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Optical study of Spintronics in III-V semiconductors

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. Outline.

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Optical study of Spintronics in III-V semiconductors

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  1. Optical study of Spintronics in III-V semiconductors Xiaodong Cui University of Hong Kong

  2. Collaborators • Spin Dynamics • Magneto-photocurrent Dr. Yang Chunlei Mr. Dai Junfeng Theorist: Dr. Lu Hai-Zhou Prof. Shen Shun-Qing Prof. Zhang Fu-Chun

  3. Outline • Time resolved Kerr-rotation spectroscopy in the Spin dynamics study • Spin Photocurrent in two dimensional electron gases of InGaAs

  4. Kerr Rotation spectroscopy Classical picture: Change in the polarization state when a linearly polarized light reflected from a strong magnet. Magnetization ↔Bound currents boundary conditions E M

  5. Microscopic origin – selection rule mj=-1/2 mj=+1/2 2 1 3 -1/2 +1/2 mj=-3/2 mj=+3/2 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.

  6. M2 Ti:Sapphire M1 I1 I2 M4 M3 I3 I4 YAG PBS1 M5 M6 M7 PEM Chopper M8 Pump M9 Sample M10 BS1 Probe /2 Plate f1 f2 L3 BS2 L5 M11 PBS2 L4 DET LA1 LA2 DET: Twin detector I: Iris M: Mirror L: lens PBS: polarized beam splitter LC: lock-in amplifier

  7. 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

  8. g factor study by Kerr rotation spectroscopy z y x Torque driving precession Spin projection along Z

  9. (a) GaAs thin film g=-0.42 (T=5K) (b) GaAs 2DEG g=-0.36 (T=5K) (c) GaAsN/GaAs quantum well (N~1.5%) g=+0.97

  10. GaAsN/GaAs quantum well Phase shift is determined by the experimental configuration  For g>0 Phase term gBBt/ħ+ for B>0 gBBt/ħ- for B<0

  11. Another Approach – magnetic field scan at fixed time delay Magnetic field shift is determined by the experimental configuration  • Advantage against time scan: • time shift in time scan ~ ps • magnetic shift in field scan ~ 102-103 Gauss

  12. Electric current and spin current The electric current The spin current

  13. Generation of Spin current • Spin injection • Spin polarized charge current • Non-local spin injection • Optical injection Intra-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

  14. Generation and Detection of Spin current -- Spin Hall effect Converting to charge current Converting to magnetization Valenzuela, S. O. & Tinkham, M. Nature 442, 176–179 (2006). Awschalom, Science 306, 1910–1913 (2004) Kimura, Phys. Rev. Lett, 98, 156601 (2007) Wunderlich; Phys. Rev. Lett. 94, 047204 (2005) Wunderlich, Nature Physics, 5,675 (2009)

  15. Zero-bias spin separation Ganichev et al., Nature Physics 2, 609 (2006). Intra-band excitation with linearly polarized THz radiation Spin dependent excitation and relaxation process

  16. (001) C2V symmetry H=(xky- ykx) Incident light: 0.8eV Linearly polarized light (Band edge excitation) Rashba coefficient =4.3E10-12 eVm

  17. J(Bx, By, )= C0By + CxBxsin2 + CyBycos2

  18. (c)

  19. Estimate the spin current • Measurement of Photocurrent with Hall Effect J~ 1.5X10-2A/m at 1mW • Estimate the spin current from SdH oscillation • Estimate the ratio of field induced charge current Vs. zero field spin current

  20. The magnetic field induced charge current vs. pure spin current Magnetic field induced charge current density ~ Pure Spin photocurrent density(ħ) ~ The ratio ~ 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

  21. 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.

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