Nonadiabatic coupling of the dynamical structure to the superconductivity in YSr ₂ Cu _2.75 Mo _0.25 O _7.54 and Sr ₂ CuO _3.3

Abstract

A crucial issue in cuprates is the extent and mechanism of the coupling of the lattice to the electrons and the superconductivity. Here we report Cu K edge extended X-ray absorption fine structure measurements elucidating the internal quantum tunneling polaron (iqtp) component of the dynamical structure in two heavily overdoped superconducting cuprate compounds, tetragonal YSr2Cu2.75Mo0.25O7.54 with superconducting critical temperature, Tc = 84 K and hole density p = 0.3 to 0.5 per planar Cu, and the tetragonal phase of Sr2CuO3.3 with Tc = 95 K and p = 0.6. In YSr2Cu2.75Mo0.25O7.54 changes in the Cu-apical O two-site distribution reflect a sequential renormalization of the double-well potential of this site beginning at Tc, with the energy difference between the two minima increasing by ∼6 meV between Tc and 52 K. Sr2CuO3.3 undergoes a radically larger transformation at Tc, >1-Å displacements of the apical O atoms. The principal feature of the dynamical structure underlying these transformations is the strongly anharmonic oscillation of the apical O atoms in a double-well potential that results in the observation of two distinct O sites whose Cu–O distances indicate different bonding modes and valence-charge distributions. The coupling of the superconductivity to the iqtp that originates in this nonadiabatic coupling between the electrons and lattice demonstrates an important role for the dynamical structure whereby pairing occurs even in a system where displacements of the atoms that are part of the transition are sufficiently large to alter the Fermi surface. The synchronization and dynamic coherence of the iqtps resulting from the strong interactions within a crystal would be expected to influence this process.

Extremely Overdoped Superconducting Cuprates via High Pressure Oxygenation Methods

“…Insofar as the structures of these YBCO variants differ from the parent compound much less than that of Sr 2 CuO 3.3 , it is expected that the coupling of their dynamic structure to the superconductivity is also much less extreme. As in YBCO [98][99][100], EXAFS measurements of YSCO-Mo also find a two-site Cu-O ap distribution that is modified across the superconducting transition of the type that we have called an Internal Quantum Tunneling Polaron [75]. It differs from that in the parent in several respects.…”

“…An external oxidizer can be used in HPO synthesis [50,58,73,74], but is more often employed in HPO treatment of pre-formed but oxygen-deficient products [23,55,57,58,60,61]. Intermediate APO steps are commonly used to minimize the amount of oxidizer needed at the HPO step, thus minimizing residue and maximizing the often very limited amount of the HPO sample [30,50,56,57,61,75]. In externally oxygenated treatment, the oxygen donor material can either be mixed in with the other material or separated from it by a thin gold foil or other barrier [25,54].…”

Oxide superconductors—light on a continuing mystery

“…A flood of theoretical models followed, which are far too numerous to list, but many of those attempted to describe the coupling in these materials to phonons or some other effective bosons (spin fluctuations, excitons). This paper by Conradson et al (2) reminds us that we cannot think of any coupling mechanism in the weak coupling limit to understand the cuprate superconductors. While many investigations use diffraction techniques or variations of diffraction, other investigations employ EXAFS (extended X-ray absorption fine structure spectroscopy), which probes the nearneighbor statics and dynamics at the Cu K edge.…”

Discovery of complex oxides via automated experiments and data science

“…Training data are particularly limited in high-order composition spaces (e.g., at least three cation oxides), which offer opportunities for tuning multiple properties through formation of a phase, i.e., a crystal structure or substitutional alloy, that contains all three cations. The vast number of potential high-order compositions exceeds current methods of discovery or prediction ( 6 – 9 ), and prediction of substitutional alloy phases and their properties remains a substantial challenge ( 10 , 11 ).…”

“…While approximations to computational modeling of alloys have been developed ( 24 – 27 ), alloys in high-order composition space comprise a dramatically underexplored class of materials for discovery efforts. We know from the examples of high-temperature superconductors and catalysis that extremely valuable properties are obtainable via substitutional alloying in high-order composition spaces ( 8 , 28 , 29 ).…”