  With infrared ellipsometry and transport measurements we investigated the
electrons at the interface between LaAlO3 and SrTiO3. We obtained a sheet
carrier density of Ns~5-9x 10E13 cm^-2, an effective mass of m*~3m_e, and a
strongly frequency dependent mobility. The latter are similar as in bulk
SrTi1-xNbxO3 and therefore suggestive of polaronic correlations of the confined
carriers. We also determined the vertical density profile which has a strongly
asymmetric shape with a rapid initial decay over the first 2 nm and a
pronounced tail that extends to about 11 nm.
