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Spin Lifetimes in n-Doped Quantum Wells and Dots: Insights from Longitudinal Pump/Probe Method

Explore the physics of spin relaxation and coherence times in quantum wells and dots. Investigate how relaxation changes with dot size and probe tuning. Utilize the longitudinal Pump/Probe method to study self-assembled quantum dots and GaAs quantum wells, revealing nuclear polarization and spin dragging phenomena. Analyze data on spin lifetimes, including bi-exponential decays, to gain insights into the behavior of spins at different temperatures. Discuss the need for more data and theoretical understanding in collaboration with experts in the field. This research, funded by NASA EPSCoR program, achieves significant findings in spin dynamics.

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Spin Lifetimes in n-Doped Quantum Wells and Dots: Insights from Longitudinal Pump/Probe Method

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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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