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.