Origin of band and localized electron states in photoemission of NiO

Abstract

In a variety of model studies it has been shown that the problem of a single hole in a Mott insulator can be quite well addressed by assuming that all that matters is the interaction between the propagating hole and the spin waves of the insulator. NiO has been often taken as the archetypical example of a Mott insulator and recent angular resolved photoemission studies have revealed that holes in this material share both itinerant and localized aspects that are very hard to understand either in conventional band-structure theory or from purely localized approaches. Starting from a strongly coupled electronic multiband Hubbard model, we derive a generalized strong-coupling spin-fermion model. The model includes the multiplet structure of the electronic excitations and describes the interaction of the O(2p) holes moving in oxygen bands with the spins localized on Ni ions. In linear spin-wave order we find an effective Hamiltonian describing the scattering of the bandlike holes on the spin waves. This problem is solved in rainbow order, and we find that the outcomes resemble well the experimental findings. In contrast to earlier impurity interpretations stressing spatial locality, we find that momentum dependencies are dominating the hole dynamics. I. SPIN WAVES AND PHOTOEMISSION OF NiOThe discovery of the high-T c superconductors triggered a revival in the interest of the electronic structure of the transition-metal oxides. Several interesting electronic and magnetic properties observed in these materials are caused by strong electron correlations, driven by the large Coulomb interaction U between the 3d electrons. Not surprisingly, band-structure theory fails in even the most elementary aspects. For instance, the calculations performed within local ͑spin͒ density approximation ͓L͑S͒DA͔ predict that some of the transition-metal oxides ͑such as FeO and CoO͒ are metals, while in reality they are large-gap Mott insulators. Even if the insulating character, as for NiO, is correctly reproduced, the value of the gap is too small by an order of magnitude.1 Some time ago, it seemed that this problem was basically solved in approaches that emphasized the interactions. It was assumed that the physics was essentially local; single electron momentum was supposed to be destroyed completely and instead one could limit the description to ͑atomic͒ interactions and short-range quantum delocalization. In this way the momentum integrated spectral functions could be explained in some detail.2,3 Most importantly, the so-called satellites seen in the photoemission spectra ͑spectral weight showing up at large excitation energies͒, originally discussed in terms of spectroscopic artifacts, were identified to correspond with Hubbard bands, showing that Hubbard models can be taken quite literally, even on energy scales for which they were not intended.Because this ''Hubbard band structure'' was now accessible experimentally, 4,5 surprises followed. It turned out that other bands could show up in between the 3d derived Hubbard bands-in the l...

Dependence of photoemission spectra on the charge-transfer gap inCuO2planes

“…These states are rather weakly coupled by the respective hybridization elements, and therefore almost do not contribute to the coherent QP states, as shown in Ref. 19.…”

“…Two oxygen subbands, (k) with ϭ1,2, are given by the interoxygen hopping processes, and by the three-site hopping terms which originate from the canonical transformation ͑2.4͒, and are ϳt dp /⌬ and ϳt dp /(UϪ⌬). 19 The acoustic magnons branch is given by q ϭ2J(1 Ϫ␥ q 2 ) 1/2 , with ␥ q ϭ(1/z)͚ ␦ e iq␦ (zϭ4 in the 2D case͒. The full expressions for the vertex M k,q and effective oxygen bands (k) can be found in Ref.…”

“…Integrating out the d 10 upper-and d 8 lower band we derived before an effective model for the planar oxygen states. 19 In order to make our approach more realistic in the relevant parameter regime, we incorporate in the present paper the itinerant character of Cu spins in the bare spin-fermion model by wave function corrections which are treated with the help of a Dyson equation. 21 We treat the excitations of the spin system within the linear spin-wave ͑LSW͒ theory and calculate the spectral functions in the SCBA.…”

“…To allow for a theoretical study of the entire oxygen-like valence band we formulated a spin-fermion model 18,19 where the spin of the oxygen carriers is no longer quenched. Thus, we consider below a realistic three-band Hubbard model 20 which takes into account Cu(3d x 2 Ϫy 2) and O(2 p ) states playing a dominant role in CuO 2 planes.…”

Novel mesoscale defect structure on NiO(100) surfaces by atomic force microscopy

“…Its bulk antiferromagnetic nature lends itself to use in coupled layer magnetic storage media [38], [39] and [40] and the magnetic behavior of NiO nanoparticles is suitable for spin valves important in the rapidly developing fi eld of spintronics [41], [42], [43] and [44]. NiO surface properties have been well investigated [19], [45], [46], [47], [48], [49], [50], [51], [52], [53] and [54] and the band structure [55], [56], [57], [58], [59], [60] and [61] and crystallography of the ideal surface [62] and [63] are well-known. The NiO(1 0 0) cleavage plane [45], [62], [63] and [64] can produce, to within reasonable tolerances, well-ordered, stoichiometric substrates although cleaved surfaces have been reported to contain randomly-spaced step defects, approximately 25-100 nm apart, with the step heights in multiples of 2.1 Å, the Ni-O nearest-neighbor distance [52] and [54].…”