X-ray resonant Raman scattering in NiO: Resonant enhancement of the charge-transfer excitations

Kao, C.‐C.; Caliebe, W.; Hastings, J. B.; Gillet, Jean‐Michel

doi:10.1103/physrevb.54.16361

Cited by 111 publications

(108 citation statements)

References 24 publications

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“…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].…”

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confidence: 55%

Resonant Inelastic X-Ray Scattering Study of Charge Excitations inLa2CuO4

Kim

Hill

Burns³

et al. 2002

Phys. Rev. Lett.

112

102

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We report a resonant inelastic x-ray scattering study of the dispersion relations of charge transfer excitations in insulating La2CuO4. These data reveal two peaks, both of which show two-dimensional characteristics. The lowest energy excitation has a gap energy of ∼ 2.2 eV at the zone center, and a dispersion of ∼ 1 eV. The spectral weight of this mode becomes dramatically smaller around (π, π). The second peak shows a smaller dispersion (∼ 0.5 eV) with a zone-center energy of ∼ 3.9 eV. We argue that these are both highly dispersive exciton modes damped by the presence of the electron-hole continuum.PACS numbers: 74.25.Jb, 74.72.Dn, 78.70.Ck, Understanding strongly correlated electron systems, such as the cuprate superconductors, remains at the heart of much of current condensed matter research. As a first step toward elucidating electron correlation effects in these systems, it is important to study the behavior of elementary excitations using various spectroscopic tools. For example, angle resolved photoemission spectroscopy (ARPES) has become an indispensable probe for studying the excitation spectrum of a single quasiparticle [1,2], while inelastic neutron scattering has been invaluable in the investigation of low-energy collective modes (1 ∼ 100 meV), such as phonons and magnons [3]. However, to date only limited information has been available on collective excitations at an energy scale on the order of ∼ 1 eV. This is unfortunate, since in this energy range, the electron dynamics are governed directly by the various hopping integrals and by the Coulomb interaction. Thus, an investigation of the dispersion relations of such collective charge excitations would yield invaluable information for any microscopic theory of charge dynamics in the copper oxides.In this Letter, we present a detailed study of the momentum-dependence of the charge excitations in La 2 CuO 4 , utilizing resonant inelastic x-ray scattering (RIXS). In the simplest picture of this insulating cuprate, the ground state consists of one hole per copper ion (Cu 2+ ), and the low-lying electronic excitations include excitons formed via a charge-transfer (CT) process, in which charge is moved from the oxygen onto the copper [4,5,6]. Specifically, an electron-hole pair, created by exciting an electron from the valence band -the ZhangRice (ZR) band [7] -to the conduction band across the CT gap, can form a bound exciton state as a result of the Coulomb interaction. This CT exciton is expected to have a large dispersion, since it has zero spin and can move without disturbing the antiferromagnetic order of the copper oxide plane. Consistent with this, our highresolution measurements have enabled us to identify an exciton-like feature at 2.2 eV with a dispersion of 1 eV. In addition, a second peak at slightly higher energy is also observed. This has a zone-center energy of 3.9 eV and a dispersion of 0.5 eV. Finally, we also discuss a dramatic reduction in spectral weight of the CT exciton observed near the (π π) position.The RIXS technique in the hard...

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confidence: 55%

Resonant Inelastic X-Ray Scattering Study of Charge Excitations inLa2CuO4

Kim

Hill

Burns³

et al. 2002

Phys. Rev. Lett.

112

102

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“…3 Recently, a new K-edge RIXS experiment has been carried out, at Taiwan beam line in SPring-8. 10 They have observed the spectra as a function of energy loss by tuning the incident photon energy at 8351 eV, which corresponds to the Ni K-edge absorption peak.…”

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confidence: 99%

“…2 Recently, taking advantage of strong synchrotron sources, the resonant inelastic x-ray scattering (RIXS) has become a powerful tool to probe charge excitations in solids. 3,4,5,6,7,8,9 In transition-metal compounds, K-edge resonances are widely used to observe momentum dependence, because corresponding x-rays have wavelengths the same order of lattice spacing. The process is described as a second-order optical process that the 1s core electron is prompted to an empty 4p state by absorbing photon, then charge excitations are created in order to screen the core-hole potential, and finally the photo-excited 4p electron is recombined with the core hole by emitting photon.…”

Section: Introductionmentioning

confidence: 99%

Analysis of resonant inelastic x-ray scattering at theKedge in NiO

2007

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We analyze the resonant inelastic x-ray scattering (RIXS) spectra at the Ni K edge in an antiferromagnetic insulator NiO by applying the theory developed by the present authors. It is based on the Keldysh Green's function formalism, and treats the core-hole potential in the intermediate state within the Born approximation. We calculate the single-particle energy bands within the Hartree-Fock approximation on the basis of the multi-orbital tight-binding model. Using these energy bands together with the 4p density of states from an ab initio band structure calculation, we calculate the RIXS intensities as a function of energy loss. By taking account of electron correlation within the random phase approximation (RPA), we obtain quantitative agreement with the experimental RIXS spectra, which consist of prominent two peaks around 5 eV and 8 eV, and the former shows considerable dispersion while the latter shows no dispersion. We interpret the peaks as a result of a band-to-band transition augmented by the RPA correlation.

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“…In these experiments on highly correlated electron systems, the incident x-ray energy is tuned through an absorption edge of the sample to exploit the resonant condition in the scattering [64,65]. For example, large resonant enhancement of charge transfer excitations was observed in NiO [66]. A configuration-interaction cluster model of NiO is used to interpret the scattering cross section and the strong incident energy dependence.…”

Section: Dynamic Response Of Strongly Correlated Electronic Systemsmentioning

confidence: 99%

Inelastic x-ray scattering at ultrahigh pressures

Mao

Kao

Hemley

2001

J. Phys.: Condens. Matter

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Inelastic x-ray scattering (IXS) provides high-pressure research with an arsenal of analytical capabilities for key measurements that were previously unattainable, and high-pressure research provides IXS with numerous applications where the technique has unique advantages over other methods. High-pressure investigations can now be conducted using non-resonant IXS, resonant IXS, nuclear resonant IXS and x-ray emission spectroscopy with energy resolutions of 100 meV to 1 eV for electronic transitions and 1 to 10 meV for phonon studies. By pressure-tuning materials over a wide range, we are able to investigate fundamental properties of the electron gas, strongly correlated electron systems, high-energy electronic excitations and phonons in energy and momentum space. The results have important implications for a variety of materials problem applications as well as providing basic information for understanding the deep interior of the Earth and other planets.

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X-ray resonant Raman scattering in NiO: Resonant enhancement of the charge-transfer excitations

Cited by 111 publications

References 24 publications

Resonant Inelastic X-Ray Scattering Study of Charge Excitations inLa2CuO4

Resonant Inelastic X-Ray Scattering Study of Charge Excitations inLa2CuO4

Analysis of resonant inelastic x-ray scattering at theKedge in NiO

Inelastic x-ray scattering at ultrahigh pressures

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