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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, UT ), C. Piermarocchi (MS) , P. Chen ( UCB ) , L. J. Sham (UCSD) , A.H. MacDonald (UT).

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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, UT), C. Piermarocchi(MS), P. Chen(UCB), L. J. Sham (UCSD), A.H. MacDonald(UT) Paramagnetic semiconductor (II,Mn)VI can become ferromagnetic when illuminated by coherent unpolarized light of frequency below the semiconductor band-gap.

  2. EG EF Properties of the 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 • Mn-Mn interaction: only first neighbors. • For x=0.012 • 0.002 coupled to nn (2%) • 0.01 is free (80%) -PARAMAGNET If doped with holes, FERROMAGNET at Tc<2 Kelvin

  3. Coherently photo-induced ferromagnetism • Laser features: • Frequency below gap: =EG-L>0 • No Photocarriers, no doping • Intensity (=dcvE0>0.1 meV) • Polarization state: not relevant

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

  5. Selection Rules jsdcMn<M> L B <M>=0 jpdcMn<M> Exchange Interaction. Giant Spin Splitting

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

  7. What is the density matrix of the laser driven (II,Mn)-VI semiconductor? Entropic Penalty Paramagnetic Gain (Optical Energy) Functional of Carrier Density Matrix

  8.  Rotating Frame RWA EU(k) L EL(k) Density matrix: effect of the laser > >(T1)-1 Coherent Occupation

  9. No absorption= No real carriers eff=  -|J|>0

  10. Microscopic Theory: Relevant Interactions • (*) Linear Response: Good if > • OK, since  >|J|> and |J|>20 meV

  11. Microscopic Theory: Bosonic Model

  12. 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

  13. 1.50 1.00 0.50 Transition Temperature for (Zn0.988,Mn0.012) S • Tc2 • Tc -3 Linear response fails there

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

  15. 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 !!!!

  16.  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)

  17. Conclusions • New way of making semiconductors ferromagnetic • Ferromagnetism mediated by virtual carriers • Originated by optical coherence • Possible at T>1 Kelvin (with the right laser)

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

  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

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