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Coherently induced ferromagnetism in Diluted Magnetic Semiconductors Southampton, OCES9-SCES2 September 7 st 2005 Joaquín Fernández-Rossier Dept. Física Aplicada, Univ de Alicante,Spain jfrossier@ua.esjfrossier@ua.es. Slides in www.ua.es/personal/jfrossier/ 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)
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Coherently induced ferromagnetism in Diluted Magnetic Semiconductors Southampton, OCES9-SCES2 September 7 st 2005 Joaquín Fernández-Rossier Dept. Física Aplicada, Univ de Alicante,Spain jfrossier@ua.esjfrossier@ua.es. Slides in www.ua.es/personal/jfrossier/ (Zn,Mn)S
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Coherently induced ferromagnetism in Diluted Magnetic Semiconductors Southampton, OCES9-SCES2 September 7 st 2005 Joaquín Fernández-Rossier Dept. Física Aplicada, Univ de Alicante,Spain jfrossier@ua.esjfrossier@ua.es. Slides in www.ua.es/personal/jfrossier/ Magnetic Order Induced by subgap Laser radiation (Zn,Mn)S
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Magnetic Impurities Localized Electrons Nuclei E laser >E G REAL population of electrons and holes Carrier Mediated Exchange Interactions Optically Induced Exchange Interactions
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Magnetic Impurities Localized Electrons Nuclei VIRTUAL electrons and holes Carrier Mediated Exchange Interactions COHERENTLY Induced Exchange Interactions E laser <E G C. Piermarocchi, P. Chen, L.J. Sham and D. G. Steel, PRL89, 167402 (2002)
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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 0508235 (2005 ) G. Chiappe, JFR, et al., cond-mat 0407639 (2004 ) JFR, C. Piermarocchi, P. Chen, A. H. MacDonald, L. J. Sham, Phys. Rev. Lett 93, 127201, (2004)
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OUTLINE DMS ORKKY: macro and micro Coherently Induced Ferromagnetism CAVITY-Spin-doped Dot
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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
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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
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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 )
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EXCHANGE INTERACTIONS Superexchange (AF) Conduction Band Valence Band EFEF 1 2
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OPTICAL EXCHANGE INTERACTION. MACROSCOPIC THEORY
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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
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=0 L j e c Mn j h c Mn B 100 meV PHOTON ENERGY (eV) (II,Mn)-VI Bands DEPEND on Mn magnetization
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Confined Levels depend on Mn state EXPERIMENTS: L. Besombes et al., PRL 93, 207403, (2004) Y. Léger et al. PRL. 95, 047403 (2005) THEORY: J. Fernández-Rossier, cond-mat/0508235 CdTe nanocrystal +1Mn SINGLE SPIN DETECTION !!! 2S+1=6
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CdTe+ 1Mn Quantum Dot: Carrier interacts with 1 Mn J. Fernández-Rossier, cond-mat/0508235 Bulk (II,Mn)VI: carrier interacts with many Mn.. BECAUSE OF EXCHANGE L j e c Mn j h c Mn
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OPTICAL EXCHANGE INTERACTION. microSCOPIC THEORY
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Microscopic Theory: HAMILTONIAN Mean Field, VC aprox, HF-Pairing JFR, C. Piermarocchi, P. Chen, A. H. MacDonald, L. J. Sham, Phys. Rev. Lett 93, 127201, (2004) KEY PARAMETERS
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E U (k) E L (k) Rotating Frame RWA Coherent Occupation Microscopic Theory: Density Matrix
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RESULTS for Zn 0.988 Mn 0.012 S Hamiltonian + Density Matrix + approximations yield U(M) (reactive energy), S(M) (entropy)
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012 M -1.45 -1.44 -1.43 -1.42 -2 (b) -0.4 -0.2 0 -K B TS T=115 mK T=105 mK (a) -2012 M -1.2 U 00.51 T /T C 0 1 2 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 0.012 ) S G
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1.50 1.00 0.50 Transition Temperature for (Zn 0.988,Mn 0.012 ) S Linear response fails there
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Transition Temperature for (Zn 0.988,Mn 0.012 ) S Also from ORKKY+ Mean Field ORKKY: C. Piermarocchi, P. Chen, L.J. Sham and D. G. Steel PRL89, 167402 (2002)
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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, 127201, (2004)
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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
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Ferromagnetic Transition Time 012 M Gibbs Free Energy 012 M 012 M Laser off Laser On T L <T 1 Laser On T L >T 1
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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
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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
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Cavity Dot System 1P,0X 0X,0P 1X,0P Photon LOWER ENERGY EXCITED STATE: Half and Half Exciton
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Cavity Dot System Exciton 1P,0X 0X,0P 1X,0P Photon 1P+ 0X 1X, 0P LOWER ENERGY EXCITED STATE MOSTLY Photon
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Cavity Dot System Exciton 1P,0X 0X,0P 1X,0P Photon LOWER ENERGY EXCITED STATE MOSTLY Exciton
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Single Spin conditional Cavity Tuning 1P,0X 0X,0P 1X(+1),0P(-) Mn(-5/2) Photon LOWER ENERGY EXCITED STATE MOSTLY Exciton
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Single Spin conditional Cavity Tuning 1P,0X 0X,0P Photon LOWER ENERGY EXCITED STATE MOSTLY Exciton 1X(+1),0P(-) Mn(+5/2)
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Cavity + QD exciton + 2 Mn G. Chiappe, JFR, et al., condmat 2004
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Cavity –QD exciton – 2 Mn G. Chiappe, JFR, et al., condmat 2004 Single Cavity mode, Single exciton
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CAVITY DOT spin correlation T= 1 Kelvin REGION I REGION III REGION II
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1.50 1.00 0.50 BULK Tc (ORKKY) CAVITY DOT spin correlation T= 1 Kelvin
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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
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CONCLUSIONS New mechanism for ferromagnetism: coherently photoinduced Cavity + Spin Doped Dot: non-trivial spin-photon-exciton correlations Email: jfrossier@ua.es Slides available in www.ua.es/personal/jfrossier/
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