Nano-resolved strain control and polar modulation of the Mott transition in a bilayer ruthenate

The 4d transition metal oxides of the Ca_n+1Ru_nO_3n+1perovskite family have recently garnered interest for their correlated electron physics and strong sensitivity to external stimuli like strain, temperature, and even electric current. These place the n=1,2 members in a rich phase diagram of competing and tunable metallic, magnetic, and insulating phases. The bilayer ruthenate Ca₃Ru₂O₇ exhibits a structural distortion producing a polar metal and, under Ti substitution for Ru, a polar Mott insulator. We report cryogenic (T<100K) infrared nano-imaging (nano-IR) of 5% and 10% Ti-doped crystals revealing spontaneous patterns of striped phase coexistence through the thermal first-order Mott transition, and demonstrate nucleation and manipulation of metallic stripes through in situ uniaxial strain. Energy-resolved nano-IR imaging combined with surface work function mapping reveal suppression or enhancement of the Mott insulating state at polar domain boundaries in the crystal. Verified by second-harmonic polarimetry and transmission electron microscopy, we find the polar orientation and state of charge at these polar domain walls can selectively modulate the Mott transition, thus opening new routes towards manipulation of this canonical insulator-metal transition at sub-micron scales.