  Understanding the nature of charge carriers at the LaAlO3/SrTiO3 interface is
one of the major open issues in the full comprehension of the charge
confinement phenomenon in oxide heterostructures. Here, we investigate
thermopower to study the electronic structure in LaAlO3/SrTiO3 at low
temperature as a function of gate field. In particular, under large negative
gate voltage, corresponding to the strongly depleted charge density regime,
thermopower displays record-high negative values of the order of 10^4 - 10^5
microV/K, oscillating at regular intervals as a function of the gate voltage.
The huge thermopower magnitude can be attributed to the phonon-drag
contribution, while the oscillations map the progressive depletion and the
Fermi level descent across a dense array of localized states lying at the
bottom of the Ti 3d conduction band. This study is the first direct evidence of
a localized Anderson tail in the two-dimensional (2D) electron liquid at the
LaAlO3/SrTiO3 interface.
