Transcript "T1 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
John S. Colton
Brigham Young University
Students:
(grad) Tyler Park
(undergrads) Ken Clark
Daniel Craft
David Meyer
Phil White
Jane Cutler
Funding acknowledgement:
NASA EPSCoR program
Talk for APS March Meeting
Feb 27, 2012
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 lprobe 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
(independent gating)
• Colton et al., Solid State Comm. 152, 410 (2012)
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
Timing Diagram
12 ms
PEM:
RCP pump
~ 1 ms
LCP pump
RCP pump
(~10 periods)
Pump:
Spin polarization (expected):
Probe:
scan relative delay, lockin signal @ PEM frequency
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)
Spin Decay
Photoluminescence
T1 from fit: ~5 ms
(using EOM to modulate pump
laser in place of PEM)
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
Kerr rotation signal (V)
GaAs Quantum Well: 14 nm, lightly doped
Wavelength Dependence, cw response
High energy
side: light hole
exciton
Low energy
side: heavy
hole exciton
Probe wavelength adjusted
until response from QW seen
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
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
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
Summary of Spin Lifetimes: 5K
not reproducible
lower energy
side of peak
higher energy
side of peak
Same
value
(T2*)
No wavelength pattern
J.S. Colton, T1 spin lifetimes in n-doped quantum wells and dots
1.5K: Bi-exponential Decays
black = raw data
red = single exp fit: 265 ns
blue = 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 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!