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Bulk Band Gap and Surface State Conduction Observed in Voltage-Tuned Crystals of the Topological Insulator<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>Bi</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Se</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math>
We report a transport study of exfoliated few monolayer crystals of topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ in an electric field effect geometry. By doping the bulk crystals with Ca, we are able to fabricate devices with sufficiently low bulk carrier density to change the sign of the Hall density with the gate …