Apical oxygen vibrations dominant role in d-wave cuprate superconductivity and its interplay with spin fluctuations

Abstract

Microscopic theory of a high T c cuprate Bi 2 Sr 2 CaCu 2 O 8+x based on main pairing channel of electrons in CuO planes due to 40mev lateral vibrations of the apical oxygen atoms in adjacent the SrO ionic insulator layer is proposed. The separation between the vibrating charged atoms and the 2D electron gas creates the forward scattering peak leading in turn to the d -wave pairing within Eliashberg formalism. The phonon mode naturally explain the kink in dispersion relation observed by ARPES and the and effect of the O 16 → O 18 isotope substitution in the normal state. To describe the pseudogap physics a single band fourfold symmetric t − t ′ Hubbard model, with the hopping parameters t ′ ∼ − 0.17 t and the on site repulsion e U ∼ 6t. It described the Mott insulator at low doping, while at higher dopping the pseudogap physics (still strongly correlated) can be be approximated by the symmetrized mean field model and with renormalized U incorporating screening. The location of the transition line T *between the locally antiferromagnetic pseudogap and the paramagnetic overdoped phases and susceptibility (describing spin fluctuations coupling to 2DEG) are also obtained within this approximation. The superconducting d - wave gap mainly due to the phonon channel but is assisted by the spin fluctuations (15%–20%). The dependence of the gap and T c on doping and effect of the isotope substitution are obtained and is consistent with experiments.

Correlation of strontium anharmonicity with charge-lattice dynamics of the apical oxygens and their coupling to cuprate superconductivity

“…Far from being passive [56], our calculations demonstrate that this coupling and consequent synchronized, antiphase motions of the Oap atoms and their associated charges have substantial effects on the dynamics and charge distributions in the other domains of the materials, notably the CuO 2 planes where the charge on the oxygen is highly correlated with T c [79]. The extension of the localized wave function on Oap to multiple IQTPs and sites in the CuO 2 plane promotes electron-electron interactions that go far beyond the acknowledged special role of the Oap atom in controlling the carrier density [79][80][81] or, in the extreme case, the suggestion that pairing could occur through possible lateral vibrations [55] (although these are not observed in EXAFS). Furthermore, depending on the details of short-and long-range interactions between IQTPs, assemblies of finite densities of them will likely result in mesoscopic patterns, including clusters and stripes, and possibly dynamical charge density waves (CDW) [82].…”

“…The collective behavior of the charges in the CuO 2 planes that is manifested as the numerous ordered phases poses the same question as the calculations: what are the ramifications of coupling the Oap atoms, which would be linked through the Ba/Sr atoms, as in figure 1? This possibility has been mostly neglected as unlikely because of the absence of any experimental data and the inert charge of the alkaline earth cations [55]. However, it could result in functionality for the entire Ba/Sr-Oap plane in the YBa 2 Cu 3 O 7±δ -type (YBCO) or Bi/Hg/Tl-based compounds and the La 1−x Ba/Sr x − Oap layer in the La 2 CuO 4 -type (LCO) compounds analogous to the other discrete sections of the structures.…”

Possible enhancement of the superconducting Tc due to sharp Kohn-like soft phonon anomalies

“…For acoustic phonons, in the limit of small wave-vector and finite temperature, we can take the linewidth to be quadratic, Γ(q) = Dq 2 , where D is the damping constant. This diffusive damping is a direct consequence of momentum conservation derivable using effective field theory and hydrodynamics [56,57] and it is independent of whether the solid is amorphous or crystalline 6 . On the contrary, for optical phonons, the leading term in the relaxation rate Γ(q) is wave vector independent, Γ(q) = Γ 0 , and is known as Klemens damping [60].…”

“…This model was subsequently adapted a few years later by Plakida and co-workers [4] to provide an early explanation for the high T c in the La(Y)BaCuO cuprates discovered in 1986 by Bednorz and Müller [5]. In this later version of the model, the structural instability in the form of a two-level anharmonic well is attributed to rotational motions of the so-called apical oxygen in the layered perovskite structure, the importance of which has been widely established [6]. The enhancement of T c is attributed to the fact that highly anharmonic motions, such as those performed by the apical oxygens, are characterized by a large displacement d, much larger than the mean squared harmonic displacements ⟨u 2 ⟩.…”

Superconducting dome in ferroelectric-type materials from soft mode instability

“…The role of structural instability, and its influence on superconductivity, similar to what happens in ferroelectrics 20 , is also a highly debated topic in the context of high-temperature superconductors, both cuprates 21,22 and the recently discovered hydrides under high pressure [23][24][25] , and, in general, in strongly coupled superconductors 26 . For example, it is well known that, before the discovery of cuprates, high-T c superconductivity was sought in A15 intermetallic compounds which exhibit soft-mode structural instability 27 .…”