University of FLORIDA electron spin resonance study in a wide parabolic quantum well Russ Bowers Associate Professor of Chemistry University of Florida, Gainesville and National High Magnetic Field Lab

University of FLORIDA Acknowledgements Dr. Josh Caldwell please visit poster!! Dr. Josh Caldwell – presently at Naval Research Lab – please visit poster!! Dr. Alexey Kovalev Professor Guennadii Gusev Dr. Alexey Kovalev – presently at Pennsylvania State University Dept. of Electrical Engineering Professor Guennadii Gusev, Departamento Física dos Materiais e Mecânica, University of Sao Paulo, Brazilfunding National Science Foundation grant DMR CNPq (Brazil) / NSF International Cooperation Program. University of Florida Division of Sponsored Research.

University of FLORIDA “The ability to create nanometer-sized nuclear spin distributions combined with long solid-state nuclear spin lifetimes has important implications for the future of dense information storage, both classical and quantum. In addition, control over highly localized interactions between conduction electrons and lattice nuclei may provide a means to manipulate such information.” - D. Awschalom PRL (2003) 91 article no

University of FLORIDA Poggio et al, PRL (2003) 91 article no g-factor variation in a PQW time-resolved optical Faraday rotation detection realization of electrically tuned g-factor in a PQW

University of FLORIDA Weisbuch and Hermann, PRB B 15 (1977) 816. g-factor variation in bulk Al x Ga 1-x As conduction electron spin resonance data In a PQW, the local g-factor varies along z due to variation of Al fraction.

University of FLORIDA Can g-factor control be integrated with conventional solid state electronics? The Awschalom demonstration employed optical techniques… The answer is YES!! but in the context of EDESR?

University of FLORIDA H.W. Jiang and E. Yablonivitch demonstrated g-factor tuning in a gated Al 0.3 Ga 0.7 As/GaAs heterostructure. However, the tuning range was relatively small… about +.8% or -0.5%. H.W. Jiang and Eli Yablonovitch PRB, 64 (2001) article no (R). first demonstration of electrically detected g-factor tuning

University of FLORIDA Remotely doped wide parabolic quantum well (WPQW) Potential V=az 2 mimics a uniform 3D charge n + proportional to the curvature. Potential V=az 2 mimics a uniform 3D charge n + proportional to the curvature. Effective potential in loaded well has no central barrier Effective potential in loaded well has no central barrier  wide, uniform layer of high mobility carriers modest gate voltage can induce large displacements of wavefunction modest gate voltage can induce large displacements of wavefunction  permits tuning of the electronic g-factor over wide range. unique properties for spin-based semiconductor devices unique properties for spin-based semiconductor devices  i.e. spintronics, spin quantum computing model system to study 3D many body effects in high magnetic fields. model system to study 3D many body effects in high magnetic fields.  high mobility, uniform electron density g-factor tuning in the context of EDESR has yet to be demonstrated in a WPQW. g-factor tuning in the context of EDESR has yet to be demonstrated in a WPQW.

University of FLORIDA A.J. Rimberg and R.M. Westervelt PRB 40 (1989) Parabolic potential mimics a uniform change n +, electrons attempt to screen

University of FLORIDA (a)Calculation of the bound states and electron density distribution for a 4000Å WPQW (AG662). (b) Calculated g-factor in each subband i.

University of FLORIDA 4000Å WPQW structure (sample AG662) after illuminationbefore illumination x=.29 x=0.2 x=0.1 x=0.0 x=0.1 x=0.2 x=.29 Al.31 Ga.69 As 4000Å 100Å W e ~ 3000Å  4000Å 25Å GaAs/Al x Ga 1-x As superlattice

University of FLORIDA The tilt angle  might be expected to have a big effect in a WPQW high parallel field (  = 90 o ) high perpendicular field (  = 0 o ) Magnetic confinement dominates over the quantum well confinement because the cyclotron diameter << well width quasi-3D Identical to the energy levels of a 3DES  quasi-3D. Usual expression for a 2DES at high magnetic field (neglecting spin…) z B 1 Tesla

University of FLORIDA Gusev et al. PRB, 65 (2003) article no at high field, bulk Landau levels. cyclotron spacing >> subband spacing

University of FLORIDA Shayegan et al. Appl. Phys. Lett. 53 (1998) 791. at high field, bulk Landau levels.

University of FLORIDA transport properties 4000Å WPQW (AG662) depopulaton of lowest LL 2DES 3DES

University of FLORIDA In high B , changing the density increases the occupancy of the higher subbands. averaging assumes electron exchange rate is fast compared to electron Larmor frequency. high density, g factor decreases due to contribution of excited states low density, g-factor determined by ground state

University of FLORIDA V xx  -wave field coax x y z B ac current rf-field coax experimental configuration

University of FLORIDA 10M  GHz GHz GHz,12Hz rf coil B0B GHz,12 Hz. YIG Oscillator Doubling amp RF Synth. modulator coax ~50 MHz Oxford Heliox 3 He probe Hall bar patterned sample mounted on rotation stage PC  -wave antenna B rf Doubling amp In 12 Hz ref out Output Lockin #1 In 5V 530 Hz out Output Lockin #2

University of FLORIDA 300Å GaAs/Al 0.1 Ga 0.9 As square well EA124 (60 o ) ~5.5 T,  =1 WPQW (AG662) 6.5T,  =90 o (parallel field) |g| g-factor temperature dependence -0.6 % change

University of FLORIDA -0.6 % change in g-factor over 2  10 K range Lower g-factor at higher T is consistent with increased i>1 sublevel occupancy.

University of FLORIDA WPQW (AG662), 6.5T 300 Å QW (EA124,) ~5.5 T, =1,  =60 o temperature dependence of ESR linewidth Trend in line broadening is opposite for B vs. perpendicular orientation.

University of FLORIDA regions of lower g-factor become occupied  decreasing g-factor  g-factor broadening z partially filled well completely full well EFEF EFEF n(z ) increased density

University of FLORIDA WPQW (AG662) 300 Å QW (EA124,) ~5.5 T, =1,  =60 o up down  R xx (amplitude) temperature dependence  Much weaker T-dependence in parallel orientation.  Signal could be observed all the way to 10 K.  in 2DES, similar to narrow QW sample.

University of FLORIDA g-factor anisotropy in a 150Å GaAs/Al 0.35 Ga 0.65 As QW M. Dobers, K. v. Klitzing, G. Weimann, Solid State Comm. 70 (1989) N=0

University of FLORIDA Energy spectrum: Linear relationship between g-factor and Landau level energy was assumed: Spatial extent of wave function. G-factor anisotropy in a 2DES is due to non-parabolicity of conduction band diamagnetic term

University of FLORIDA g-factor anisotropy ~ 5%. monotonic decrease with  g-factor anisotropy in the WPQW g=g 0 -cB =36.65 GHz

University of FLORIDA g-factor decreases monotonically with increasing tilt angle.  diamagnetic term dominates at all angles in lowest landau level. especially true for WPQW  much larger in 400nm well compare to 15nm well qualitative interpretation: Anisotropy of g-factor in B || smaller than expected  calculated g 0 should be actual:  selectivity for detection in central layers of high mobility

University of FLORIDA dynamic nuclear polarization in PQW DNP in the q3DES closely resembles DNP in the 2DES WPQW and 30nm QW Direction of DNP establishes g<0.

University of FLORIDA T 1n in parallel and perpendicular fields

University of FLORIDA Overhauser shift decay in high parallel field Temperature independent nuclear spin relaxation in q3DES

University of FLORIDA 2DES, Korringa law: 2DES q3DES 3DES, T 1n temperature independent. where for kT <<  A. Berg, M. Dobers, R.R. Gerhardts, K. v. Klitzing PRL, 24 (1990) 2563.

University of FLORIDA A. Berg, M. Dobers, R.R. Gerhardts, K. v. Klitzing PRL, 24 (1990) WPQW (AG662) 150Å Al.35 Ga.65 As/GaAs QW filling factor dependence of T 1n around n=1

University of FLORIDA Fig  R xx (  MHz 75 As 69 Ga 71 Ga But no 27 Al signal!!! Frequency swept MDENDOR - see poster for details

University of FLORIDA  R xx (  ) (a) AG662 (b) AG662 (c) EA124 (d) EA124 Fig. 12.

University of FLORIDA OPNMR MDENDOR The solid blue curve is the angle dependence observed in a sample subjected to planar stress. Green symbols represent the splitting in the 30nm QW. Splitting due to selective detection in the band bending region?

University of FLORIDA Representative spectra illustrating the filling factor dependence of the 75 As spectrum in EA124. The lowest Landau level, illustrating the change in EFG with filling factor.

University of FLORIDA conclusions Increased linewidth in parallel field vs. perpendicular field. g-factor anisotropy, ~ 5% g-factor decreases monotonically over 0  90 o. in parallel field, detection is selective to higher mobility layers near center. Mechanism of EDESR may be different in q3DES than in the 2DES No Al-27 NMR despite good S:N on other isotopes. Korringa Relaxation in 2DES, T-independent Relaxation in q3DES broadening in MDENDOR spectrum could result from heterogeneous band bending across z.

University of FLORIDA application of electric field V gate g-factor tuning range in a WPQW is large

University of FLORIDA 5s 7s 10s 60s Effect of illumination on the g-factor Increasing the density is expected to decrease the g-factor However, results inconsistent…