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)

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

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

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

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, (2002)

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)

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

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

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

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 )

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

OPTICAL EXCHANGE INTERACTION. MACROSCOPIC THEORY

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

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

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

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

OPTICAL EXCHANGE INTERACTION. microSCOPIC THEORY

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

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

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

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

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

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)

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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