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, 084307 (2012)

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

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, 081310R (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)

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 10-100 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

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

Wavelength Summary Peak positions: LH HH

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

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

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

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?

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

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, 084307 (2012)