  The electronic band gap in conventional semiconductor materials, such as
silicon, is fixed by the material's crystal structure and chemical composition.
The gap defines the material's transport and optical properties and is of great
importance for performance of semiconductor devices like diodes, transistors
and lasers. The ability to tune its value would allow enhanced functionality
and flexibility of future electronic and optical devices. Recently, an
electrically tunable band gap was realized in a 2D material - electronically
gated bilayer graphene [1-3]. Here we demonstrate the realization of an
electrically tunable band gap in a 3D antiferromagnetic Mott insulator Sr2IrO4.
Using nano-scale contacts between a sharpened Cu tip and a single crystal of
Sr2IrO4, we apply a variable external electric field up to a few MV/m and
demonstrate a continuous reduction in the band gap of Sr2IrO4 by as much as
16%. We further demonstrate the feasibility of reversible resistive switching
and electrically tunable anisotropic magnetoresistance,which provide evidence
of correlations between electronic transport, magnetic order, and orbital
states in this 5d oxide. Our findings suggest a promising path towards band gap
engineering in 5d transition-metal oxides that could potentially lead to
appealing technical solutions for next-generation electronic devices.

Mesoscale and Nanoscale Physics;Materials Science