Central peak in the pseudogap of high T c superconductors

Abstract

We study the effect of antiferromagnetic (AF) correlations in the three-band Emery model, with respect to the experimental situation in weakly underdoped and optimally doped BSCCO. In the vicinity of the vH singularity of the conduction band there appears a central peak in the middle of a pseudogap, which is in an antiadiabatic regime, insensitive to the time scale of the mechanism responsible for the pseudogap. We find a quantum low-temperature regime corresponding to experiment, in which the pseudogap is created by zero-point motion of the magnons, as opposed to the usual semiclassical derivation, where it is due to a divergence of the magnon occupation number. Detailed analysis of the spectral functions along the (π, 0)-(π, π) line show significant agreement with experiment, both qualitative and, in the principal scales, quantitative. The observed slight approaching-then-receding of both the wide and narrow peaks with respect to the Fermi energy is also reproduced. We conclude that optimally doped BSCCO has a well-developed pseudogap of the order of 1000 K. This is only masked by the narrow antiadiabatic peak, which provides a small energy scale, unrelated to the AF scale, and primarily controlled by the position of the chemical potential. I. INTRODUCTIONThe Fermi surface phenomena in the high-T c cuprates, and especially BSCCO, have been extensively investigated, and a broad consensus has developed concerning their main features. The Fermi surface is large and hole-like, with a simple topology of a rounded square, or barrel, centered around the M-point [1]. Single-particle phenomenology is routinely invoked on the ARPES spectra, thus the 'self-energy' and 'damping' are often extracted from the main peak as if it were a coherent, weakly perturbed quasiparticle [2]. However, in the underdoped regime and below a temperature scale T * , the metallic state is surfeit with low-energy correlations, about whose relevance for either the pseudogap, or the superconducting mechanism itself, there is no general agreement at present. Various experimental observations at low energy have been interpreted in terms of stripes [3], paramagnons [4], phonons [5], and superconducting fluctuations above T c [6]. All these correlations are at present the object of intense scrutiny, mainly with a view to ascertaining whether they enhance or suppress superconductivity.Theoretical understanding of the measured electronic spectral functions of high-T c superconductors has received significant attention in the context of these efforts [7,8,9,10,11,12,13,14,15]. Physically, conduction occurs in the copper oxide planes, so the most important electronic states are directly accessible to surface probes such as ARPES. This naturally allows for a concentration of theoretical effort, especially because the observed spectra offer some outstanding puzzles of their own. Such a long-standing issue is the appearance of the pseudogap [16], observed near the vH points for underdoped systems, and its connection with the AF gap at lower doping on the o...

Electronic Raman scattering in a multiband model for cuprate superconductors

“…15,43 The effects of the AF correlations on the Raman spectral functions is approximated here by replacing the coupling of the conduction band electrons to the AF fluctuations by their coupling to the Q AF mode, which is taken as frozen in. The effect of bosons with the wave vectors close to Q AF on the quasiparticle dispersion is thus neglected, i.e., the pseudogap is replaced by the gap ⌬͑k͒ involved in H AF , 23,24 …”

“…23 The associated pseudogap energy ⌬ AF is well below 0.1 eV. Until now, the bosonic effects of paramagnons were estimated only by omitting the band broadening due to bosonic charge fluctuations.…”

“…This amounts to the use of the MFSB theory, supplemented by the coupling of the Fermi liquid to the paramagnons. 23 Such an approximation conserves the 0.1 eV energy scales in the band dispersion and allows for the ͑in͒elastic scattering on paramagnons. The corresponding inelastic processes turn out to be more important 23 on the holedoped side than on the electron-doped side of the "nonmagnetic normal state" extrapolated close below the superconducting T c .…”

“…23 Such an approximation conserves the 0.1 eV energy scales in the band dispersion and allows for the ͑in͒elastic scattering on paramagnons. The corresponding inelastic processes turn out to be more important 23 on the holedoped side than on the electron-doped side of the "nonmagnetic normal state" extrapolated close below the superconducting T c . The whole hierarchy of energy scales, and especially the assertion that the relevant nonmagnetic energy scales are larger than ⌬ AF , which itself is larger than T c , is obviously of essential importance for the understanding of high-T c superconductivity.…”

Instanton Sector of Correlated Electron Systems as the Origin of Populated Pseudo-gap and Flat “Band” Behavior: Analytic Solution

“…In the center of the pseudo-gap there are fermionic states with the density of states that decreases exponentially with temperature according to "quadratic Arrenius" law ∼ k ′ = exp{−const · (ε F /2T ) 2 }, where const = exp{−2ε F /|g|}. These in-gap states are also observed experimentally [8] in high-T c cuprates. Also in qualitative accord with the ARPES experiments in these compounds [3,5,6] we find at the edge of the pseudo-gap a "flat band" in the fermionic dispersion, see creasing parameter k ′ in Eq.…”