Evidence for Polaron Formation in High-Temperature Superconducting Cuprates: Experiment and Theory

Abstract

We provide compelling support for the key role played by polaron formation to the physics of cuprate superconductors, which is evidenced above the pseudogap temperature T * , is the origin of the pseudogap phase itself and persists in the superconducting phase. Experimental and theoretical results are compared and show convincing agreement with each other.

Evidence for a temperature dependent anisotropy of the superconducting state parameters in underdopedSmBa2Cu3Ox

“…We wish to point out that in all torque measurements a nearly reversible signal was obtained In principle, the origin of the observed temperature dependence of the anisotropy parameter in Sm123 may be related to one of at least five situations: (i) multiband superconductivity, [60][61][62][63][64][65][66][67][68][69] (ii) Fermi surface anisotropy, [70][71][72] (iii) unconventional pairing and anisotropy of the superconducting energy gap, [73][74][75][76][77][78][79][80] (iv) strong coupling, [81][82][83] (v) real limitations of AGLT in the case of highly underdoped superconductors due to their strictly layered structure. Nonlocality, which becomes observable when the mean free path becomes larger than the superconducting coherence length, may be a necessary ingredient for the situations…”

“…81 This produces an increase in H c2 compared to using the bare band Fermi velocity, but the shift being relatively temperature-independent cancels out of h(T). 82 A strong polaron coupling approach, proposed as a further extension of the BCS phonon-mediated superconductivity for the cuprates, 82,83 could possibly be included in this scenario. However, when detailed calculations are made, 83 strong coupling gives only minor modifications to H c2 (T) and cannot explain the observed strong temperature dependence of the anisotropy parameter.…”

Isotope Effect in the Translation-Invariant Bipolaron Theory of High-Temperature Superconductivity

“…In papers [2,13] this disagreement with the developed theory and the BCS is explained by the fact that in many HTSC materials at low temperatures, the main role belongs to non-adiabaticity effects, leading to high values of β(0). With a simple model, such as that developed in this paper, one cannot hope to reproduce all the details (see, for example, [21][22][23][24][25]), and our goal was only the qualitative explanation of the most spectacular features. The theory cannot provide agreement for small doping where the experimental value for some samples is much larger than the BCS limit of α = 0.5 and can have even negative values of α.…”

Local Electron-Lattice Interactions in High-Temperature Cuprate Superconductors

“…The last three terms are the pure lattice part, the electron-lattice interactions with the AFM holes, and the doped holes, respectively. The terms in (1) are explicitly given by [43][44][45][46][88][89][90]]…”