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T 1 spin lifetimes in n-doped quantum wells and dots

T 1 spin lifetimes in n-doped quantum wells and dots. John S. Colton Brigham Young University. Students: (grad) Tyler Park (undergrads) Ken Clark David Meyer Daniel Craft Phil White Jane Cutler . Funding acknowledgement: NASA EPSCoR program.

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T 1 spin lifetimes in n-doped quantum wells and dots

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  1. T1 spin lifetimes in n-doped quantum wells and dots John S. Colton Brigham Young University Students: (grad)Tyler Park (undergrads) Ken Clark David Meyer Daniel Craft Phil White Jane Cutler Funding acknowledgement: NASA EPSCoR program Talk for APS March Meeting Feb 27, 2012

  2. Motivations • Which materials are best? • T1 = longitudinal lifetime, aka “spin relaxation time”, provides useful upper limit for T2, aka “spin coherence time” • What do we learn about the physics? • Example: in QDs, tuning lprobe will change subset of dots being studied. How will relaxation change with dot size?

  3. Longitudinal Pump/Probe Method • Colton et al., Solid State Comm. 152, 410 (2012) (independent gating)

  4. LCP pump RCP pump RCP pump Timing Diagram 12 ms PEM: ~ 1 ms (~10 periods) Pump: Spin polarization (expected): Probe: scan relative delay, lockin signal @ PEM frequency

  5. Data on bulk n-GaAs sample From Colton et al., Solid State Comm. 152, 410 (2012) T1 from fit: 359 ns

  6. Self-Assembled Quantum Dots (InAs on GaAs) Spin Decay Photoluminescence T1 from fit: ~5 ms (using EOM to modulate pump laser in place of PEM)

  7. GaAs Quantum Well: 14 nm, lightly dopedWavelength Dependence, cw response Kerr rotation signal (V) Low energy side: heavy hole exciton High energy side: light hole exciton Probe wavelength adjusted until response from QW seen

  8. Not all directions created equal… Black = scan up,  Green = scan down,  block pump laser, value shifts down—“spin dragging” 50% duty 10% duty 5% duty 1% duty

  9. Summary of Spin Lifetimes: 5K not reproducible lower energy side of peak higher energy side of peak Same value (T2*) No wavelength pattern

  10. 1.5K: Bi-exponential Decays black = raw data red = single exp fit: 265 ns blue = bi-exp fit: 226 ns, 1652 ns

  11. Conclusions • Quantum Dots: • T1 > 5 ms! • Need much more data, including wavelength study to probe different dot sizes • Quantum Well: • Probable nuclear polarization and “spin dragging” • Spin lifetimes from 50-250 ns at 5K; possibly up to 2 ms at 1.5 K • Under some conditions we get two lifetimes • bistability at 5K and large fields • bi-exponential at 1.5 K • Need to understand theory, discussions with Sophia Economou of NRL • Measurements over 5 orders of magnitude in T1!

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