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Dynamical Local Lattice Instability Triggered High T_{c} Superconductivity

100%

Ranninger J.

Acta Physica Polonica A

2015

vol. 127

issue 2

176-184

High T_{c} cuprate superconductors are characterized by two robust features: their strong electronic correlations and their intrinsic dynamical local lattice instabilities. Focusing on exclusively that latter, we picture their parent state in form of a quantum vacuum representing an electronic magma in which bound diamagnetic spin-singlet pairs pop in and out of existence in a Fermi sea of itinerant electrons. The mechanism behind that resides in the structural incompatibility of two stereo-chemical configurations Cu^{II}O₄ and Cu^{III}O₄ which compose the CuO₂ planes. It leads to spontaneously fluctuating Cu-O-Cu valence bonds which establish a local Feshbach resonance exchange coupling between bound and unbound electron pairs. The coupling, being the only free parameter in this scenario, the hole doping of the parent state is monitored by varying the total number of unpaired and paired electrons, in chemical equilibrium with each other. Upon lowering the temperature to below a certain T*, bound and unbound electron pairs lock together in a local quantum superposition, generating transient localized bound electron pairs and a concomitant opening of a pseudo-gap in the single-particle density of states. At low temperature, this pseudo-gap state transits via a first order hole doping induced phase transition into a superconducting state in which the localized transient bound electron pairs get spatially phase correlated. The mechanism driving that transition is a phase separation between two phases having different relative densities of bound and unbound electron pairs, which is reminiscent of the physics of ⁴He-³He mixtures.

Polarons in axial transport in single-layer high-Tc superconductors

63%

Andreev A., Tsintsarska S., Dimitrova-Ivanovich M., Polyanski I., Georgiev M., Gochev A.

Open Physics

2004

vol. 2

issue 2

329-356

The temperatureT dependencies ρ(T) of normal state electric resistivitiesρ c (axial) andρ ab (in plane) of single-layer high-T c superconductors show common trends: AsT is raised, the resistivity first drops steeply before it starts rising αT above an apparent semiconductor-to-metal crossoverT cross. To analyze ρ(T) we plottT/ρ againstT at various dopingsx for bothρ c andρ ab.T/ρ is inversely proportional to the traversal time across a potential barrier as an ionic particle drifts in an electric field. We findT/ρ in good agreement with theT dependence of the quantum rate of migrating particles: AsT is raised, a zero-point rate at the lowestT is extended to a nearly flat plateau before a thermally activated branch sets in. We also find evidence for the admixture of 1- & 2-phonon absorptions below the Arrhenius range. These features shape the semiconductor-like branch below Tcross. AboveT cross a metallic-like branch sets in, its αT character deriving from the field coupling of the migrating particle. Our analysis suggests that metal physics may not suffice if ionic features play a role in transport. We attribute our conclusions to the drift of strong-coupling polarons along Cu−O bonds. These “bond polarons” originate from carrier scattering by double-well potentials associated with the bonds. A bond polaron dissociates to a free hole as it passes onto a neighboring O-O link.

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1 Acta Physica Polonica A

1 Open Physics

1 2015

1 2004

1 Andreev A.

1 Dimitrova-Ivanovich M.

1 Georgiev M.

1 Gochev A.

1 Polyanski I.

1 Ranninger J.

1 Tsintsarska S.

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Dynamical Local Lattice Instability Triggered High T_{c} Superconductivity

Polarons in axial transport in single-layer high-Tc superconductors