Elevated Tc levels in YBa2Cu3O6.5 modeled on a 3-dimensional doped bond structure of chain and plane pairs

TL;DR

This paper models the elevated superconducting transition temperatures in YBa2Cu3O6.5 using a 3D doped bond structure involving chain and plane pairs, explaining metastable states and predicting higher Tc levels.

Contribution

It introduces a 3D bond model of chain and plane pairs to explain and predict elevated Tc levels in YBa2Cu3O6.5 and similar compounds.

Findings

Shot quenching produces metastable states with increased Tc.

Laser pulsed materials show aspects of higher doping levels.

Predicted Tc levels based on bond order principles.

Abstract

The complex phenomenology of shot quenched YBa2Cu3O6.5 with Tc=100K and 200K levels is compared with laser pulsed analogs with an eye on explaining the presumed Tc=552K of the latter. Shot quenching can produce metastable states with pronounced increases in plane metric and cell volume, accompanied by a rough doubling of Tc to a 100K level of an orthorhombic with 3-fold O coordinated chain Cu (O3 type). These states decay over a non-superconducting transition range to the conventional Tc=50K level of O24. We consider the plane expanded laser pulsed materials to contain aspects of O42 plane n-doped counterparts of the O3 n-doped version of shot quench preparations. In addition, we assume that highly charged p-doped chains of 4-fold O coordination form hole pairs at trijugate position, allowing close approach of the apical O to the electron-doped planes. They are now capable of participating in the bonding with the plane pairs at corresponding 3a0/2 location. The overall pair number is therefore multiplied, and the coupling strengthened, by limited 3-D effects within the Plane-Chain-Plane sandwich. The latter can be seen as an extended chemical bonding system that has the potential to equilibrate contractive and expansive pairs and so obviate the need for distinction of doping type as it may exchange it dynamically. It is argued that indications for a Tc=200K level on shot quenching has a related origin and represents one in series of predicted Tc levels based on bond order principles. Predictions are made where similar effects can be expected in other compound classes.