  Interlayer coupling effects between high mobility two-dimensional
superconductors are studied in bilayer delta-doped SrTiO3 heterostructures. By
tuning the undoped SrTiO3 spacer layer between the dopant planes, clear tunable
coupling is demonstrated in the variation of the sheet carrier density, Hall
mobility, superconducting transition temperature, and the temperature- and
angle-dependences of the superconducting upper critical field. Systematic
variation is found between one effective (merged) two-dimensional
superconductor to two decoupled two-dimensional superconductors. In the
intermediate coupled regime, a crossover is observed due to coupling arising
from inter-sub-band interactions.
