Resonant Inelastic X-Ray Scattering from Valence Excitations in Insulating Copper Oxides

Abstract

We report resonant inelastic x-ray measurements of insulating La2CuO4 and Sr2CuO2Cl2 taken with the incident energy tuned near the Cu K absorption edge. We show that the spectra are well described in a shakeup picture in 3rd order perturbation theory which exhibits both incoming and outgoing resonances, and demonstrate how to extract a spectral function from the raw data. We conclude by showing q-dependent measurements of the charge transfer gap.PACS numbers: 78.70. Ck, 74.25.Jb Inelastic x-ray scattering (IXS) has shown promise as a practical probe of electronic excitations in condensed matter because of its broad kinematic range and direct coupling to the electron charge. However, since x-rays are strongly absorbed in high density materials, successful applications of the technique have been limited to low-Z systems [1,2,3,4,5].Several recent studies [6,7,8,9], have shown that by that by tuning the incident energy near an x-ray absorption edge a Raman effect could be measured, despite the high absorption, because of the resonant enhancement. These studies have emphasized the role of coulomb interactions in the scattering process. Since it involves coupling between highly excited virtual states and strongly correlated valence states, it is important to characterize the resonance process well for the technique to be useful.With emphasis on systematics, we have measured resonant inelastic x-ray scattering at moderate resolution (∆E=0.9 eV) near the CuK absorption edge in the high-T c parent insulator La 2 CuO 4 (LCO) as a function of incident photon energy. Based on the changes of inelastic intensity and peak position with incident energy we show that the scattering is well described in a "shakeup" picture in 3rd order perturbation threoy [10]. We also present higher resolution measurements (∆E=0.45 eV) on another insulator, Sr 2 CuO 2 Cl 2 (SCOC), as a function of momentum transfer, q, which show some new features, such as the 2 eV optical charge transfer gap.Experiments were carried out at the X21 wiggler line at the National Synchrotron Light Source and the 3ID (SRI-CAT) undulator line at the Advanced Photon Source. At X21 the energy resolution was 0.45 eV and typical count rates were 0.4 Hz. At 3ID with 0.9 eV resolution 9 Hz was typical. The scattered light was collected with a spherical, diced, Ge(733) analyzer and imaged onto a detector. Energy analysis was done by rotating the analyzer and translating the detector in coincidence. The momentum transfer was varied by rotating the entire apparatus around the scattering center (exact experimental geometries are indicated in the figure captions). −1 parallel toĉ. The upper frame shows the raw spectra plotted against transferred energy (curves are offset for clarity). In the lower frame the open and filled circles are the inelastic peak height and position, respectively, plotted against incident energy. The black line is the fluorescence yield which peaks at the Cu1s → 4p energy.The LCO and SCOC crystals were grown by techniques described previously [11,12]....

Resonant Inelastic X-Ray Scattering Study of Charge Excitations inLa2CuO4

“…Note that both excitations have a direct gap, that is, a minimum energy at the zone center. This observation of q-dependent excitations in the 2 ∼ 4 eV range is broadly consistent with the recent RIXS studies of Ca 2 CuO 2 Cl 2 [11] and of Sr 2 CuO 2 Cl 2 [10], although in these experiments, the coarse energy resolution prevented the authors from being able to resolve any double peak features. In addition, the amount of dispersion observed here along the Cu-O bond direction (∼ 1.0 eV) is almost twice that observed for Ca 2 CuO 2 Cl 2 and in fact is close to that observed in the quasi-one-dimensional cuprate Sr 2 CuO 3 [12].…”

“…As first observed in NiO [8], large enhancement of the inelastic x-ray scattering signal is obtained when the incident x-ray energy is tuned near the transition metal K-edge. To date, a number of insulating cuprates have been studied [9,10,11,12]. While these previous studies provided detailed information on the local resonance processes, they provided only qualitative information on the momentum(Q)-dependence of the CT excitations, mainly due to poor signal-to-noise ratio.…”

Resonant inelastic x-ray scattering study of overdopedLa2−xSrxCuO4

“…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.…”

Resonant Inelastic X-Ray Scattering of the Holon-Antiholon Continuum inSrCuO2

“…We note that these three resonance energies correspond to the peaks in x-ray absorption spectra near the Cu Kedge [15]. The intensity at 6 eV likely arises from both the Cu Kβ 5 emission and a charge-transfer type excitation commonly found in the RIXS studies of cuprates [11,12]. Detailed studies of the E i -dependence will be reported elsewhere, and here we fix E i = 8982 eV, and focus on the momentum dependence of the 3 eV feature.…”

“…In addition, this gives an element specificity that is valuable in studying complex materials such as the cuprate compounds [4,11,12,13,14]. The RIXS experiments were carried out at the Advanced Photon Source on the…”