  We have found that there is more than one type of conducting carriers
generated in LaAlO3/SrTiO3 heterostructures by comparing the sheet carrier
density and mobility from optical transmission spectroscopy with those from
dc-transport measurements. When multiple types of carriers exist, optical
characterization dominantly reflects the contribution from the high-density
carriers whereas dc-transport measurements may exaggerate the contribution of
the high-mobility carriers even though they are present at low-density. Since
the low-temperature mobilities determined by dc-transport in the LaAlO3/SrTiO3
heterostructures are much higher than those extracted by optical method, we
attribute the origin of high-mobility transport to the low-density conducting
carriers.
