Applying an electric field F to the superlattice structure causes the Hamiltonian to exhibit an additional scalar potential eφ (z) = − eFz that destroys the translational invariance. In this case, given an eigenstate with wavefunction Φ Φ 0 ( z ) and energy E 0 , then the set of states corresponding to wavefunctions Φ Φ j ( z ) = Φ Φ 0 ( z − − j d ) are eigenstates of the Hamiltonian with energies E j = E 0 − jeFd. These states are equally spaced both in energy and real space and form the so-called Wannier–Stark ladder. The potential Φ Φ 0 ( z ) is not bounded for the infinite crystal, which implies a continuous energy spectrum. Nevertheless, the characteristic energy spectrum of these Wannier–Stark ladders could be resolved experimentally.