Charge transfer in high-Tc superconducting superlattices.

Abstract

A previously developed model of charge transfer in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$/Pr${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (YBCO-PBCO) superlattices is described in detail, refinements of the approach are discussed, and extensions are made to barrier layers other than PBCO, e.g., ${\mathrm{Pr}}_{0.7}$${\mathrm{Y}}_{0.3}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and ${\mathrm{Pr}}_{0.5}$${\mathrm{Ca}}_{0.5}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. An expression for the energy of a supercell as a function of the charges transferred from the barrier layers to the YBCO is set up and minimized subject to the constraint of charge neutrality of the supercell. The approximate distribution of the transferred charge between the chain and Cu${\mathrm{O}}_{2}$ plane regions within a given YBCO unit cell is found by invoking symmetry arguments and results from ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ alloys. The dependence of the superconducting transition temperature on the hole density in the Cu${\mathrm{O}}_{2}$ planes is taken from a spinpolaron theory of high-${T}_{c}$ superconductivity which gives a good fit to the experimentally measured ${T}_{c}(x)$ function in bulk YBCO and ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$. Satisfactory agreement with the measured ${T}_{c}$ values at the midpoint of the resistive transition region for a variety of YBCO superlattices is found. Experimental evidence in support of charge transfer as the main mechanism determining ${T}_{c}$ is cited. It is emphasized that pairing interactions between YBCO unit cells are not the origin of high-${T}_{c}$ superconductivity in these materials.