Upload
justice-scarlett
View
220
Download
6
Tags:
Embed Size (px)
Citation preview
Coherently induced ferromagnetism inDiluted Magnetic Semiconductors
Southampton, OCES9-SCES2 September 7st 2005
Joaquín Fernández-RossierDept. Física Aplicada, Univ de Alicante,Spain
[email protected]. 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)
Coherently induced ferromagnetism inDiluted Magnetic Semiconductors
Southampton, OCES9-SCES2 September 7st 2005
Joaquín Fernández-RossierDept. Física Aplicada, Univ de Alicante,Spain
[email protected]. Slides in www.ua.es/personal/jfrossier/
(Zn,Mn)S
Coherently induced ferromagnetism inDiluted Magnetic Semiconductors
Southampton, OCES9-SCES2 September 7st 2005
Joaquín Fernández-RossierDept. Física Aplicada, Univ de Alicante,Spain
[email protected]. Slides in www.ua.es/personal/jfrossier/
Magnetic OrderInduced by subgapLaser radiation
(Zn,Mn)S
Magnetic ImpuritiesLocalized ElectronsNuclei
Elaser>E
G
REAL population of electrons and holesCarrier Mediated Exchange Interactions
,k ,k',' k
'11 SS '22 SS
Optically Induced Exchange Interactions
Magnetic ImpuritiesLocalized ElectronsNuclei
VIRTUAL electrons and holesCarrier Mediated Exchange Interactions
,k ,k',' k
'11 SS '22 SS
22 EPn
COHERENTLY Induced Exchange Interactions
Elaser<E
G
C. Piermarocchi, P. Chen, L.J. Sham and D. G. Steel, PRL89 , 167402 (2002)
SYSTEM 1BULK diluted magnetic semiconductors (DMS) PARAMAGNETIC to FERROMAGNETIC transition
SYSTEM 23D 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)
OUTLINE•DMS•ORKKY: macro and micro•Coherently Induced Ferromagnetism•CAVITY-Spin-doped Dot
B C
Al Si
N O
P S
Ga Ge
In Sn
As Se
Sb
II
Zn
Cd
Hg
IV VIII VI
TeII-VIZn-SeZn-S Cd-Te
EF
II-VI Semiconductors
B C
Al Si
N O
P S
Ga Ge
In Sn
As Se
Sb Te
Zn
Cd
Hg
Mn
EF
(II,Mn)-VI PARAMAGNETIC Semiconductors
(II,Mn)-VI(Zn,Mn)-Se(Zn,Mn)-S (Cd,Mn)-Te
Zn: Ar: 3d10 4s2
Mn: Ar: 3d5 4s2
B C
Al Si
N O
P S
Ga Ge
In Sn
As Se
Sb Te
Zn
Cd
Hg
Mn
EF
(II,Mn)-VI PARAMAGNETIC Semiconductors
(II,Mn)-VI(Zn,Mn)-Se(Zn,Mn)-S (Cd,Mn)-Te
Mn: neutral impurity, SPIN S=5/2 (3d5)
EXCHANGE INTERACTIONS
iihih
iieie
iiii
AF
rSMJ
rSMJ
MMiiJH
)(
)(
)',( '',
Superexchange (AF)
Conduction Band
Valence Band
EF
1
2
he JJ41
OPTICAL EXCHANGE INTERACTION.MACROSCOPIC THEORY
Macroscopic Explanation of optical ferromagnetism
EEU L'
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
<M>=0
L
jecMn<M>
jhcMn<M>
B
100 meV
PH
OT
ON
EN
ER
GY
(eV
)
(II,Mn)-VI
Bands DEPEND on Mn magnetization
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
CdTe+ 1Mn Quantum Dot:Carrier interacts with 1 Mn
J. Fernández-Rossier, cond-mat/0508235
MSJH
I
Ik MkSkJ
MJcMnkk
Bulk (II,Mn)VI: carrier interacts with many Mn
.. BECAUSE OF EXCHANGE
LjecMn<M>
jhcMn<M>
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, 127201, (2004)
KEY PARAMETERS
2
23
0
na
Ed
E
cv
LG
k
kkH
2
1
kU
kL
kE
EH
0
0
2
1
EU(k)
EL(k)
Rotating FrameRWA
00
01
2
2
vuv
uvu Coherent
Occupation
Microscopic Theory: Density Matrix
L
01
T
RESULTS for Zn0.988 Mn0.012 S
Hamiltonian + Density Matrix + approximations yield U(M) (reactive energy),S(M) (entropy)
MneMnh cJcJ ,,,
0 1 2M
-1.45
-1.44
-1.43
-1.42
-2
(b)
-0.4
-0.2
0-K
BT
S T=115 mKT=105 mK
(a)
-2 -1 0 1 2M
-1.2
-1
U
0 0.5 1T /TC
0
1
2
M
=26 meV, TC=780 mK
=41 meV, TC=114 mK
=71 meV, TC=22 mK
Results for (Zn0.988,Mn0.012) S
G
1.50
1.00
0.50
Transition Temperature for (Zn0.988,Mn0.012) S
Linear response fails there
3
2
cT
1.02
23
na
Transition Temperature for (Zn0.988,Mn0.012) S
Also from ORKKY+ Mean Field
ji
jiORKKY SSjiJH,
),(
ORKKY:C. Piermarocchi, P. Chen, L.J. Sham and D. G. SteelPRL89 , 167402 (2002)
Isothermal transitions for (Zn,Mn) S
T=0.5 K
Switching ferromagnetis
m on and off
!!!
JFR, C. Piermarocchi, P. Chen, A. H. MacDonald, L. J. Sham,Phys. Rev. Lett 93, 127201, (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
0 1 2M
Gib
bs
Free E
nerg
y
0 1 2M
0 1 2M
Laser off Laser OnTL<T1
Laser OnTL>T1
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 d’Aubigné, T. DietlPhys. Rev. B56, R1664 (1997)
II-VI
Cavity Dot System
1P,0X
0X,0P
1X,0P
Photon
LOWER ENERGY EXCITED STATE:Half and Half
Exciton
0
Cavity Dot System
Exciton
1P,0X
0X,0P
1X,0P Photon
1P+0X
1X,0P
LOWER ENERGY EXCITED STATEMOSTLY Photon
0
Cavity Dot System
Exciton 1P,0X
0X,0P
1X,0P
Photon
LOWER ENERGY EXCITED STATEMOSTLY Exciton
0
Single Spin conditional Cavity Tuning
1P,0X
0X,0P
1X(+1),0P(-)Mn(-5/2)
Photon
LOWER ENERGY EXCITED STATEMOSTLY Exciton
Single Spin conditional Cavity Tuning
1P,0X
0X,0P
Photon
LOWER ENERGY EXCITED STATEMOSTLY Exciton
1X(+1),0P(-)Mn(+5/2)
Cavity + QD exciton + 2 Mn
G. Chiappe, JFR, et al., condmat 2004
Cavity –QD exciton – 2 Mn
2,1'''' )()(
21
),,(
IIeh
ehhehe
dc
MccIJddIJ
bcddcb
ddEccEbbHF
G. Chiappe, JFR, et al., condmat 2004
Single Cavity mode, Single exciton
CAVITY DOT spin correlationT= 1 Kelvin
121 MM
REGION I
REGION III
REGION II
0
0
1.50
1.00
0.50
BULK Tc (ORKKY)
CAVITY DOT spin correlationT= 1 Kelvin
121 MM
OutlookIncoherent exciton coupling (magnetic polarons)
Experiments and theory
Virtual excitons (ORKKY)
Theory
Polariton exciton (QORKKY)
TheoryExperiment: 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
Email: [email protected] available in www.ua.es/personal/jfrossier/