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Coherently photo-induced ferromagnetism in diluted magnetic semiconductors

Coherently photo-induced ferromagnetism in diluted magnetic semiconductors. J. Fernandez-Rossier ( University of Alicante, Spain ) , C. Piermarocchi (MS) , P. Chen ( UCB ) , L. J. Sham (UCSD) , A.H. MacDonald (UT). 2004 American Physical Society March Meeting, Montreal.

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Coherently photo-induced ferromagnetism in diluted magnetic semiconductors

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  1. Coherently photo-induced ferromagnetism in diluted magnetic semiconductors J. Fernandez-Rossier (University of Alicante, Spain), C. Piermarocchi(MS), P. Chen(UCB), L. J. Sham (UCSD), A.H. MacDonald(UT) 2004 American Physical Society March Meeting, Montreal

  2. Coherently photo-induced ferromagnetism in diluted magnetic semiconductors Paramagnetic semiconductor (II,Mn)VI can become ferromagnetic when illuminated by coherent unpolarized light of frequency below the semiconductor band-gap. (cond-mat/0312540) J. Fernandez-Rossier (University of Alicante, Spain), C. Piermarocchi(MS), P. Chen(UCB), L. J. Sham (UCSD), A.H. MacDonald(UT) 2004 American Physical Society March Meeting, Montreal

  3. EG EF Materials: Diluted paramagnetic (II(1-x),Mnx)-VI (II(1-x),Mnx)-VI (Zn(1-x),Mnx)-Se (Zn(1-x),Mnx)-S (Cd(1-x),Mnx)-Te • Direct gap semiconductor • Mn d-electrons -> localized spins • Mn-Mn interaction: only first neighbors. • X=1%, most spins INDEPENDENT • Paramagnetic material • Ferromagnetic when doped with holes Tc<2 Kelvin

  4. Coherently photo-induced ferromagnetism • Laser features: • Frequency below gap: =EG-L>0 • No Photocarriers, no doping • Intensity (=dcvE0>1 meV) • Polarization state: not relevant

  5. Reactive optical energy, due to matter-laser interaction: • U depends on <M> • Ferromagnetism minimizes U (M) • But entropy favors <M>=0 Depends on bandstructure Macroscopic Explanation of optical ferromagnetism Real part of retarded Optical Response function Electric Field of the Laser Band structure depends on magnetic state

  6. <M>=0 Spin unpolarized case Detuning 

  7. jsdcMn<M> L jpdcMn<M> Spin polarized case Spin dependent Detuning 

  8. Microscopic Theory • Determination of steady state Density matrix for laser driven semiconductor. Electron-Hole COHERENCE • Determination of • Minimization of

  9. Dilute exciton limit: analytical results Density of virtual excitons

  10. Results for (Zn0.988,Mn0.012) S (a) (b) ) -3 0 T=115 mK meV nm T=105 mK -0.2 -2 S (10 -1.42 -0.4 ) ) -3 -1 -3 meV nm meV nm -1.2 -1.43 -2 -2 U (10 -2 -1 0 1 2 G (10 M -1.44 2 d =26 meV, T =780 mK M C 1 d =41 meV, T =114 mK C -1.45 d =71 meV, T =22 mK C 0 0 0.5 1 0 1 2 T /T M C

  11. 1.50 1.00 0.50 Transition Temperature for (Zn0.988,Mn0.012) S • Tc2 • Tc -3 Dilute exciton limit fails there

  12. Isothermal transitionsfor (Zn,Mn) S T=0.5 K Switching ferromagnetism on and off !!!

  13. Conclusions • New way of making semiconductors ferromagnetic • Indirect exhange interaction mediated by virtual carriers • Originated by e-h coherence • Possible at T>1 Kelvin (with the right laser)

  14. Materials and Lasers • Important material properties: • Robust Excitons • Not much Mn (x=1%) • (Zn,Mn)S, (Zn,Mn)Se • (Zn,Mn)O ?? • Laser properties: • Tunable, around material band-gap • Intense lasers • Tc <50 mK with cw laser • Pulse duration longer than • Switching time • Switching time: interesting question !!!!

  15.  jsd jsd   jpd jpd   jsd jpd ORKKY vs coherently photo-induced FM The SAME than Bosonic Model (*) C. Piermarocchi, P. Chen, L.J. Sham and D. G. Steel PRL89 , 167402 (2002)

  16. Always absorbing Always coherent T PM T=1.5 K PM Coherent PM T=2.0 K Absorbing FM FM FM (/J) Phase Diagram

  17. Reactive optical energy, due to matter-laser interaction: Macroscopic Explanation of optical ferromagnetism • U depends on <M>: U(M) • Ferromagnetism (<M>0) minimizes U (M) • But entropy favors <M>=0 Real part of retarded Optical Response function Electric Field of the Laser

  18. Reactive optical energy, due to matter-laser interaction: Macroscopic Explanation of optical ferromagnetism • U depends on <M>: U(M) • Ferromagnetism (<M>0) minimizes U (M) • But entropy favors <M>=0 Real part of retarded Optical Response function Electric Field of the Laser Spin dependent Detuning 

  19. No absorption= No real carriers= Optical Coherence: eff=  -|J|>0, where Microscopic Theory: Relevant Interactions * Linear Response: Good if >

  20. Carrier mediated ferromagnetism Functional of carrier density matrix Paramagnetic gain Entropic Penalty What is the density matrix of the laser driven (II,Mn)-VI semiconductor?

  21. Coherently photo-induced ferromagnetism Diluted paramagnetic semiconductor V V VI VI IV IV II II III III B B C C N N O O EG EG Zn Zn Al Al Si Si P P S S Cd Cd Ga Ga Ge Ge As As Se Se • Laser features: • Frequency below gap: =EG-L>0 • No Photocarriers • Intense (=dcvE0>0.1 meV) • Non circularly polarized Mn Hg Hg EF EF (II,Mn)-VI (Zn,Mn)-Se (Zn,Mn)-S (Cd,Mn)-Te II-VI Zn-Se Zn-S Cd-Te In In Sn Sn Sb Sb Te Te

  22. V VI IV II III B C N O Zn Al Si P S Cd Ga Ge As Se Mn Hg In Sn Sb Te

  23. This prediction is a logical consequence of: • Experimentally established facts • Theoretical concepts in agreement with experiments

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