Giant effective charges and piezoelectricity in gapped graphene

Since the first realization of reversible charge doping in graphene via field-effect devices, it has become evident how the induction of a gap could further enhance its potential for technological applications. Here we show that the gap opening due to a sublattice symmetry breaking has also a profound impact on the polar response of graphene. By combining ab initio calculations and analytical modelling we show that for realistic band-gap values (Delta less than or similar to 0.5 eV) the piezoelectric coefficient and the Born effective charge of graphene attain a giant value, independent on the gap. In particular the piezoelectric coefficient per layer of gapped mono- and bilayer graphene is three times larger than that of a large-gap full polar insulator as hexagonal Boron nitride (h-BN) monolayer, and 30% larger than that of a polar semiconductor as MoS2. This surprising result indicates that piezoelectric acoustic-phonons scattering can be relevant to model charge transport and charge-carrier relaxation in gated bilayer graphene. The independence of the piezoelectric coefficient and of the Born effective charge on the gap value follows from the connection between the polar response and the valley Chern number of gapped Dirac electrons, a relation previously identified for polar h-BN and made possible by the effective gauge-field description of the electron-lattice/ strain coupling in these systems. In the small gap limit, where the adiabatic ab initio approximation fails, we implement analytically the calculation of the dynamical effective charge, and we establish a universal relation between the complex effective charge and the so-called Fano profile of the phonon optical peak. Our results provide a general theoretical framework to understand and compute the polar response in narrow-gap semiconductors, but may also be relevant for the contribution of piezoelectric scattering to the transport properties in Dirac-like systems.