T 1 spin lifetimes in n-doped quantum wells and dots
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Transcript of T 1 spin lifetimes in n-doped quantum wells and dots
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
T1 spin lifetimes in n-doped quantum wells and dots
Talk for APS March MeetingFeb 27, 2012
John S. ColtonBrigham Young University
Funding acknowledgement:NASA EPSCoR program
Students: (grad) Tyler Park (undergrads) Ken Clark David Meyer
Daniel Craft Phil WhiteJane Cutler
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
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 probe will change subset of dots being studied. How will relaxation change with dot size?
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
Longitudinal Pump/Probe Method
• Colton et al., Solid State Comm. 152, 410 (2012)
(independent gating)
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
Timing Diagram
(~10 periods)
Pump:
Probe:
scan relative delay, lockin signal @ PEM frequency
PEM: RCP pump RCP pumpLCP pump
Spin polarization (expected):
12 s
~ 1 s
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
Data on bulk n-GaAs sample
T1 from fit: 359 ns
From Colton et al., Solid State Comm. 152, 410 (2012)
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
Self-Assembled Quantum Dots (InAs on GaAs)
T1 from fit: ~5 ms
Photoluminescence
Spin Decay
(using EOM to modulate pump laser in place of PEM)
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
GaAs Quantum Well: 14 nm, lightly dopedWavelength Dependence, cw response
Probe wavelength adjusted until response from QW seen
Low energy side: heavy hole exciton
High energy side: light hole exciton
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J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
Not all directions created equal…
50% duty10% duty 5% duty1% duty
Black = scan up, Green = scan down,
block pump laser, value shifts down—“spin dragging”
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
Summary of Spin Lifetimes: 5K
higher energy side of peak
lower energy side of peak
No wavelength pattern
Same value (T2
*)
not reproducible
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
1.5K: Bi-exponential Decays
black = raw datared = single exp fit: 265 nsblue = bi-exp fit: 226 ns, 1652 ns
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
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 s
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!