Energy shift and conduction-to-valence band transition mediated by a   time dependent potential barrier in graphene
  We investigate the scattering of a wave packet describing low-energy electrons in graphene by a time-dependent finite step potential barrier. Our results demonstrate that, after Klein tunneling through the barrier, the electron acquires an extra energy which depends on the rate of change the barrier height in time. If such a rate is negative, the electron loses energy and ends up as a valence band state after leaving the barrier, which effectively behaves as a positively charged quasi-particle 
Andrey Chaves, D. R. da Costa, G. O. de Sousa, J. M. Pereira Jr., and   G. A. Farias
cond-mat.mes-hall