Theory of the sp-d coupling of transition metal impurities with free carriers in ZnO.

The [Formula: see text] exchange coupling between the spins of band carriers and of transition metal (TM) dopants ranging from Ti to Cu in ZnO is studied within the density functional theory. The [Formula: see text] corrections are included to reproduce the experimental ZnO band gap and the dopant levels. The p-d coupling reveals unexpectedly complex features. In particular, (i) the p-d coupling constants [Formula: see text] vary about 10 times when going from V to Ni, (ii) not only the value but also the sign of [Formula: see text] depends on the charge state of the dopant, (iii) the p-d coupling with the heavy holes and the light holes is not the same; in the case of Fe, Co and Ni, [Formula: see text]s for the two subbands can differ twice, and for Cu the opposite sign of the coupling is found for light and heavy holes. The main features of the p-d coupling are determined by the p-d hybridization between the d(TM) and p(O) orbitals. In contrast, the s-d coupling constant [Formula: see text] is almost the same for all TM ions, and does not depend on the charge state of the dopant. The TM-induced spin polarization of the p(O) orbitals contributes to the s-d coupling, enhancing [Formula: see text].

Electronic properties of Bi2Se3 dopped by 3d transition metal (Mn, Fe, Co, or Ni) ions.

Topological insulators are characterized by the existence of band inversion and the possibility of the realization of surface states. Doping with a magnetic atom, which is a source of the time-reversal symmetry breaking, can lead to realization of novel magneto-electronic properties of the system. In this paper, we study effects of substitution by the transition metal ions (Mn, Fe, Co and Ni) into Bi2Se3 on its electric properties. Using the ab inito supercell technique, we investigate the density of states and the projected band structure. Under such substitution the shift of the Fermi level is observed. We find the existence of nearly dispersionless bands around the Fermi level associated with substituted atoms, especially, in the case of the Co and Ni. Additionally, we discuss the modification of the electron localization function as well as charge and spin redistribution in the system. Our study shows a strong influence of the transition metal-Se bond on local modifications of the physical properties. The results are also discussed in the context of the interplay between energy levels of the magnetic impurities and topological surface states.