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

Ker

r ro

tatio

n si

gnal

(V

)

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!