Dispersion anomalies induced by the low-energy plasmon in the cuprates

Abstract

We discuss the characteristic effects of the electron plasmon interaction resulting from the ∼ 1 eV plasmon, which is a universal feature in the cuprates. Using the framework of a one-band tight binding model, we identify signatures of this low energy plasmon in the electronic structure of metallic overdoped Bi2212 as well as half-filled insulating SCOC. The electron-plasmon interaction is found to yield renormalizations near the Fermi energy in reasonable accord with experimental observations, and to produce dispersion anomalies at higher energies.PACS numbers: 71.38.Cn, 71.45.Gm Quasiparticle dispersions near the Fermi energy in the cuprates are renormalized (reduced) by a factor of Z ∼ 0.3 − 0.6[1] in comparison to the band theory predictions based on the conventional LDA picture, interrupted by the appearance of low energy 'kinks' or dispersion anomalies in the 50-70 meV energy range, which arise from the coupling of the electronic system with phonons[2] and/or magnetic modes [3]. Interestingly, the 'spaghetti' of various hybridized and unhybridized Cu and O bands starting around 1 eV in the cuprates seems to remain more or less unrenormalized. [4] Recalling that optical [5] and electron energy loss spectroscopy (EELS) experiments [6,7] have shown the presence of dispersive plasmons in the cuprates lying at ∼1 eV, we explore the effects of such a low energy plasmon in this Letter. The plasmon is found to induce renormalizations in the low energy regime in both the metallic and the insulating cuprates, which are in substantial accord with the corresponding experimental results. We emphasize that plasmons in 3D materials typically lie at ∼10 eV and therefore have little influence on the electronic states in the 0-1 eV range.We approach the problem within the framework of a one-band tight-binding model and consider the limiting cases of a metal with the example of overdoped Bi 2 Sr 2 CaCu 2 O 8 (Bi2212) [8,9] and the insulator with the example of half-filled Sr 2 CuO 2 Cl 2 (SCOC) [10]. The dielectric functions computed via the random phase approximation (RPA) yield loss functions and the associated plasmon dispersions in both Bi2212 and SCOC, which are in reasonable accord with the corresponding EELS data. The generic effects of the electron-plasmon interaction on the electronic structure and how the low energy plasmon in the metal and the insulator induces characteristic dispersion anomalies and renormalizations of the spectrum are then delineated via a first order calculation of the electronic self energy.We discuss first the metallic case of overdoped Bi2212. Here the magnetic gap has collapsed fully and we take the bare electronic dispersion to be given by a one-band tight-binding model fitted to the LDA-based band structure of Bi2212 in the vicinity of the Fermi energy (E F ) as follows[1]:where c α (na) = cos(nk α a) for α = x or y, n is an integer, and a is the in-plane lattice constant. Hopping parameters appropriate for Bi2212 (neglecting bilayer splitting) [1] are: t=360 meV, t ′ = −100 meV, t ′′...

High-Energy Anomaly in the Angle-Resolved Photoemission Spectra ofNd2−xCexCuO4:

“…This difference was interpreted in terms of a shift of the chemical potential [14]. The experimental studies were accompanied by numerous theoretical papers [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31].…”

“…For the phenomenon of the HEA, a number of explanations have been suggested including Mott-Hubbard models with a transition from the coherent quasi-particle dispersion to the incoherent lower Hubbard band [5,18,21,26,32], a disintegration of the low-energy branch into a holon and spinon band due to a spin charge separation [2], a coupling to spin fluctuations [9,17,19,29,33], a coupling to phonons [7], string excitations of spinpolarons [32], a bifurcation of the quasi-particle band due to an excitation of a bosonic mode of charge 2e [22], and a coupling to plasmons [16]. These are all intrinsic interpretations in terms of many-body interactions leading to a change of the spectral function.…”

Insights on the cuprate high energy anomaly observed in ARPES

“…[7] However, the shallow, dispersing band near the Fermi level, together with the cross-over at the HEA energy scale seen in both hole-and electron-doped cuprates, [8,11,12] originates from intrinsic band renormalization effects and not extrinsic mechanisms that merely serve to change the appearance of features with changing experimental conditions. [27] Those theoretical scenarios based on weak coupling to high energy bosonic modes [4,6,[20][21][22]27] have an appeal based on the kink-like appearance of the HEA, recalling earlier efforts aimed at explaining the origin of the low-energy kink in cuprates. [14][15][16][17] While coupling to these modes, such as spin fluctuations, would generally satisfy the energy scale for the HEA in holedoped compounds, it fails to account for the dichotomy in energy scales between hole-and electron-doped materials.…”

“…[9][10][11][12] Taken together with results from hole-doped and half-filled parent materials, these findings raise questions about the origin or mechanism of the HEA, given the ap-parent dichotomy in energy scales between electron-and hole-doped compounds, and what role, if any, many body effects may play. Focusing primarily on the HEA in holedoped compounds, a number of theories have been advanced including spin-charge separation, [3] in-gap bandtails, [18] spin polarons, [19] coupling to spin fluctuations, [6,20,21] phonons, [4] or plasmons, [22] strong correlation or "Mott" physics, [2,[23][24][25][26] and even extrinsic effects associated with photoemission matrix elements. [7] Photoemission matrix elements clearly play a role in modifying the appearance of the HEA, complicating the analysis.…”

Two-Mode Variational Monte Carlo Study of Quasiparticle Excitations in Cuprate Superconductors

“…This high energy anomaly has intrigued many theoretical studies and disputes. [2,[10][11][12][13][14][15][16][17] A central concern is whether the anomaly is caused by extrinsic effects due to phonons, or is an intrinsic property of the strongly correlated electrons themselves. We summarize the main important issues that will concern us in this Letter: 1) the doping-independent Fermi velocity, 2) two energy scales in the quasiparticle spectral function, and 3) a suppression of the low energy spectral weight near the zone center.…”