  Tunneling between two parallel, two-dimensional electron gases (2DEGs) in a
complex oxide heterostructure containing a large, mobile electron density of ~
3x10^14 cm^-2 is used to probe the subband structure of the 2DEGs.
Temperature-dependent current-voltage measurements are performed on
SrTiO3/GdTiO3/SrTiO3 junctions, where GdTiO3 serves as the tunnel barrier, and
each interface contains a high-density 2DEG. Resonant tunneling features in the
conductance and its derivative occur when subbands on either side of the
barrier align in energy as the applied bias is changed, and are used to analyze
subband energy spacings in the two 2DEGs. We show that the results agree
substantially with recent theoretical predictions for such interfaces.
