Optical study of Spintronics in III-V semiconductors Xiaodong Cui University of Hong Kong.
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Transcript of Optical study of Spintronics in III-V semiconductors Xiaodong Cui University of Hong Kong.
Optical study of Spintronics in III-V semiconductors
Xiaodong Cui
University of Hong Kong
Collaborators
• Spin Dynamics • Magneto-photocurrent
Dr. Yang Chunlei Mr. Dai Junfeng
Theorist:
Dr. Lu Hai-Zhou
Prof. Shen Shun-Qing
Prof. Zhang Fu-Chun
Time resolved Kerr-rotation spectroscopy in the Spin dynamics study
Spin Photocurrent in two dimensional electron gases of InGaAs
Outline
Kerr Rotation spectroscopy
Classical picture: Change in the polarization state when a linearlypolarized light reflected from a strong magnet.
Magnetization ↔Bound currents boundary conditions
nKBB ˆ21 E
M
• Microscopic origin – selection rule
mj=-3/2 -1/2
mj=-1/2
+1/2 mj=+3/2
mj=+1/2
mj=-1/2 mj=+1/2
Pump beam: Creating Spin Polarization via Optical injection.
Probe beam: A linearly polarized light is a superposition of a left and right circularly Polarized lights.
31
2
M: Mirror I: Iris
PBS: polarized beam splitter
DET: Twin detector
LC: lock-in amplifier L: lens
f1 f2
LA1 LA2DET
M1
YAG
Ti:SapphireM2
M3M4
M5M6M7
M8
M9
M10
M11
Sample
I1 I2
I3I4
PEMChopper
Pump
Probe
PBS2
PBS1
BS1
BS2
/2 PlateL3
L4
L5
g-factor
Existing techniques to study g factor:
Electric transport Low temperature, high requirements for sample quality
Electron spin resonance unpaired electron
Magneto-photoluminescence complex origins, signal reflects information of exciton
Kerr-rotation spectroscopy Magnitude, NO sign information
x
y
z
BSgBT B
)/( Torque driving precession
)/cos()/exp()( 0 BtgtStS BSZSpin projection along Z
(b) GaAs 2DEG g=-0.36 (T=5K)
(c) GaAsN/GaAs quantum well (N~1.5%)g=+0.97
(a) GaAs thin filmg=-0.42 (T=5K)
)/cos()/exp()( 0 BtgtStS BSZ
GaAsN/GaAs quantum well
Phase shift is determined by the experimental configuration
For g>0Phase term gBBt/ħ+ for B>0
gBBt/ħ- for B<0
)/cos()/exp()( 0 BtgtStS BSZ
Another Approach – magnetic field scan at fixed time delay
Magnetic field shift is determined by the experimental configuration
)/cos()/exp(0 BtgtS BS
Advantage against time scan: • time shift in time scan ~ ps• magnetic shift in field scan ~ 102-103 Gauss
Electric current and spin current
The electric current 0 jjeJ c
The spin current 02
jjJ s
Generation of Spin current
Spin injection Spin polarized charge current Non-local spin injection
Optical injectionIntra-band Linearly polarized light: Ganichev et al., Nature Physics 2, 609 (2006).
Inter-band Linearly polarized light (one photon, two photon):
H. Zhao et al., PHYSICAL REVIEW B 72, 201302 2005; Phys. Rev. Lett. 96, 246601 (2006).Bhat et al., Phys. Rev. Lett. 85, 5432 (2000).
Spin pumping (ferromagnetic resonance)
Spin Hall effect
Generation and Detection of Spin current -- Spin Hall effect
Converting to magnetization Converting to charge current
Awschalom, Science 306, 1910–1913 (2004)
Wunderlich; Phys. Rev. Lett. 94, 047204 (2005)
Valenzuela, S. O. & Tinkham, M. Nature 442, 176–179 (2006).
Kimura, Phys. Rev. Lett, 98, 156601 (2007)
Wunderlich, Nature Physics, 5,675 (2009)
Ganichev et al., Nature Physics 2, 609 (2006).
Zero-bias spin separation
Intra-band excitation with linearly polarized THz radiation Spin dependent excitation and relaxation process
Incident light: 0.8eV Linearly polarized light (Band edge excitation)Rashba coefficient =4.3E10-12 eVm
C2V symmetry
H=(xky- ykx)
(001)
J(Bx, By, )= C0By + CxBxsin2 + CyBycos2
(c)
Estimate the spin current
Measurement of Photocurrent with Hall Effect J~ 1.5X10-2A/m at 1mW
Estimate the spin current from SdH oscillation
mAn
nJJ S /104~ 4
Estimate the ratio of field induced charge current Vs. zero field spin current
TeslaBJBJ Sx /107.1)0(/)( 2
The magnetic field induced charge current vs. pure spin current
yx
xk h
kk
mk
kEV
2
*
sincos)(
)(1
2sin2sin2cos2cos sincos0, cvcvcvkcv
kh /Magnetic field induced charge current density ~
kvks yxxyx
x ,,, /21/
Pure Spin photocurrent density(ħ) ~
The ratio ~Bg
mk
B*
22
2
*/mk
In our case, Fermi energy ~ 10-1~10 -2eV (n=9E11cm-1), Zeeman energy hu=1.2E-4 eV/Telsa (g= -0.4)
The Ratio ~ 10-2 ~10-3 /Tesla
Conclusion
Magnetic field induced photocurrent via direct inter-band transition by a linearly polarized light
Our experiments support that the spin photocurrent could be generated by linearly polarized light absorption in material with spin-orbit coupling.
The conversion of spin current to magnetic field induced photocurrent is around 10-2~10-3 per Tesla.