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Optical study of Spintronics in III-V semiconductors
Xiaodong Cui University of Hong Kong
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Collaborators Spin Dynamics • Magneto-photocurrent
Dr. Yang Chunlei Mr. Dai Junfeng Theorist: Dr. Lu Hai-Zhou Prof. Shen Shun-Qing Prof. Zhang Fu-Chun
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Conventional electronic devices ignore the spin property and rely strictly on the transport of the electrical charge of electrons Adding the spin degree of freedom provides new effects, new capabilities and new functionalities
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No Need for Moving Spin: Potential for Low Power Dissipation!
Present Applications: Giant Magneto Resistivity in Ferromagnetic materials. Charge Spin No Need for Moving Spin: Potential for Low Power Dissipation!
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Outline Time resolved Kerr-rotation spectroscopy in the Spin dynamics study Spin Photocurrent in two dimensional electron gases of InGaAs
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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
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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.
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PBS: polarized beam splitter LC: lock-in amplifier L: lens
DET M1 YAG Ti:Sapphire M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 Sample I1 I2 I3 I4 PEM Chopper Pump Probe PBS2 PBS1 BS1 BS2 /2 Plate L3 L4 L5 I: Iris DET: Twin detector M: Mirror PBS: polarized beam splitter LC: lock-in amplifier L: lens
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g-factor Kerr-rotation spectroscopy
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
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g factor study by Kerr rotation spectroscopy
z y x Torque driving precession Spin projection along Z
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(a) GaAs thin film g= (T=5K) (b) GaAs 2DEG g= (T=5K) (c) GaAsN/GaAs quantum well (N~1.5%) g=+0.97
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Phase shift is determined by the experimental configuration
GaAsN/GaAs quantum well Phase shift is determined by the experimental configuration For g>0 Phase term gBBt/ħ+ for B>0 gBBt/ħ- for B<0
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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 ~ Gauss
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Spin flip mechanisms Electron-impurity scattering -- Elliott and Yafet (EY) mechanism Electron-hole exchange scattering -- Bir–Aranov–Pikus (BAP) Spin-orbital coupling -- D’yakonov–Perel’ (DP) mechanism
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Sample Descriptions Monolayer (ML) and Sub-Monolayer InAs embedded in GaAs A) (001) oriented: 1/3 ML, 1/2 ML, 1ML GaAs GaAs sub monolayer (1/3, 1/2…) InAs 1 monolayer InAs B) (311) oriented: 1ML GaAs 1 monolayer InAs
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Electronic states of Monolayer and sub-monolayer InAs
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Lateral size reduction strongly suppressed spin relaxation process
Low pumping (pump/probe =1 , about 5x1016 /cm3 in GaAs) T=77K, Laser: 1.52 eV, B=0T and 0.82 T Lateral size reduction strongly suppressed spin relaxation process
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Electric current and spin current
The electric current The spin current
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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, ; Phys. Rev. Lett. 96, (2006). Bhat et al., Phys. Rev. Lett. 85, 5432 (2000). Spin pumping (ferromagnetic resonance) Spin Hall effect
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Generation and Detection of Spin current -- Spin Hall effect
Converting to magnetization Converting to charge current Valenzuela, S. O. & Tinkham, M. Nature 442, 176–179 (2006). Awschalom, Science 306, 1910–1913 (2004) Kimura, Phys. Rev. Lett, 98, (2007) Wunderlich; Phys. Rev. Lett. 94, (2005) Wunderlich, Nature Physics, 5,675 (2009)
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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
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C2V symmetry H=(xky- ykx) (001)
Incident light: 0.8eV Linearly polarized light (Band edge excitation) Rashba coefficient =4.3E10-12 eVm
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J(Bx, By, )= C0By + CxBxsin2 + CyBycos2
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(c)
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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
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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 ~ ~10-3 /Tesla
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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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