Large-shift Raman scattering in insulating parent compounds of cuprate superconductors

Salamon, David; Liu, Ran; Klein, Miles V.; Karlow, M. A.; Cooper, S. L.; Cheong, Sang‐Wook; Lee, W. C.; Ginsberg, D. M.

doi:10.1103/physrevb.51.6617

Cited by 48 publications

(51 citation statements)

References 53 publications

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“…44,45 Although for Nd 2 CuO 4 only the A 2g (d x 2 −y 2 → d xy ) Cu crystal-field excitation has been observed around 1.4 eV, 43 the crystal-field excitations of tetragonal Sr 2 CuO 2 Cl 2 have been extensively studied 10,11,42,44 and their energies should be similar to those of Nd 2 CuO 4 . For the latter, Raman selection rules suggest that the resolution-limited feature we observe at 1.65 eV (Fig.…”

Section: Discussionmentioning

confidence: 99%

“…In principle, d → d excitations provide a testing ground for the validity of conventional Raman selection rules within RIXS, and such excitations have been well studied with soft X-ray RIXS at the Cu L- [8][9][10]40,41 and M-edges, 11 with optical absorption, 42 with large-shift Raman scattering, 43 and with different theoretical methods. 44,45 Although for Nd 2 CuO 4 only the A 2g (d x 2 −y 2 → d xy ) Cu crystal-field excitation has been observed around 1.4 eV, 43 the crystal-field excitations of tetragonal Sr 2 CuO 2 Cl 2 have been extensively studied 10,11,42,44 and their energies should be similar to those of Nd 2 CuO 4 .…”

Section: Discussionmentioning

confidence: 99%

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Polarization dependence and symmetry analysis in indirectK-edge RIXS

et al. 2010

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We present a study of the charge-transfer excitations in undoped Nd 2 CuO 4 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 CuO 2 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 back-scattering. 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 E g and B 1g charge-transfer excitations to non-bonding oxygen 2p bands, above 2.5 eV energy-loss, and to an E g d → d excitation at 1.65 eV.Raman scattering and optical spectroscopy have enabled tremendous contributions to the study of condensed matter systems. Both probes use ∼ 1 eV light and are limited to essentially zero momentum transfer. In order to investigate the charge response of a material throughout the Brillouin zone, photons in the X-ray regime must be used. 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, 1-5 this resonant technique is relatively new and there is still much to learn about the details of its cross section.One of the strengths of conventional Raman scattering derives from the fact that the technique allows the determination of the symmetry of excitations by selecting the polarization of the incident and scattered photons. Only polarization dependent soft (or direct 6 ) X-ray RIXS studies, at the O K-edge or the Cu L-and M-edges of the cuprates, 7-11 have shown that photon polarization can be used to select different electronic excitations. In these cases, the photon polarization effects are understood to come from the direct interaction of the photoelectron, excited into the valence system, with the valence electrons. This is why this type of RIXS is commonly referred to as direct. 6 On the other hand, tuning the incident energy to the Cu K-edge excites the photoelectron into the Cu 4p band approximately 10-20 eV above the 3d valence levels. The relatively large separation in energy from the valence levels as well as the large spatial extent of the 4p orbital are believed to prevent the photoelectron from interacting with the valence system, and this type of RIXS is in turn referred to as indirect. 6 Accordingly, theoretical models of this type of RIXS accordingly do not include possible interactions of the 4p photoelectron with the valence system, 12-15 which is commonly referred to as the '4p-as-spectator' approximation.RIXS investigations of ...

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Polarization dependence and symmetry analysis in indirectK-edge RIXS

et al. 2010

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“…Experimentally, optical spectroscopy and Raman scattering studies have been carried out to address the nature of the excitation spectrum near this CT gap [10,11,12,13]. Ohana et al directly observed the coupling of the electronic excitations to the lattice system in the resonance of phonon Raman lines at laser energies around the CT energy (2.14 eV) and a low-energy shoulder at 1.9 eV [10].…”

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confidence: 99%

“…We note that charge transfer with the surrounding O2p states of the same symmetry would also be involved [8,13]. This peak assignment is consistent with earlier studies using different experimental techniques, such as large-shift Raman scattering [13], Cu M-edge RIXS [24], and L-edge RIXS [26]. In addition, the 1.8 eV shoulder was also observed near the zone boundary in a recent K-edge RIXS study [20], and likewise attributed to a crystal field excitation.…”

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confidence: 99%

“…The strong magnetic interaction in this system has been pointed to as the origin of the exciton binding energy [2] as well as the large dispersion of the excitation [4]. Indeed the apparently large dispersion observed in Sr 2 CuO 2 Cl 2 using electron energy loss spectroscopy (EELS) was attributed to a CT exciton with a small effective mass [9].Experimentally, optical spectroscopy and Raman scattering studies have been carried out to address the nature of the excitation spectrum near this CT gap [10,11,12,13]. Ohana et al directly observed the coupling of the electronic excitations to the lattice system in the resonance of phonon Raman lines at laser energies around the CT energy (2.14 eV) and a low-energy shoulder at 1.9 eV [10].…”

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Charge-transfer exciton inLa2CuO4probed with resonant inelastic x-ray scattering

et al. 2008

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We report a high-resolution resonant inelastic x-ray scattering study of La2CuO4. A number of spectral features are identified that were not clearly visible in earlier lower-resolution data. The momentum dependence of the spectral weight and the dispersion of the lowest energy excitation across the insulating gap have been measured in detail. The temperature dependence of the spectral features was also examined. The observed charge transfer edge shift, along with the low dispersion of the first charge transfer excitation are attributed to the lattice motion being coupled to the electronic system. In addition, we observe a dispersionless feature at 1.8 eV, which is associated with a d-d crystal field excitation.PACS numbers: 78.70. Ck, 74.25.Jb, As the prototypical parent insulating compound, La 2 CuO 4 has drawn much attention as a starting point for the study of high-temperature superconductors. La 2 CuO 4 is well described as an antiferromagnetic insulator with a d x 2 −y 2 hole localized at the Cu site due to the strong electron correlations. In particular, this material is classified as a charge-transfer (CT) insulator, since the lowest energy charge excitation across the insulating gap corresponds to transferring an electron from oxygen to a neighboring copper. This CT excitation creates a Cu 1+ ion and an oxygen 2p hole that forms a ZhangRice singlet (ZRS)with the neighboring copper spin, the two copper sites forming a bound state [1]. Over the years, exciton formation of these electron-hole pairs has been considered by several authors as a possible low energy elementary excitation in this half-filled copper oxide plane [2,3,4,5,6,7,8]. The strong magnetic interaction in this system has been pointed to as the origin of the exciton binding energy [2] as well as the large dispersion of the excitation [4]. Indeed the apparently large dispersion observed in Sr 2 CuO 2 Cl 2 using electron energy loss spectroscopy (EELS) was attributed to a CT exciton with a small effective mass [9].Experimentally, optical spectroscopy and Raman scattering studies have been carried out to address the nature of the excitation spectrum near this CT gap [10,11,12,13]. Ohana et al. directly observed the coupling of the electronic excitations to the lattice system in the resonance of phonon Raman lines at laser energies around the CT energy (2.14 eV) and a low-energy shoulder at 1.9 eV [10]. Falck et al. found that a polaron model was able to reproduce their reflectivity data very well for a wide temperature range [11]. However, despite extensive experimental and theoretical studies to date, the nature of the CT exciton is still controversial. Outstanding questions include whether the electron-hole pair actually form a bound exciton state, or if they remain as a resonance state within the particle-hole continuum. Another question relates to the relative role played by magnetic interactions and/or phonons in the exciton formation and dispersion. For each of these questions, valuable information can be gained by studying the momentum d...

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