Guillaume Chabot-Couture

Demonstrating the Model Nature of the High-Temperature Superconductor HgBa2CuO4+d

The compound HgBa{sub 2}CuO{sub 4+{delta}} (Hg1201) exhibits a simple tetragonal crystal structure and the highest superconducting transition temperature (T{sub c}) among all single Cu-O layer cuprates, with T{sub c} = 97 K (onset) at optimal doping. Due to a lack of sizable single crystals, experimental work on this very attractive system has been significantly limited. Thanks to a recent breakthrough in crystal growth, such crystals have now become available. Here, we demonstrate that it is possible to identify suitable heat treatment conditions to systematically and uniformly tune the hole concentration of Hg1201 crystals over a wide range, from very underdoped (T{sub c} = 47 K, hole concentration p {approx} 0.08) to overdoped (T{sub c} = 64 K, p {approx} 0.22). We then present quantitative magnetic susceptibility and DC charge transport results that reveal the very high-quality nature of the studied crystals. Using XPS on cleaved samples, we furthermore demonstrate that it is possible to obtain large surfaces of good quality. These characterization measurements demonstrate that Hg1201 should be viewed as a model high-temperature superconductor, and they provide the foundation for extensive future experimental work.

Incident energy and polarization-dependent resonant inelastic x-ray scattering study of La2CuO4

We present a detailed Cu K-edge resonant inelastic X-ray scattering (RIXS) study of the Mott insulator La2CuO4 in the 1-7 eV energy transfer range. As initially found for the high-temperature superconductor HgBa2CuO4+d, the spectra exhibit a multiplet of weakly-dispersive electron-hole excitations, which are revealed by utilizing the subtle dependence of the cross section on the incident photon energy. The close similarity between the fine structures for in-plane and out-of-plane polarizations is indicative of the central role played by the 1s core hole in inducing charge excitations within the CuO2 planes. On the other hand, we observe a polarization dependence of the spectral weight, and careful analysis reveals two separate features near 2 eV that may be related to different charge-transfer processes. The polarization dependence indicates that the 4p electrons contribute significantly to the RIXS cross section. Third-order perturbation arguments and a shake-up of valence excitations are then applied to account for the final-energy resonance in the spectra. As an alternative scenario, we discuss fluorescence-like emission processes due to 1s ->4p transitions into a narrow continuum 4p band.

Charge-transfer excitations in the model superconductor HgBa2CuO4+δ

We report a Cu -edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2-8 eV range in the structurally simple compound HgBa(2)CuO(4+delta) at optimal doping (T(c)=96.5 K). The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly dispersive (<0.5 eV) electron-hole excitations, including a mode at 2 eV. The observation of this 2 eV excitation suggests the existence of a remnant charge-transfer gap deep in the superconducting phase. Quite generally, our results, which include additional data for the Mott insulator La(2)CuO(4), demonstrate the importance of exploring the incident photon-energy dependence of the RIXS cross section.

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

The results of model calculations using exact diagonalization reveal the orbital character of states associated with different Raman loss peaks in Cu K-edge resonant inelastic x-ray scattering (RIXS) from La₂CuO₄. The model includes electronic orbitals necessary to highlight the nonlocal Zhang-Rice singlet, charge transfer, and d-d excitations, as well as states with apical oxygen 2p(z) character. The dispersion of these excitations is discussed with prospects for resonant final state wave-function mapping. A good agreement with experiments emphasizes the substantial multiorbital character of RIXS profiles in the energy transfer range 1-6 eV.

Polarization dependence and symmetry analysis in indirect K -edge RIXS

We present a study of the charge-transfer excitations in undoped Nd2CuO4 using resonant inelastic x-ray scattering RIXS at the Cu K-edge. At the Brillouin zone center, azimuthal scans that rotate the incidentphoton polarization within the CuO2 planes reveal weak fourfold oscillations. A comparison of spectra taken in different Brillouin zones reveals a spectral weight decrease at high-energy loss from forward- to backscattering. We show that these are scattered-photon polarization effects related to the properties of the observed electronic excitations. Each of the two effects constitutes about 10% of the inelastic signal while the “4p-as-spectator” approximation describes the remaining 80%. Raman selection rules can accurately model our data, and we conclude that the observed polarization-dependent RIXS features correspond to Eg and B1g charge-transfer excitations to non-bonding oxygen 2p bands, above 2.5 eV energy-loss, and to an Eg d →d excitation at 1.65 eV.

Lattice coupling and Franck?Condon effects in K-edge resonant inelastic x-ray scattering

In this paper, we discuss our recent Cu K-edge resonant inelastic x-ray scattering (RIXS) results for CuB2O4, a model system consisting of electronically isolated CuO4 plaquettes, which exhibits anomalies in the incident photon energy dependence that are not captured by electron-only models. We show that these anomalous features can be qualitatively described by extending the electron-only considerations to include the lattice degrees of freedom. Our findings have important general implications for the interpretation of K-edge RIXS results for electronically higher-dimensional transition metal oxides.

Resonant inelastic x-ray scattering in electronically quasi-zero-dimensional CuB 2 O 4

We present a resonant inelastic scattering (RIXS) study using of

Crystal Growth and Characterization of the Model High‐Temperature Superconductor HgBa2CuO4+δ

{\text{CuB}}_{2}{\text{O}}_{4}

Erratum: Publisher's Note: Resonant inelastic x-ray scattering in electronically quasi-zero-dimensional CuB2O4 Physical Review B - Condensed Matter and Materials Physics (2009) 80 (092509))

, which contains a lattice of

Universality of momentum-dependent charge-transfer excitations in the undoped cuprates measured by resonant inelastic X-ray scattering

{\text{CuO}}_{4}