1. Brigham Young University
Long-lived electron spins in a modulation doped (100) GaAs quantum well
John S. Colton
Brigham Young University
Students:
(grad) Tyler Park
(undergrads) Ken Clark David Meyer
Daniel Craft Phil White
Jane Cutler
Sample: Allan Bracker, NRL
Talk for APS March Meeting
Mar 21, 2013
Ref: J. Appl. Phys. 112, (2012)

2. Importance of spin lifetimes
DiVincenzo criteria, 1998
Kikkawa & Awschalom, long lifetimes in GaAs, 1998
Greilich et al., Mode locking, 2006
Balocchi et al., Electrical control of lifetimes, 2011
Connections
Implementations—spin control
Theory—understanding spin

3. Time-resolved Kerr rotation (cw laser)
QW sample (Bracker, NRL)
GaAs wells
2.8 nm well
4.2 nm well
AlGaAs barriers
Tischler et al, Phys Rev B 66, R (2002)
6.2 nm well
8.4 nm well
14 nm well
GaAs substrate
Time-resolved Kerr rotation (cw laser)
Solid State Comm. 152, 410 (2012)

4. What do we expect?
Lack of bulk inversion symmetry  spin splitting of CB (“DP”)
D’yakonov and Kachorovskii (1986), QWs
Bastard & Ferreira (1992), ionized impurity
scattering
ts = 2.5 ns for 15 nm well at 10 K
perhaps longer at 1.5 K
Bastard (1992), high fields (6-15 T)
ts = 1-2 ns for 9 nm well, ~B1/2
Terauchi et al (1999), 300 K, DK verified in part
Lau (2001, 2002), 100+ K
matched experiments of Terauchi, and Kikkawa & Awschalom
also T2  T1
Various T2* measurements: a few ns in GaAs, longer in II-VI
Longest lifetimes in any QWs: ~35 ns

5. Doped QWs: Selection Rules
Singlet trion:
e + e + h(either  or , either hh or lh)

6. Wavelength Summary
Peak positions: LH HH

7. Spin Decays!
pump and probe overlapped
5 K
ts = 169 ns
ts = 85 ns

8. Spin Lifetimes, Summary
ts > 1000 ns!
Very long lifetimes!!
Lifetimes increase monotonically with B (one exception), but not B1/2 dependence

9. Dynamic Nuclear Polarization, and removal
polarization “pinned”
polarization removed
Fields of 4T and higher

10. Selection Rules and Wavelength Dependence
Singlet trion:
e + e + h(either  or , either hh or lh)
What about exciton?
(II-VI: probes delocalized
electron spins)
HH exciton?

11. Evidence of Two Spin Populations?
Compare: II-VI QWs
localized? l1
“LHT” l2
delocalized?
Zhukov et al, PRB 2007
(see also Hoffman et al, PRB 2006)
Localized vs. Delocalized

12. Conclusions QW lifetimes via pump-probe
T1 lifetimes far longer than seen before… >1000 ns!
Previously in QWs, 35 ns max!
Not too far from Bastard & Ferreira theory (impurity-related scattering)
Probably also very long T2 lifetime!
Saw DNP, suppressed with NMR
Saw evidence of two spin populations
localized vs. delocalized
seen by others in II-VI QWs
Reference: Colton et al., J. Appl. Phys. 112, (2012)