Transcript Слайд 1 - TU Muenchen

SAINT-PETERSBURG STATE
UNIVERSITY
EXPERIMENTAL STUDY OF SPIN
MEMORY IN NANOSTRUCTURES
ROMAN V.
CHERBUNIN
1. About a possibility of the “spin devices” creation.
CONTENTS
2. Optical orientation - method of creation of the nonequilibrium spins in semiconductors and
semiconductor structures.
3. Detection of a non-equilibrium spin in semiconductors
by the study
a) of luminescence.
b) of reflection (Faraday effect).
4. Wavelength dependence of Kerr effect .
5. Spin lifetime. Possible mechanisms of spin relaxation.
6. Our experiments. Time-resolved observation of a spin
dynamics. Modification of a pump-probe method.
7. Future trends of a “spin devices” . Adaptation of
quantum dots for this applications.
OUR DREEMS
“LONG-TIME” SPIN MEMORY
“FAST” SPIN MEMORY
(DURIND ONE CLOCK)
GAP
GAP
QW
QW
SUBSTRATE
SUBSTRATE
We have a two dimensional
array of electrons in a quantum
We have a two dimensional array of
excitons in a quantum well. This
well. This electrons content our
information in it’s polarization. To
save information electrons have do
not move and do not change
polarization.
excitons also can content information in
it’s polarization. But their lifetime is
about 10-11s, and it is shorter than any
other time.
If we study spin polarization using light, it isn't ease to separate
electron and exciton spin polarization.
OPTICAL ORIENTATION IN GaAs QUANTUM WELLS
Conduction band
Conduction band:
I = 0; S =1/2
J =1/2 ; m = ±1/2
Valence band:
I = 1; S =1/2

1
2


3

2
1
2


J =1/2 ; m = ±1/2
1
2

1
2

3
2
AlGaAs
GaAs
AlGaAs
Valence band
J =3/2 ;
m =±1/2 (light hole)
m=±3/2 (heavy hole)
In quantum wells, to take
a spin polarization in a
case of photo inducted
carriers is quite simple.
Irradiation by circular
polarized light produce
100% spin polarization.
Absorption of circular polarized light in a bulk GaAs result in an
appearance of two kinds of excitons: (-3/2,+1/2) and (+1/2,-1/2). In a
narrow quantum wells they energy levels are different, and we can excite
them separately.
Laser
Glan
prism
Retarder λ/4
Sample in
cryostat
CRATION OF ELECTRON SPIN POLARIZATION
Electrons
from donors
Photo
inducted
electron –
hole pairs
Circularly
polarized light
Circularly polarized light creates
polarized electrons and holes.
Hole lost spin orientation in a few
picoseconds (10-12). If there are
electrons from donors, they could
recombine with holes as well as
photo inducted ones. The
remained after recombination
electrons would have a preferred
spin orientation. This orientation
would not 100%, but for our study
even 10% is still enough.
Note1: If electron – hole pairs are being produced in a bound state (exciton), they are
also polarized free electrons, but this process is more complicated and is not enough
studied.
SPIN POLARIZATION MEASUREMENT
Electrons
from donors
Где
Photo
inducted
electron –
hole pairs
Elliptically
polarized light
(luminescence)
With light cooperate electron - hole
pairs, whose total angular moment
does not exceed unit. Electron with
spin 1/2 and hole with spin 3/2 can
have the total moment 1 or 2
depending on their mutual
orientation. As it was mentioned
above, a hole in time about ten
picoseconds loses orientation. But
the electron - hole pair can be
highlighted only at that moment
when hole orientation will return to
an initial state. Thus, the
luminescence will be polarized the
same as an absorbed light .
USE FARADAY EFFECT FOR SPIN POLARIZATION MEASUREMENT
Nonlinear Faraday effect
Simple Faraday effect
Medium in a
magnetic field
m =+1
α
σ-
σ+
α ~ Δn ~ B
Refractive index
σ+
Photon energy
α
m =+1
m =-1
ΔE = μB g B
m =-1
Refractive index
Pumped
medium
Photon energy
σ-
Different level
population makes
refractive indexes
for σ + and σ –
polarized light
different. It results
in light rotation.
SPIN RELAXAOTION
 The main mechanism for the hole spin relaxation is interaction with phonons.
 The reason of high efficiency
of this mechanism is that a hole
has orbit angular momentum.
 There are many reasons for spin relaxation in semiconductor.
All of them are based on particle motion.
current
magnetic field
 Magnetic momentum
rotate slowly around the
magnetic field
MODIFICATION
OF PUMP-PROBE
PUMP-PROBE
SETUP SETUP
Ti:S PULSED
Laser
Pulse duration: 100 fs.
Repetition rate: 80 MHz
(12,5 ns).
Delay
length: 30
cm (600 ps)
Half mirror
Static delay
Variable delay
Probe shaper
Pump shaper
(Monochromator & Glan) (Monochromator & Glan)
Lens
Sample in
cryostat
Retarder λ/4
Lens
Balance photo detector
T = 10 K.
Light power:
800 W cm-2
Sensitivity:
10-6 rad
(10-8 grad).
DETAILS. BALANSE PHOTO DETECTOR
I 0  E0
2
Ey
E0
α
-U
Ey
Iy
+U
Ix
Ix - Iy

Ex
Ex
Signal (i)

iI 0 cos 2 (  450 )  sin 2 (  450 )  2 I 0
EXPERIMENTAL RESULTS
Quantum well without free electrons
τ 2  200 ps
Δα
Δα
τ1  10 ps
Delay
0
40
80
Delay (ps)
Δα
A
B
804
806
808
810
Wavelength (nm)
Thank you
for
your attention
The End
© Copyright. All rights reserved. Saint-Petersburg, 2005