Pair Fermi contour and repulsion-induced superconductivity in cuprates

Abstract

The pairing of charge carriers with a large pair momentum is considered in connection with hightemperature superconductivity of cuprate compounds. The possibility of pairing arises due to some essential features of quasi-two-dimensional electronic structure of cuprates: ͑i͒ The Fermi contour with strong nesting features. ͑ii͒ The presence of an extended saddle point near the Fermi level. ͑iii͒ The existence of some ordered state (for example, antiferromagnetic) close to the superconducting one as a reason for the appearance of a ''pair'' Fermi contour resulting from carrier redistribution in momentum space. In an extended vicinity of the saddle point, the momentum space has hyperbolic ͑pseudoeuclidean͒ metrics; therefore, the principal values of the two-dimensional reciprocal reduced effective mass tensor have unlike signs. At small momenta of the relative motion of a pair with a large pair momentum, the pairing is sensitive just to the sign and value of the effective mass and not to only the value of the Fermi velocity as in the case of Cooper pairing. The nesting of the Fermi contour results in an increase of the statistical weight of the pair with a large total momentum due to an extension of the momentum space domain which corresponds to permissible values of the relative motion momentum. The rearrangement of holes in momentum space results in the rise of a ''pair'' Fermi contour which may be defined as the zero-energy line for the relative motion of the pair. The superconducting gap arises just on this line. The pair Fermi contour formation inside the region of momentum space with hyperbolic metrics results in not only superconducting pairing but in a rise of a quasistationary state in the relative motion of the pair. Such a state has rather small decay, and may be related to the pseudogap regime of underdoped cuprates. It is concluded that pairing in cuprates may be due to screened Coulomb repulsion. The pairing mechanism and the pair Fermi contour conception may provide a qualitative interpretation for the key experimental facts relating to cuprates.

Propeller-Like Low Temperature Fermi Surface of Ba1-xKxFe2As2 from Magnetotransport and Photoemission Measurements

“…The low energy electronic band dispersion, and in particular the shape of the Fermi surface (FS), are decisive properties of any superconductor, as they are forming premises for theoretical description of the superconductivity. 1) Naturally, the issue of the FS geometry of iron-arsenide high-T c family was addressed in many theoretical [2][3][4] and experimental [5][6][7][8][9][10][11][12][13] studies. Primarily the FS was assumed to consist of two hole-like sheets around the center of FeAs Brillouin zone and two electron-like sheets at the corners.…”

Phase Separation of Electrons Strongly Coupled with Phonons in Cuprates and Manganites

“…(1)) or its strong-coupling bipolaronic extension [2,3] (right-hand upper corner in Fig. (1)) , but also "excitonic" [4,5], "plasmonic" [6,7], "magnetic" [8,9], "kinetic" [10], or purely "coolombic" due to a mirror-nested Fermi surface [11]. The BCS theory like any mean-field theory is rather universal, so that it describes well the cooperative quantum phenomenon of superconductivity even with these non-phononic mechanisms, if the coupling is weak (left-hand lower corner in Fig.…”

About the role of 2D screening in high temperature superconductivity

“…Up to our knowledge, the idea that the Coulomb potential in a 2D crystal system can contribute to produce, not only screening but even binding between pairs, was firstly advanced by D. Mattis 8 . Also various authors have been also considering the effects of screening in superconductivity along various directions of thinking 9,11,12,13,14,15 .…”