Electrical instability of donor states in elongated quantum ring induced by external fields

摘要

The energy spectrum, dipole momentum, and polarizability induced by an off-axis donor in InAs/GaAs elongated volcano-shaped quantum dot in the presence of external electric and magnetic fields are theoretically investigated. Our calculations, carried out by using the adiabatic approximation and the Kane model, which takes into account the effect of non-parabolicity of the conduction band, show that estimated donor energies are essentially lower than corresponding values obtained by means of the effective mass approximation. The mixing of the donor's energies, the variation of the dipole moment, the electric polarizability and the charge distribution induced by the electric and magnetic fields are consistently described with our simple formalism, in which the adiabatic potential is related directly to the morphology of the elongated quantum ring. In this structure, the charge density in two lowest "bonding" and "anti-bonding" states tends to align along of one of two symmetry axes. The donor-electron Coulomb attraction leads to an increase of the energy gap between these states and provides a strong polarization of the anti-bonding state. The out-of-plane magnetic field decreases the overlapping of the corresponding wave functions, while the in-plane electric field increases their mixing and provides the reordering of the energy levels. We have found that the dependency of the polarizability on the external electric field in these conditions displays a phase transition to a donor state with a giant dipole moment.