Experimental signatures of phase interference and subfemtosecond time dynamics on the incident energy axis of resonant inelastic x-ray scattering

Abstract

Core hole resonance is used in X-ray spectroscopy to incisively probe the local electronic states of many-body systems. Here, resonant inelastic X-ray scattering (RIXS) is studied as a function of incident photon energy on Mott insulators SrCuO2 and NiO to examine how resonance states decay into different excitation symmetries at the transition metal M-, L-and K-edges. Quantum interference patterns characteristic of the two major RIXS mechanisms are identified within the data, and used to distinguish the attosecond scale scattering dynamics by which fundamental excitations of a many-body system are created. A function is proposed to experimentally evaluate whether a particular excitation has constructive or destructive interference in the RIXS cross-section, and corroborates other evidence that an anomalous excitation is present at the leading edge of the Mott gap in quasi-one dimensional SrCuO2.

Extending resonant inelastic X-ray scattering to the extreme ultraviolet

“…The principle electronic excitations observed in Xray absorption spectroscopy (XAS) and RIXS with p-symmetry core holes on Co [7,16], Ni [6,8,14,15,17,18,36], and Cu [8,9] are closely identified with excitation matrix elements obtained from AM based calculations for a high-spin 2+ valence atomic state perturbed by the crystal field of surrounding oxygen atoms. The compounds CoO and NiO have a rock salt crystal structure, with an octahedral crystal field on the transition metal site, and strong antiferromagnetic superexchange interactions through σ -bonded oxygen atoms.…”

“…The Mott insulators CoO, NiO, and SrCuO 2 were chosen as model materials for this study, and provide examples of resonance from strongly correlated 3d 7 , 3d 8 , and 3d 9 systems, respectively. The principle electronic excitations observed in Xray absorption spectroscopy (XAS) and RIXS with p-symmetry core holes on Co [7,16], Ni [6,8,14,15,17,18,36], and Cu [8,9] are closely identified with excitation matrix elements obtained from AM based calculations for a high-spin 2+ valence atomic state perturbed by the crystal field of surrounding oxygen atoms.…”

“…The Cu atoms of SrCuO 2 are found in a quasi-1D [11,13,21,37] network of copper oxide plaquettes that lack apical oxygens. The ground state includes a single hole in the 3d x 2 −y 2 orbital [36], and the energies of dd crystal field states are known from previous RIXS measurements [8].…”

“…Shake-up excitations include double magnons (see Discussion in Bisogni et al [28]) and charge transfer excitations at the M-and L-edges (see Figure 4 below), and can have a distinctive pattern of incident energy dependence in RIXS spectra [8]. These excitations have a fractional contribution to scattering intensity in the RIXS spectrum that scales roughly as I ∼ K 2 / Ŵ 2 when core hole lifetime is short (|K/ Ŵ| ≪ 1) due to destructive interference in the numerator of the Kramers-Heisenberg scattering equation (see discussion of "indirect" RIXS in Ament et al [4]).…”

“…The lifetime of shallow core hole resonance states accessed in the EUV has a complex dependence on incident photon energy [6,[31][32][33], and can provide the option to access both long and short core hole lifetimes at the same resonance edge. The effective shake-up perturbation caused by shallow core holes also tends to be weaker, and can exclude some classes of excitations that are not directly created by the action of the scattering photon operators (e.g., multi-magnons, phonons, charge transfer modes) [3,4,7,8,10,28,34,35]. Atomic multiplet (AM) models of RIXS can potentially work better in the EUV, because angular momentum entanglement through multipolar Coulomb interactions is stronger than at the L-edge, and protects multiplet symmetries.…”