Seminarium Fizyki Materii Skondensowanej

The virtual crystal approximation is a building block of condensed matterphysics which allows to apply the methods of group theory to randomalloys. In the case of (Ga,Mn)As, however, even a qualitative descriptionof the experimental magnetic anisotropy is not possible within the grouptheory for zinc-blend crystals.In our work [1] we demonstrate that this apparent failure of the grouptheory is associated with a non-random distribution of the magneticconstituent of the alloy. We show that the effect of the non-randomnesscan be parameterized by effective strains, introduced earlier asphenomenological parameters. We determine the magnitude of these effectivestrains either ab initio or from second order perturbation theory in theeffective mass approach. The effect of a finite size of supercellsintroduced commonly to make ab initio computations feasible is discussed.[1] M. Birowska, C. Śliwa, J. A. Majewski, T. Dietl, Origin of BulkUniaxial Anisotropy in Zinc-Blende Dilute Magnetic Semiconductors, Phys.Rev. Lett. 108, 237203 (2012).