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Long-lived electron spins in a modulation doped (100) GaAs quantum well

Long-lived electron spins in a modulation doped (100) GaAs quantum well. John S. Colton Brigham Young University. Students: (grad) Tyler Park (undergrads) Ken Clark David Meyer Daniel Craft Phil White Jane Cutler . Sample: Allan Bracker, NRL.

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Long-lived electron spins in a modulation doped (100) GaAs quantum well

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  1. Long-lived electron spins in a modulation doped (100) GaAs quantum well John S. Colton Brigham Young University Students: (grad)Tyler Park (undergrads) Ken Clark David Meyer Daniel Craft Phil White Jane Cutler Sample: Allan Bracker, NRL Talk for APS March Meeting Mar 21, 2013 Ref: J. Appl. Phys. 112, 084307 (2012)

  2. Importance of spin lifetimes • DiVincenzo criteria, 1998 • Kikkawa & Awschalom, long lifetimes in GaAs, 1998 • Greilich et al., Mode locking, 2006 • Balocchi et al., Electrical control of lifetimes, 2011 • Connections • Implementations—spin control • Theory—understanding spin

  3. QW sample (Bracker, NRL) GaAs wells Time-resolved Kerr rotation (cw laser) 2.8 nm well 4.2 nm well AlGaAs barriers Tischler et al, Phys Rev B 66, 081310R (2002) 6.2 nm well 8.4 nm well 14 nm well GaAs substrate Solid State Comm. 152, 410 (2012)

  4. What do we expect? • Lack of bulk inversion symmetry  spin splitting of CB (“DP”) • D’yakonov and Kachorovskii (1986), QWs • Bastard & Ferreira (1992), ionized impurity scattering • ts = 2.5 ns for 15 nm well at 10 K • perhaps 10-100 longer at 1.5 K • Bastard (1992), high fields (6-15 T) • ts = 1-2 ns for 9 nm well, ~B1/2 • Terauchi et al (1999), 300 K, DK verified in part • Lau (2001, 2002), 100+ K • matched experiments of Terauchi, and Kikkawa & Awschalom • also T2 T1 • Various T2* measurements: a few ns in GaAs, longer in II-VI • Longest lifetimes in any QWs: ~35 ns

  5. Doped QWs: Selection Rules • Singlet trion: e + e + h(either  or , either hh or lh)

  6. LH HH Wavelength Summary Peak positions:

  7. Spin Decays! pump and probe overlapped 5 K ts = 169 ns ts = 85 ns

  8. Spin Lifetimes, Summary • Very long lifetimes!! • Lifetimes increase monotonically with B (one exception), but not B1/2 dependence ts > 1000 ns!

  9. Dynamic Nuclear Polarization, and removal • Fields of 4T and higher polarization “pinned” polarization removed

  10. Selection Rules and Wavelength Dependence • Singlet trion: e + e + h(either  or , either hh or lh) What about exciton? (II-VI: probes delocalized electron spins) HH exciton?

  11. Evidence of Two Spin Populations? Compare: II-VI QWs • Localized vs. Delocalized localized? l1 “LHT” l2 delocalized? Zhukov et al, PRB 2007 (see also Hoffman et al, PRB 2006)

  12. Conclusions • QW lifetimes via pump-probe • T1 lifetimes far longer than seen before… >1000 ns! • Previously in QWs, 35 ns max! • Not too far from Bastard & Ferreira theory (impurity-related scattering) • Probably also very long T2 lifetime! • Saw DNP, suppressed with NMR • Saw evidence of two spin populations • localized vs. delocalized • seen by others in II-VI QWs Reference: Colton et al., J. Appl. Phys. 112, 084307 (2012)

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