Flexoelectricity is the linear response of polarization to a strain gradient.
Here we address the simplest class of dielectrics, namely elemental cubic
crystals, and we prove that therein there is no extrinsic (i.e. surface)
contribution to flexoelectricity in the thermodynamic limit. The flexoelectric
tensor is expressed as a bulk response of the solid, manifestly independent of
surface configurations. Furthermore, we prove that the flexoelectric responses
induced by a long-wavelength phonon and by a uniform strain gradient are
identical.Comment: 5 pages, 1 figure (2 panels
