Charge-Transfer Excitations in the Model SuperconductorHgBa2CuO4+δ

Abstract

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.

Polarization dependence and symmetry analysis in indirectK-edge RIXS

“…35,36 Furthermore, calculated RIXS spectra using one-band Hubbard models are automatically limited to states of x 2 − y 2 local symmetry (the ZRS combination of oxygen orbitals is an x 2 − y 2 combination of the O p σ orbitals), so that the symmetry of the chargetransfer excitations is limited to A 1g at Q = 0. 37 On the other hand, non-A 1g transitions have been suggested to explain new features in measured RIXS spectra: charge-transfers to nonbonding bands 4 and local d → d excitations 38 are both examples of such non-A 1g transitions. While it remains unclear if the polarization-based Raman selection rules can be applied to interpret indirect RIXS spectra at the Brillouin zone center (zero reduced q), we test their applicability by comparing their predictions with our data.…”

“…Charge-transfers from the non-bonding oxygen bands to the upper Hubbard band, which have been used to explain the multiplet of inelastic features between 2 and 6 eV, 4 are expected to be of non-A 1g symmetry. As studied with ARPES, [46][47][48] O 2p non-bonding bands have approximately 1.5 eV more binding energy than the ZRS band.…”

“…X-ray Raman scattering, more commonly referred to as resonant inelastic X-ray scattering (RIXS), allows the measurement of the momentum dependence of charge excitations. Even though it has successfully been used to study the physics of a wide array of systems, [1][2][3][4][5] this resonant technique is relatively new and there is still much to learn about the details of its cross section.…”

Resonant inelastic x-ray scattering study of overdopedLa2−xSrxCuO4

“…7 Recently, there has been a number of RIXS studies of doped cuprate superconductors. 6,7,8,11 For the La 2−x Sr x CuO 4 (LSCO) system, Kim et al 6 showed that the two features of the CT excitations in LCO are also observed for x = 0.05, but only a single feature is observed at ∼ 4 eV for x = 0.17. In addition, a continuum-like excitation appears for x = 0.17 below the charge transfer excitation energy (ω < 2 eV) at the zone center (0 0) and at the zone boundary (π 0).…”

“…This technique was initially applied to NiO; 1 in that case a large enhancement of the inelastic signal with incident photon energy was observed near the Ni K edge. More recently this technique has been applied to various cuprates including high-T c materials 2,3,4,5,6,7,8,9,10,11 and other highly-correlated electron systems. 12,13,14 A recent RIXS study near the Cu K edge in undoped La 2 CuO 4 (LCO) has revealed three features in its charge excitation spectrum.…”