Mott gap excitations in twin-free YBa2Cu3O7-delta (Tc=93 K) studied by resonant inelastic x-ray scattering

Self-doping processes between planes and chains in the metal-to-superconductor transition of YBa2Cu3O6.9

The interplay between the quasi 1-dimensional CuO-chains and the 2-dimensional CuO2 planes of YBa(2)Cu(3)O(6+x) (YBCO) has been in focus for a long time. Although the CuO-chains are known to be important as charge reservoirs that enable superconductivity for a range of oxygen doping levels in YBCO, the understanding of the dynamics of its temperature-driven metal-superconductor transition (MST) remains a challenge. We present a combined study using x-ray absorption spectroscopy and resonant inelastic x-ray scattering (RIXS) revealing how a reconstruction of the apical O(4)-derived interplanar orbitals during the MST of optimally doped YBCO leads to substantial hole-transfer from the chains into the planes, i.e. self-doping. Our ionic model calculations show that localized divalent charge-transfer configurations are expected to be abundant in the chains of YBCO. While these indeed appear in the RIXS spectra from YBCO in the normal, metallic, state, they are largely suppressed in the superconducting state and, instead, signatures of Cu trivalent charge-transfer configurations in the planes become enhanced. In the quest for understanding the fundamental mechanism for high-Tc-superconductivity (HTSC) in perovskite cuprate materials, the observation of such an interplanar self-doping process in YBCO opens a unique novel channel for studying the dynamics of HTSC.

Lattice model of resonant inelastic x-ray scattering in metals: relation of a strong core hole to the x-ray edge singularity

We show how the classic approach of Nozières and di Domenicis for treating the edge singularity in x-ray absorption and emission can be generalized to treat the more complex case of the resonant inelastic x-ray scattering (RIXS) process, including effects of the intermediate states involved therein in the presence of the core hole. We solve our lattice model essentially exactly (numerically) to obtain a novel form of edge singularity at the RIXS threshold energy. Our RIXS spectrum naturally includes both the well and poorly screened spectral components and their dispersions and allows its separation into pair and multiple-pair excitations.

Real-space visualization of remnant Mott gap and magnon excitations

We demonstrate the ability to visualize real-space dynamics of charge gap and magnon excitations in the Mott phase of the single-band Hubbard model and the remnants of these excitations with hole or electron doping. At short times, the character of magnetic and charge excitations is maintained even for large doping away from the Mott and antiferromagnetic phases. Doping influences both the real-space patterns and long timescales of these excitations with a clear carrier asymmetry attributable to particle-hole symmetry breaking in the underlying model. Further, a rapidly oscillating charge-density-wave-like pattern weakens, but persists as a visible demonstration of a subleading instability at half-filling which remains upon doping. The results offer an approach to analyzing the behavior of systems where momentum space is either inaccessible or poorly defined.

Charge excitations in cuprate and nickelate in resonant inelastic x-ray scattering

We analyze the resonant inelastic x-ray scattering (RIXS) spectra at the Cu and Ni K-edges in La(2)CuO(4) and La(2)NiO(4), respectively. We make use of the Keldysh-Green function formalism, in which the RIXS intensity is described by a product of the incident-photon-dependent factor and the density-density correlation function in the 3d states. The former factor is calculated using the 4p density of states given by an ab initio band structure calculation and the latter using the wavefunctions given by a Hartree-Fock calculation of a multiorbital tight-binding model. The initial state is described within the Hartree-Fock approximation and the electron correlations on charge excitations are treated within the random phase approximation. The calculated RIXS spectra reproduce well several characteristic features in the experiments. Although several groups have interpreted the RIXS peaks as bound excitons, our calculation indicates that they should be interpreted as band-to-band excitations augmented by electron correlations. The difference in RIXS spectra between La(2)CuO(4) and La(2)NiO(4) is explained from this point of view.