Nature, Vol 458, 2009 Leon Camenzind FMM University of Basel,

Transcription

1 Nature, Vol 458, 2009 Leon Camenzind University of Basel,

2 Outlook Part I: Transient (Spin)-Grating Spectroscopy Part II: Theory of Persistent Spin Helix (PSH) Experimental results

3 Part I Transient (spin)-grating spectroscopy PRL, Vol 76, 1996

4 Transient-Grating Spectroscopy General idea: 1. Two laser beams interfere grating on sample changing optical properties 2. Probe pulse is diffracted from the property grating [1] Example: Concentration grating Two parallel linear polarized pump pulses amplitude modulation free carrier concentration grating Decay: Recombination and ambipolar diffusion In Semiconductor (2DEG): (1) Use of two (non-collinear) pulses change of optical properties due to excitons (2) Time-delayed probe pulse is diffracted by grating grating decay measurable (e.g. due to recombination) [1] A. Harata, Annu. Rev. Phys. Chem. 1999

5 Spin Grating I Optical Orientation effect Two pump pulses with crossed linear polarizations uniform amplitude but Electric field polarization is spatially modulated across excitation region Linear and circular polarized regions on sample Heavy holes excitons are spin polarized Spin polarization waves Wavevector: q = 2π/Λ (Λ is function of pump beams angle) Diffraction of circularly polarized probe pulse is spin dependent Spin grating lifetime can be measured A. Miller, PRL

6 Spin Grating II Initial condition Sinusoidal variation of Sz as a initial condition is equivalent to two equal amplitude Sy-Sz helices 1D Becomes important in combination with SOC Orenstein,SpinAps Asilomar Conference D

7 Spin Grating III Spin grating decay rate: Exponential decay fit decay rate For different spin polarisation wavelengths Diffusion constants s =55 s Motion of electrons Spin relaxation De: electron Diffusion coeff. Λ: grating spacing/wavelength (Da: ambipolar diffusion coefficient τr: recombination time)

8 Part II

9 Persistent Spin Helix Rashba (heterointerface) Dresselhaus (structural) α=β Transformation of k and diagonalisation of Hamiltonian Energy bands in new transformed spin base have interesting shifting property: Due to this property, precession in x-y-plane only depends on x+ Parameter: ~ 1μm q = =Q Persistent Spin Helix (PSH) J. Orenstein, PRL 97, 2007

10 Summary of Theory Spin orbit coupling is responsible for longer lifetimes for certain wavevectors q due to PSH formation α = β: infinite spin grating PSH Orientation of grating has to be in x'-y' direction Orenstein,SpinAps Asilomar Conference 2007

11 Sample Preparation 10 Periods GaAs QW contrast enhancement Substrate GaAs edged away for grating experiment Ti:Sapphire laser (100fs pulses) Tuning Rashba (α) couplinig: concentration-asymmetry of top/bottom d-layers but holding total donor concentration constant (8E11cm-2) Tuning Dresselhauss (β1) coupling: β1 k 2z function of QW width (d) No control over cubic Dresselhauss (β3) coupling strength

12 α β for different q q =Q Fitting For each q: two modes two exponential decays (τe,τr) One mode is enhanced due to the spin orbit field, the other one reduced Same set of SOC parameter for both decays q=0 measured with an other method, also delivers Ds α,β1,β3,ds

13 Tuning of α and β Rashba (α) tuning: d=12nm Dresselhaus (β1) tuning: a=1 Scaling with corresponding DS Max. lifetime rather at α=β1-β3

14 Temperature dependence of α = β sample d=11nm, a=0.8? Reduced mobility sample Quenching of spincoulomb drag PSH lifetime normalized to lifetime without SOC T-2.2 DS(T)? β << β : 3 1 eff β1 = β1 β3 (v/vf)2 Connection to electronelectron scattering?

15 Conclusion Persistent Spin Helix found lifetime enhancement up to 100 α=β in first order of Dresselhaus term achieved Cubic Dresselhaus term seems to be limiting factor Temperature dependence still an open question

16 Addition 1 Heterodyne Transient Grating

17 Addition 2 Rashba, Dresselhauss Fields

18 Persistent Spin Helix II J. Orenstein, PRL 97, 2007

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