Quantum well (QW) resonances in Fe(001)/MgO/Fe/MgO/Fe double barrier magnetic tunnel junctions are calculated from first principles. By including the Coulomb blockade energy due to the finite size islands of the middle Fe film, we confirm that the oscillatory differential resistance observed in a recent experiment [T. Nozaki, Phys. Rev. Lett. 96, 027208 (2006)10.1103/PhysRevLett.96.027208] originates from the QW resonances from the Delta1 band of the Fe majority-spin channel. The primary source of smearing at low temperatures is shown to be the variation of the Coulomb blockade energy.

The polarity of the ZnO film grown on sapphire using an ultrathin Ga wetting layer has been investigated by electron holography. Spontaneous polarization of the ZnO film leads to localized charges in the surface, which change the potential distribution in the vacuum side of the film. The potential distribution depends on the nature of the bounded charges and change as a function of the distance from the film surface. By studying the dependence of the potential change on the distance from the film surface, the ZnO film with very thin Ga wetting layer is determined to have the [0001] polarity.