ∼2 mm tall) on a sample. The spectrometer will use inelastically scattered X-rays to image the extended field on the sample in the incident photon energy direction (vertical), resolving the incident photon energy. At the same time it will image and disperse the scattered photons in the orthogonal (horizontal) direction, resolving the scattered photon energy. The principal challenge is to design a system that images from the flat-field illumination of the sample to the flat field of the detector and to achieve sufficiently high spectral resolution. This spectrometer provides a completely parallel resonant inelastic X-ray scattering measurement at high spectral resolution (∼30,000) over the energy bandwidth (∼5 eV) of a soft X-ray absorption resonance.

Single-band model of resonant inelastic x-ray scattering by quasiparticles in high-T(c) cuprate superconductors

We show that a simple model of noninteracting quasiparticles accurately describes resonant inelastic x-ray scattering (RIXS) experiments in the hole-doped cuprate superconductors. Band structure alone yields signatures previously attributed to collective magnetic modes, such as the dispersing peaks and nontrivial polarization dependence found in several experiments. We conclude that RIXS data can be explained without positing the existence of magnetic excitations that persist with increasing doping. In so doing we develop a formalism for RIXS in itinerant electron systems that accounts for the positively charged core hole exactly and discover a mechanism by which the core hole produces polarization dependence mimicking that of a magnetic system.

Determining the short-range spin correlations in the spin-chain Li2CuO2 and CuGeO3 compounds using resonant inelastic x-ray scattering

We report a high-resolution resonant inelastic soft x-ray scattering study of the quantum magnetic spin-chain materials Li(2)CuO(2) and CuGeO(3). By tuning the incoming photon energy to the oxygen K edge, a strong excitation around 3.5 eV energy loss is clearly resolved for both materials. Comparing the experimental data to many-body calculations, we identify this excitation as a Zhang-Rice singlet exciton on neighboring CuO(4) plaquettes. We demonstrate that the strong temperature dependence of the inelastic scattering related to this high-energy exciton enables us to probe short-range spin correlations on the 1 meV scale with outstanding sensitivity.