1. FZU 30.5.061 Comparison of Mn doped GaAs, ZnSe, and LiZnAs dilute magnetic semiconductors J.Mašek, J. Kudrnovský, F. Máca, and T. Jungwirth

2. FZU 30.5.062 Introduction: A II B VI vs. A III B V DMS New proposition: Li(Zn,Mn)As Basic picture: scaling of Tc in (Ga,Mn)As Comparison of (Zn,Mn)Se, (Ga,Mn)As, and Li(Zn,Mn)As based on density functional calculations Li(Zn,Mn)As – a detailed study Conclusions

3. FZU 30.5.063 (A II,Mn)B VI DMS (Cd,Mn)Te, (Zn,Mn)Se Isovalent substitution of Mn(A II ) Mn atoms neutral, with spin 5/2 Wide concentration range of alloys Intrinsic semiconductors (may not be the case for other transition metals)

4. FZU 30.5.064 (A II,Mn)B VI DMS … cont. Giant AF splitting of the valence band: E v (↑)>E v (↓) Antiferromagnetic exchange coupling of local moments (superexchange) Complicated phase diagram due to frustration … semimagnetic semicond. Ferromagnetic state achieved only in strongly n- type doped materials

5. FZU 30.5.065 (A III,Mn)B V DMS (Ga,Mn)As, (Ga,Mn)N Non-isovalent substitution of Mn(A III ) Limited range of alloying (x<0.1) Mn acts as single acceptor Without extra doping, (Ga,Mn)As is p-type semiconductor (E F ~ E v – 0.2eV) Local moment 5/2 + hole spin -1/2

6. FZU 30.5.066 (A III,Mn)B V DMS … cont. Strong ferromagnetic RKKY interaction of local moments due to free carriers Ferromagnetic state with T c <170K (T c increases with both x and h) Superexchange seems unimportant and is usually neglected Troubles with selfcompensation (Mn ( int ) )

7. FZU 30.5.067 Open problems Limitations of T c Dependence of T c on x and density of carriers from the microscopic theory The role of superexchange in (Ga,Mn)As Possibility of n-type FM semiconductors Optimization of the host materials

8. FZU 30.5.068 Proposition: Li(Zn,Mn)As Band structure of LiZnAs similar to ZnSe and GaAs a(LiZnAs)= 5.815Å, a(GaAs)= 5.642 Å → hybridization Easy substitution of Mn for Zn expected Number of carriers related to non-stoichiometry in Li sublattice (vac (Li) =acceptor, Li (int) =donor)

9. FZU 30.5.069 Basic picture sp 3 bands + Anderson Hamiltonian for 3d 5 -states of Mn Spin-orbit interaction neglected CPA description of the mixed crystal in the FM state E rev ≡ energy of flipping a single atomic moment In mean-field approximation, T c ~ E rev.

10. FZU 30.5.0610

11. FZU 30.5.0611 Preliminary results for (Ga,Mn)As General scaling rule: T c /x ≈ f(n/x) T c <0 for |n|<x/4 … superexchange important ! Saturation of T c for n ≈ -x FM state (T c >0) possible for both p-type and n- type !

12. FZU 30.5.0612 Detailed density-functional study LMTO-CPA, LDA (partly LDA+U) Supporting calculations: FP LAPW Densities of states & distribution of Mn d-states in (Zn,Mn)Se, (Ga,Mn)As, and Li(Zn,Mn)As Mean-field T c in rigid-band approximation T c in realistic, co-doped DMS

13. FZU 30.5.0613 DOS: (Ga,Mn)As vs. (Zn,Mn)As

14. FZU 30.5.0614 DOS: (Zn,Mn)Se vs. Li(Zn,Mn)As

15. FZU 30.5.0615 Curie temperature: rigid-band approx.

16. FZU 30.5.0616 Curie temperature: co-doped DMS

17. FZU 30.5.0617

18. FZU 30.5.0618 Summary Dependence of T c (n) similar in all (Ga,Mn)As, (Zn,Mn)Se, and Li(Zn,Mn)As: no difference of II-VI and III-V DMS Superexchange → unstable FM state for low concentration of carriers n-type ferromagnetic DMS available at high concentration of donors Seems realistic only in Li(Zn,Mn)As

19. FZU 30.5.0619 Li(Zn,Mn)As – more details Problems: Microscopic nature of Li non-stoichiometry (vac (Li) and Li (int), Li (Zn) and Zn (Li) ) Limits of solubility of Mn in LiZnAs Mn solubility in non-stoichiometric material

20. FZU 30.5.0620 Substitution of Mn for Zn not affected by non-stoichiometry

21. FZU 30.5.0621 Large formation energies of vac (Li) and Zn (Li) → substoichoimetry unprobable

22. FZU 30.5.0622 Both Li (int) and Li (Zn) possible, leading to either n- or p-type material

23. FZU 30.5.0623 Dynamical equilibrium Principal defects in Li(Zn,Mn)As: Mn (Zn), Li (Zn),and Li (int) Formation energies E s,i (x s,x i ) of Li (Zn) and Li (int) as functions of partial concentrations x s and x i. Balanced state: E s (x s,x i ) = E i (x s,x i ).

24. FZU 30.5.0624

25. FZU 30.5.0625 Partial concentrations of Li (Zn) and Li (int)

26. FZU 30.5.0626 Conclusions Both p-type and n-type ferromagnetic DMS possible with suitable doping of III-V, II-VI, and I-II-V DMS AFM exchange in compensated materials Electron-mediated FM: disorder-induced reconstruction of CB + partial occuparion of side minima + strong admixture of d-states (unclear) Li-rich Li(Zn,Mn)As seems a good candidate for n-type FM semiconductor.