CUPRATE TRILAYER C-AXIS TUNNELING HETEROSTRUCTURES

Trilayer tunneling structures consisting of cuprate electrodes and tit anate barriers were grown by atomic layer-by-layer molecular beam; epi taxy and processed into c-axis transport samples. Barriers of SrTiO3 a nd related titanates with thicknesses ranging from 4 Angstrom to 28 An gstrom (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 sampl es with five or fewer titanate unit cell barriers, indicating tunnelin g transport. Each additional titanate unit cell caused the zero bias r esistance to increase by one order of magnitude. A detailed investigat ion of the properties of the cuprate layers immediately adjacent to th e titanate layers revealed that they were depleted of charge carriers and exhibited variable range hopping transport. Thus the electron stat es in these layers were localized. The trilayer transport process is m odeled as one phonon assisted tunneling between localized states.