We describe the synthesis of bilayer graphene thin films deposited on insulating silicon carbide and report the characterization of their electronic band structure using angle-resolved photoemission. By selectively adjusting the carrier concentration in each layer, changes in the Coulomb potential led to control of the gap between valence and conduction bands. This control over the band structure suggests the potential application of bilayer graphene to switching functions in atomic-scale electronic devices.

We investigated the electronic structure of $5d$ transition-metal oxide ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ using angle-resolved photoemission, optical conductivity, x-ray absorption measurements, and first-principles band calculations. The system was found to be well described by novel effective total angular momentum $J_{\mathrm{eff}}$ states, in which the relativistic spin-orbit coupling is fully taken into account under a large crystal field. Despite delocalized Ir $5d$ states, the $J_{\mathrm{eff}}$ states form such narrow bands that even a small correlation energy leads to the $J_{\mathrm{eff}}=1/2$ Mott ground state with unique electronic and magnetic behaviors, suggesting a new class of $J_{\mathrm{eff}}$ quantum spin driven correlated-electron phenomena.