  Electrostatic doping into an $n$-type Mott insulator Sm$_{2}$CuO$_{4}$ has been successfully achieved with use of the heterojunction with an   $n$-type band semiconductor Nb-doped SrTiO$_{3}$. The junction exhibits rectifying current-voltage characteristics due to the interface band discontinuity and the formation of depleted region. The application of reverse bias electric field on this junction enables the field-effect electron doping (presumably up to 6% per Cu atom) to the Mott insulator. The electro-modulation absorption spectroscopy could clearly show a large modification of the Mott-gap state accompanying the spectral weight transfer to the lower-energy region, reminiscent of formation of a metallic state. 
Optical probe of electrostatic doping in an n-type Mott insulator