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Coherently induced ferromagnetism in Diluted Magnetic Semiconductors Southampton, OCES9-SCES2 September 7 st 2005 Joaquín Fernández-Rossier Dept. Física.

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Presentation on theme: "Coherently induced ferromagnetism in Diluted Magnetic Semiconductors Southampton, OCES9-SCES2 September 7 st 2005 Joaquín Fernández-Rossier Dept. Física."— Presentation transcript:

1 Coherently induced ferromagnetism in Diluted Magnetic Semiconductors Southampton, OCES9-SCES2 September 7 st 2005 Joaquín Fernández-Rossier Dept. Física Aplicada, Univ de Alicante,Spain Slides in Collaboration with: C. Piermarocchi (Michigan State), P. Chen (Taiwan), A. H. MacDonald (University of Texas), L. J. Sham, (UC San Diego) G. Chiappe, E. Louis, E. Anda (Alicante)

2 Coherently induced ferromagnetism in Diluted Magnetic Semiconductors Southampton, OCES9-SCES2 September 7 st 2005 Joaquín Fernández-Rossier Dept. Física Aplicada, Univ de Alicante,Spain Slides in (Zn,Mn)S

3 Coherently induced ferromagnetism in Diluted Magnetic Semiconductors Southampton, OCES9-SCES2 September 7 st 2005 Joaquín Fernández-Rossier Dept. Física Aplicada, Univ de Alicante,Spain Slides in Magnetic Order Induced by subgap Laser radiation (Zn,Mn)S

4 Magnetic Impurities Localized Electrons Nuclei E laser >E G REAL population of electrons and holes Carrier Mediated Exchange Interactions Optically Induced Exchange Interactions

5 Magnetic Impurities Localized Electrons Nuclei VIRTUAL electrons and holes Carrier Mediated Exchange Interactions COHERENTLY Induced Exchange Interactions E laser

6 SYSTEM 1 BULK diluted magnetic semiconductors (DMS) PARAMAGNETIC to FERROMAGNETIC transition SYSTEM 2 3D Optical Cavity + Quantum Dot + 2 Mn atoms Full Quantum mechanical analysis of Optical RKKY JFR, cond-mat (2005 ) G. Chiappe, JFR, et al., cond-mat (2004 ) JFR, C. Piermarocchi, P. Chen, A. H. MacDonald, L. J. Sham, Phys. Rev. Lett 93, , (2004)

7 OUTLINE DMS ORKKY: macro and micro Coherently Induced Ferromagnetism CAVITY-Spin-doped Dot

8 BC AlSi NO PS GaGe InSn AsSe Sb II Zn Cd Hg IV V III VI Te II-VI Zn-Se Zn-S Cd-Te EFEF II-VI Semiconductors

9 BC AlSi NO PS GaGe InSn AsSe SbTe Zn Cd Hg Mn EFEF (II,Mn)-VI PARAMAGNETIC Semiconductors (II,Mn)-VI (Zn,Mn)-Se (Zn,Mn)-S (Cd,Mn)-Te Zn: Ar: 3d 10 4s 2 Mn: Ar: 3d 5 4s 2

10 BC AlSi NO PS GaGe InSn AsSe SbTe Zn Cd Hg Mn EFEF (II,Mn)-VI PARAMAGNETIC Semiconductors (II,Mn)-VI (Zn,Mn)-Se (Zn,Mn)-S (Cd,Mn)-Te Mn: neutral impurity, SPIN S=5/2 (3d 5 )

11 EXCHANGE INTERACTIONS Superexchange (AF) Conduction Band Valence Band EFEF 1 2

12 OPTICAL EXCHANGE INTERACTION. MACROSCOPIC THEORY

13 Macroscopic Explanation of optical ferromagnetism Reactive optical energy, due to matter-laser interaction: U depends on M Ferromagnetism (M>0) minimizes U (M) But entropy favors M=0 Competition between reactive optical energy and entropy Electric Field of the Laser Real part of retarded Optical Response function U depends on bands Bands Depend on M

14 =0 L j e c Mn j h c Mn B 100 meV PHOTON ENERGY (eV) (II,Mn)-VI Bands DEPEND on Mn magnetization

15 Confined Levels depend on Mn state EXPERIMENTS: L. Besombes et al., PRL 93, , (2004) Y. Léger et al. PRL. 95, (2005) THEORY: J. Fernández-Rossier, cond-mat/ CdTe nanocrystal +1Mn SINGLE SPIN DETECTION !!! 2S+1=6

16 CdTe+ 1Mn Quantum Dot: Carrier interacts with 1 Mn J. Fernández-Rossier, cond-mat/ Bulk (II,Mn)VI: carrier interacts with many Mn.. BECAUSE OF EXCHANGE L j e c Mn j h c Mn

17 OPTICAL EXCHANGE INTERACTION. microSCOPIC THEORY

18 Microscopic Theory: HAMILTONIAN Mean Field, VC aprox, HF-Pairing JFR, C. Piermarocchi, P. Chen, A. H. MacDonald, L. J. Sham, Phys. Rev. Lett 93, , (2004) KEY PARAMETERS

19 E U (k) E L (k) Rotating Frame RWA Coherent Occupation Microscopic Theory: Density Matrix

20 RESULTS for Zn Mn S Hamiltonian + Density Matrix + approximations yield U(M) (reactive energy), S(M) (entropy)

21 012 M (b) K B TS T=115 mK T=105 mK (a) M -1.2 U T /T C M =26 meV, T C =780 mK =41 meV, T C =114 mK =71 meV, T C =22 mK Results for (Zn 0.988,Mn ) S G

22 Transition Temperature for (Zn 0.988,Mn ) S Linear response fails there

23 Transition Temperature for (Zn 0.988,Mn ) S Also from ORKKY+ Mean Field ORKKY: C. Piermarocchi, P. Chen, L.J. Sham and D. G. Steel PRL89, (2002)

24 Isothermal transitions for (Zn,Mn) S T=0.5 K Switching ferromagnetism on and off !!! JFR, C. Piermarocchi, P. Chen, A. H. MacDonald, L. J. Sham, Phys. Rev. Lett 93, , (2004)

25 Experimental Issues Materials: –Moderate x (avoid superexchange) –Large exciton binding energy (osc. Stre) Detection: Easy (polarized PL) Smal detuning vs unwanted heating Transition Time vs Laser Pulse duration

26 Ferromagnetic Transition Time 012 M Gibbs Free Energy 012 M 012 M Laser off Laser On T L T 1

27 Cavity-Dot ORKKY. Motivation Effect of exciton dimensionality (JFR, L. Brey, PRL 2004) Confine Photons (increase Rabi) (G. Chiappe, JFR et al., condmat 2004) Optical RKKY in the Cavity-QD system: Photons are treated quantum mechanically Mn-exciton interaction is treated exactly Photon-exciton interaction is treated exactly

28 Cavity Dot System. State of the Art J. P. Reithmaier et al., Nature 432, 197 (2004) III-V g=0.1 meV g=16 meV M. Obert, APL 84,1435 (2004) Magnetic tuning in excitonic Bragg structures of (Cd,Mn)Te/(CdTe) J. Sadowski, H. Mariett, A. Wasiela, R. André, Y. Merle dAubigné, T. Dietl Phys. Rev. B56, R1664 (1997) II-VI

29 Cavity Dot System 1P,0X 0X,0P 1X,0P Photon LOWER ENERGY EXCITED STATE: Half and Half Exciton

30 Cavity Dot System Exciton 1P,0X 0X,0P 1X,0P Photon 1P+ 0X 1X, 0P LOWER ENERGY EXCITED STATE MOSTLY Photon

31 Cavity Dot System Exciton 1P,0X 0X,0P 1X,0P Photon LOWER ENERGY EXCITED STATE MOSTLY Exciton

32 Single Spin conditional Cavity Tuning 1P,0X 0X,0P 1X(+1),0P(-) Mn(-5/2) Photon LOWER ENERGY EXCITED STATE MOSTLY Exciton

33 Single Spin conditional Cavity Tuning 1P,0X 0X,0P Photon LOWER ENERGY EXCITED STATE MOSTLY Exciton 1X(+1),0P(-) Mn(+5/2)

34 Cavity + QD exciton + 2 Mn G. Chiappe, JFR, et al., condmat 2004

35 Cavity –QD exciton – 2 Mn G. Chiappe, JFR, et al., condmat 2004 Single Cavity mode, Single exciton

36 CAVITY DOT spin correlation T= 1 Kelvin REGION I REGION III REGION II

37 BULK Tc (ORKKY) CAVITY DOT spin correlation T= 1 Kelvin

38 Outlook Incoherent exciton coupling (magnetic polarons) Experiments and theory Virtual excitons (ORKKY) Theory Polariton exciton (QORKKY) Theory Experiment: Planar Cavities with Mn Condensed exciton coupling (BEC-RKKY) Theory (GIANT POLARON) PRB 1998, Kavokin

39 CONCLUSIONS New mechanism for ferromagnetism: coherently photoinduced Cavity + Spin Doped Dot: non-trivial spin-photon-exciton correlations Slides available in


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