Coherently photo-induced ferromagnetism in diluted magneti

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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
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
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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 -gt localized spins
• Mn-Mn interaction only first neighbors.
• X1, most spins INDEPENDENT
• Paramagnetic material
• Ferromagnetic when doped with holes Tclt2 Kelvin

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Macroscopic Explanation of optical ferromagnetism
Real part of retarded Optical Response function
Electric Field of the Laser
Band structure depends on magnetic state
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Spin unpolarized case
Detuning ?
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Spin polarized case
Spin dependent Detuning ?
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Microscopic Theory

1. Determination of steady state Density matrix for
   laser driven semiconductor. Electron-Hole
   COHERENCE
2. Determination of
3. Minimization of

Interaction Aproximation
Laser Matter All orders
h-Mn, e-Mn Mean Field,Virtual Crystal
Electron-Hole All orders (Ladder)
e-e and h-h Hartree-Fock
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Dilute exciton limit analytical results
Density of virtual excitons
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Results for (Zn0.988,Mn0.012) S
(b)
(a)
)
0
-3
meV nm
T115 mK
T105 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
1
2
0
0.5
1
T /T
M
C
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Transition Temperature for (Zn0.988,Mn0.012) S

• Tc??2
• Tc ??-3

Dilute exciton limit fails there
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Isothermal transitions for (Zn,Mn) S
T0.5 K
Switching ferromagnetism on and off !!!
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Conclusions

• New way of making semiconductors ferromagnetic
• Indirect exhange interaction mediated by virtual
  carriers
• Originated by e-h coherence
• Possible at Tgt1 Kelvin (with the right laser)

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Materials and Lasers

• Important material properties
• Robust Excitons
• Not much Mn (x1)
• (Zn,Mn)S, (Zn,Mn)Se
• (Zn,Mn)O ??

• Laser properties
• Tunable, around material band-gap
• Intense lasers
• Tc lt50 mK with cw laser
• Pulse duration longer than
• Switching time
• Switching time interesting question !!!!

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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)
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Phase Diagram
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Macroscopic Explanation of optical ferromagnetism

• U depends on ltMgt U(M)
• Ferromagnetism (ltMgt?0) minimizes U (M)
• But entropy favors ltMgt0

Real part of retarded Optical Response function
Electric Field of the Laser
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Macroscopic Explanation of optical ferromagnetism

• U depends on ltMgt U(M)
• Ferromagnetism (ltMgt?0) minimizes U (M)
• But entropy favors ltMgt0

Real part of retarded Optical Response function
Electric Field of the Laser
Spin dependent Detuning ?
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Microscopic Theory Relevant Interactions
Interaction BCS Bosonic Model
Laser Matter All orders Linear response ()
h-Mn, e-Mn MF VCA MF VCA
Electron-Hole Pairing All orders
Mn-Mn AF s-exc x replaced by xeff x replaced by xeff
e-e and h-h Hartree-Fock Irrelevant (linear response)
Linear Response Good if ?gt?
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Carrier mediated ferromagnetism
Functional of carrier density matrix
Entropic Penalty
Paramagnetic gain
What is the density matrix of the laser driven
(II,Mn)-VI semiconductor?
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• This prediction is a logical consequence of
• Experimentally established facts
• Theoretical concepts in agreement with
  experiments