  We report on a metal-insulator transition (MIT) that is observed in an
electron system at the SmTiO3/SrTiO3 interface. This MIT is characterized by an
abrupt transition at a critical temperature, below which the resistance changes
by more than an order of magnitude. The temperature of the transition
systematically depends on the carrier density, which is tuned from ~ 1x10^14
cm^-2 to 3x10^14 cm^-2 by changing the SmTiO3 thickness. Analysis of the
transport properties shows non-Fermi liquid behavior and mass enhancement as
the carrier density is lowered. We compare the MIT characteristics with those
of known MITs in other materials systems and show that they are distinctly
different in several aspects. We tentatively conclude that both long range
Coulomb interactions and the fixed charge at the polar interface are likely to
play a role in this MIT. The strong dependence on the carrier density makes
this MIT of interest for field-tunable devices.
