  Interfaces of sapphire are of technological relevance as sapphire is used as
a substrate in electronics, lasers, and Josephson junctions for quantum
devices. In addition, its surface is potentially useful in catalysis. Using
first principles calculations, we show that, unlike bulk sapphire which has
inversion symmetry, the (0001) sapphire surface is piezoelectric. The inherent
broken symmetry at the surface leads to a surface dipole and a significant
response to imposed strain: the magnitude of the surface piezoelectricity is
comparable to that of bulk piezoelectrics.
