Transcript Magnetic polarons

Photo-induced ferromagnetism in
Y. Hashimoto, H. Mino, T. Yamamuro, D. Kanbara, AT. Matsusue, BS. Takeyama
Graduate School of Science and Technology, Chiba University, Chiba, Japan
AFaculty of Engineering, Chiba University, Chiba, Japan
BThe institute for Solid State Physics, University of Tokyo, Chiba, Japan
magnetic polarons
bulk-Cd0.95Mn0.05Te via exciton
Magnetic polarons
Free Exciton Magnetic Polaron (FEMP)
Mn spin
h
Exciton spin
e
A Golnic, et. al. J. Phys. C16, 6073 (1983)
M. Umehara, Phys. Rev. B 68, 193202 (2003)
Localization only by sp-d exchange interaction
Bound Magnetic Polaron (BMP)
E
e
h
Local magnetic
order surrounding
an impurity bound
exciton
What is interesting about FEMP ?
FEMP
Circular polarized light
Photo-induced magnetism
via the FEMP
BMP
Circular polarized light
No magnetism via the BMP
Dark exciton magnetic polarons
Transient absorption with
circularly polarized pump
and probe pulses.
T [a. u.]
Hole spin relaxation
Individual spin relaxation of the
electron and hole
Dark exciton formation
Hole spin flip t < 1 ps
2
1
1
2
Exciton spin relaxation
()
()
0
5
10
15
Time delay [ps]
20
h

e
G
Dark exciton may form
dark exciton magnetic polaron
via the strong p-d exchange interaction
Localization energy
Free exciton magnetic polaron (FEMP) in CdMnTe
Localization energy of
Magnetic Polaron
Alloy Potential
fluctuation
Current work :
High quality CdMnTe sample
with low Mn concentration
5 10
Mn Concentration [%]
S. Takeyama, J. of Crys. Growth,
184-185 (1998) 917-920
x = 5 ~ 10% → FEMP energy : Large
Alloy potential fluctuation : Small
CW and time-resolved
Photoluminescence
Time- and spectral-resolved
photo-induced Faraday rotation
(TR- and SR-PIFR)
Sample
Bulk-Cd1-xMnxTe x = 5%
GaAs substrate
Cd1-yMgyTe
Transparent buffer layer
Cd0.95Mn0.05Te Thickness: 0.5 mm
Quartz disk
The opaque GaAs substrate was removed.
CdMgTe layer is transparent in the
wavelength of CdMnTe’s resonance.
Absorption and Photoluminescence spectrum
Peak
position
[eV]
1.4K
Binding
Energy
[meV]
Absorption
1.6750
FX
1.6740
FEMP
1.6722
1.8
Donor-BMP
1.6657
8.3
AcceptorBMP
1.6558
18.2
Distinct PL line of the FEMP appears !!
FEMP binding energy  1.8 meV
Absorption: 4.2 K, PL: 1.4K
PL Light source：He-Ne 633nm
Temperature and magnetic field dependence of the PL spectrum
Temperature
Magnetic field
FEMP
FX
1.4K
10K
1.668
1.672
1.676
Photon Energy [eV]
Photoluminescence [a. u.]
Photoluminescence [a. u.]
FX
1.4K
0.3T
0.2T
0.1T
FEMP
0T
1.664
1.668
1.672
Photon Energy [eV]
1.676
Time-resolved photoluminescence
BMP
FEMP
Setup
T = 1.4 K
76 MHz Ti:sapphire laser
l = 400 nm
Synchronized Streak camera
FX
400
200
0
1.65
1.66
1.67
1.68
Energy [eV]
Energy
[eV]
200
1.69
Photoluminecence [Arb.Units]
[ps]
Time
Time [ps]
600
FX 1.4K
FEMP
1000
100
10
BMP
Time [ps]
[ps]
Time
1
0
200
400
600
Time [ps]
100
tBMP > tFEMP > tFX
0
1.67
1.68
Experimental setup of PIFR
Delay Stage
λ/2
B.S.
λ/4
λ/2
Pump
Sample
1.4 ~ 300K
0 ~ 6.9T
Polarization
Beam Splitter
Optical Bridge
EX absorption
76MHz
Ti:Sapphire
Laser
Laser spectrum
1.670 1.675 1.680
Energy
Probe
Pump : Probe = 10 : 1
Exciton density:
1.1 x 1016 / cm3
Lock-in
Amplifier
Fourier transfer spectrum filter
Grating
Mirror
Band edge exciton
resonance absorption
EX
lens
Mirror
Probe beam
1.670 1.675 1.680
Energy
slit
FWHM
Pump：6.2meV (2.8nm)
Probe：1.6meV (0.7nm)
Photo-induced Faraday rotation
Spectral profile
< 1 ps: hole spin relaxation
PIFR [a. u.]

1.4K

8 ps: exciton spin relaxation
0
10
20
Time delay [ps]
30
Long decay process
Longer than the repetition time of
the excitation source 13 ns
PIFR () - PIFR (-)
Temporal profile
Negative delay time
EX resonance
0
1.675
Photon Energy [eV]
PIFR spectrum at 13 ns shows the
maximum value at the EX resonance
Zeeman splitting
W. Maslana PRB 63 165318 (2001)
Possible nature of the long decay signal in PIFR
1, Ferromagnetic Mn spin orientation caused by the FEMP
Mn spins are ferromagnetically aligned via
the FEMP formation
Mn spin relaxation time in Cd0.95Mn0.05Te 100 ns
T. Strutz et.al, Phys. Rev. Lett 68, 3912 (1992)
2, Dark exciton magnetic polaron
h
e
Mn spins are ferrpmagnetically aligned
via the DEMP formation
The relaxation time of the dark exciton
is much longer than the bright exciton
Future work
The origin of the long PIFR signal
Resonant spin amplification
Direct observation of the
ferromagnetically aligned
Mn spins by means of
Resonant Spin Amplification
Bright-exciton dark-exciton
level crossing
Energy [eV]
J. M. Kikkawa, PRL 80 4313 (1998)
1.6740
1.6739
Bright exciton
Dark exciton
1.6738
1.6737
0
5 10 15 20
Magnetic Field [mT]
Summary
• Performed first time-resolved Faraday rotation
on CdMnTe which shows clear FEMP PL
Spin dynamics of holes, electrons and Mn ions
• tspin (hole) < 1 ps
• tspin (electron) ~ 8 ps
• tspin (Mn) > 13 ns
Possible evidence of photo-induced
magnetism via FEMP and DEMP formation
h
h
e
e
Dark excitonic effect ?
T [a. u.]
Transient absorption shows very long decay
Radiative decay time < 300 ps
t = 139 ps
Dark exciton ?
0
0
500
Time delay [ps]
Transient absorption spectrum
Red shift (~ 0.3 meV)
T/T
0.5
0.0
1.3 ps
EX
341 ps
BGR?
DX
13 ns
-0.5
1.67
1.68
Photon energy [eV]
1.69
Do dark excitons cause
band gap renormalization ?
FEMP structure in CdMnTe
Hole wave function:
Electron wave function:
14.4 A
64 A
Mott density: 9.1 x 1017/cm3
(In the present case, rs = 4.4)
In the hole wave function:
In the electron wave function:
NMn ~ 1
NMn ~ 100
Hole wave function
Electron wave function
MASAKATSU UMEHARA, PRB 67, 035201 (2003)
Crystal structure of CdTe
Crystal structure of the CdTe: Zinc Blend
In one unit cell,
Cd: 4 peaces
Te: 4 peaces
http://www.uncp.edu/home/mcclurem/lattice/zincblende.htm
CdTe unit cell
:
CdTe unit cell volume
6.482 A
:
Number of the CdTe unit cell:
(6.5 1010 ) 3 m3  2.751028 m3
1
 3.63  10 27 m  3
 28
3
2.75  10 m
1.4 K
11.1mW
8.0mW
5.0mW
3.5mW
2.0mW
1.4mW
MP
FX
MP
’
Ï•ª‹­“x [a.u.]
Photoluminescence [a.u.]
Excitation source: He-Ne laser
Integrated Intensity [Arb. Units.]
Super linear increase of the PL intensity in Cd0.99Mn0.01Te
In low excitation regime
8000
MP
6000
MP’
4000
2000
0
2
4
6
8
10
Excitation Power [mW]
1.595 1.600 1.605 1.610 1.615
Energy [eV]
FX
conventional Gaussian type
MP
inverse-Boltzman type
MP’
inverse-Boltzman type
MP and MP’ Line show the
super-linear increase against
the excitation power
MP ∝ I1.3
MP’ ∝ I1.3
Out line
1. What is free exciton magnetic polaron ?
2. Sample
3. Results & Discussion
PL & absorption
Photo-induced Faraday rotation
4. Conclusions
Estimation of the
dark exciton density and lifetime
rs  (
J
3
1 13
)
3
4 (aex ) n
k BT
Ry
E  (3.24  rs
3 4
)  (1  0.0478 rs  J 2 )1 4
3
rs=(3/(4*pi*(aex^3)*n))^(1/3)
print rs
J=kB*T/Ry
DE=(-3.24*rs^(-3/4))*(1+0.0478*(rs^3)*(J^2))^(1/4)
print DE
PIFR [a. u.]
What is the meaning of the negative delay region?
-13 ps
Ti:S laser
76 MHz
0 ps
+13 ps
2
1
1

G
2