CUPRATE TRILAYER C-AXIS TUNNELING HETEROSTRUCTURES

Trilayer tunneling structures consisting of cuprate electrodes and titanate barriers were grown by atomic layer-by-layer molecular beam; epitaxy and processed into c-axis transport samples. Barriers of SrTiO3 and related titanates with thicknesses ranging from 4 Angstrom to 28 Angstrom (one to seven unit cells of the titanate) were grown. While no supercurrent was observed for even the thinnest barrier, the zero bias resistance was an exponential function of barrier thickness for samples with five or fewer titanate unit cell barriers, indicating tunneling transport. Each additional titanate unit cell caused the zero bias resistance to increase by one order of magnitude. A detailed investigation of the properties of the cuprate layers immediately adjacent to the titanate layers revealed that they were depleted of charge carriers and exhibited variable range hopping transport. Thus the electron states in these layers were localized. The trilayer transport process is modeled as one phonon assisted tunneling between localized states.