  Rotation of MO6 (M = transition metal) octahedra is a key determinant of the
physical properties of perovskite materials. Therefore, tuning physical
properties, one of the most important goals in condensed matter research, may
be accomplished by controlling octahedral rotation (OR). In this study, it is
demonstrated that OR can be driven by an electric field in Sr$_2$RuO$_4$.
Rotated octahedra in the surface layer of Sr$_2$RuO$_4$ are restored to the
unrotated bulk structure upon dosing the surface with K. Theoretical
investigation shows that OR in Sr$_2$RuO$_4$ originates from the surface
electric field, which can be tuned via the screening effect of the overlaid K
layer. This work establishes not only that variation in the OR angle can be
induced by an electric field, but also provides a way to control OR, which is
an important step towards in situ control of the physical properties of
perovskite oxides.
