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
(independent gating)
Colton et al., Solid State Comm. 152, 410 (2012)

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

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

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 doped Wavelength 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: Quantum Well:
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 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!