Sensitivity Enhanced NMR and ESR Studies in GaAsAlGaAs Quantum Wells and Heterostructures

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Sensitivity Enhanced NMR and ESR Studies in
GaAs/AlGaAs Quantum Wells and Heterostructures

• Shu-chen Liu
• Department of Physics
• University of Florida
• September 2, 2003

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Principal Objective

• Develop a method to enhance NMR spectra of nuclei
  in the vicinity of a 2DES by dynamic nuclear
  polarization.

DNP-NMR

• Determine if DNP-NMR can provide greater
  signal enhancement and reduced heating in
  comparison to OPNMR.

• Evaluate the utility of DNP-NMR for probing
  the electronic states of the 2DES.

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Outline

• Description of samples
• Fabrication
• Transport properties
• Existing NMR methods
• optically pumped NMR
• thermally polarized NMR
• Proposed DNP-NMR Method
• Instrumentation Development
• Our Preliminary Results
• Summary

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Sample Fabrication and Structure
Sample EA124 Doped GaAs/Al0.1Ga0.9As Multiple
Quantum Well
Si d-doping
AlGaAs Barrier
30 nm
GaAs QW
2DES n6.9?1010 cm-2 ?4.4?105 cm2/Vs
AlGaAs Barrier
?21 layers
360 nm
Si d-doping
epoxy bond
Growth direction
Si
support
Grown by Molecular Beam Epitaxy (MBE) at Sandia
Labs
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Tilting the magnetic field
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Electronic Transport at High Magnetic Field
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Longitudinal Resistance Temperature Dependence
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Spin wave v.s. Skyrmion excitations in the 2DES
Ferromagnet ground state (all spin up)
low-lying elementary excitations
one reversed spin
(Spin wave)
Skyrmion charged spin-texture excitations, CSTEs
Spin split Landau levels
lower B field, decrease nB
excited state
ground state
increasing B
(Barrett website)
(localized spin wave)
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Outline

• Description of samples
• Fabrication
• Transport properties
• Existing NMR methods
• optically pumped NMR
• thermally polarized NMR
• Proposed DNP-NMR Method
• Instrumentation Development
• Our Preliminary Results
• Summary

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Method 1 OPNMR
 
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Optically Pumped NMR Knight Shift DataS.E.
Barrett et al., Phys. Rev. Lett. 74, 5112 (1995)
71Ga OPNMR Knight shift versus filling factor data
Knight shift equation
independent electron theory
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Method 2 Thermally polarized NMR
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Outline

• Description of samples
• Fabrication
• Transport properties
• Existing NMR methods
• optically pumped NMR
• thermally polarized NMR
• Proposed DNP-NMR Method
• Instrumentation Development
• Our Preliminary Results
• Summary

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Proposed Method DNP-NMR
Use microwave saturation of the 2DES spin
resonance to enhance NMR. Step 1 Set the
magnetic field to a value higher than the
resonant value. Step 2 Apply microwave field
to the sample. Step 3 slowly sweep the magnetic
field downward toward the resonance field
position. Step 4 continue the down sweep. Due
to polarization of the nuclei, resonance
position is pinned. Step 5 terminate the field
sweep, detect NMR. Repeat 1-5 with different
terminating fields, i.e. different filling
factors. NMR spectra are recorded as a function
of filling factor.
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Why ? Compare the DNP-NMR spectra to the OPNMR
spectra and thermally polarized NMR spectra. Can
a Knight shift be observed? Can spin diffusion
be observed? What NMR enhancement can be
obtained? Can the electron-nuclear hyperfine
coupling be measured by this method? Does DNP
polarize the same set of nuclei as OP? What
other advantages does DNP-NMR offer? Reduced
heating? Microwave irradiation does not perturb
the electron density of the sample as much as
optical irradiation. (a few mW/m2 ltlt 10-300
mW/cm2)
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DNP-NMR will provide a method to measure the
hyperfine coupling constant.
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Electron-Nuclear Cross Relaxation
Dynamic nuclear polarization
Dynamic Nuclear Polarization Overhauser Effect
Thermal Equilibrium -- Negligible net nuclear
polarization
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Proposed experiment procedure

• Microwave dynamic nuclear polarization (DNP)
  enhancement of NMR of quantum well nuclei.

Step (1) ESR occurs at B0 position,
where the ESR condition is fulfilled
h?g?BB. (? microwave frequency)
?xx
ESR position
(1) Up-sweep
B(T)
B0
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Proposed experiment procedure

• Microwave dynamic nuclear polarization (DNP)
  enhancement of NMR of quantum well nuclei.

Step (2) Down-sweep the field. The
resonance position is shifted towards lower
position upon the irradiation of
microwave. BresB0-BN. (BN induced
nuclear field)
?xx
(2) Down-sweep
ESR position
(1) Up-sweep
B(T)
B0
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Proposed experiment procedure

• Microwave dynamic nuclear polarization (DNP)
  enhancement of NMR of quantum well nuclei.

Step (3) Terminating the field- sweep at
Bf. Acquire the NMR spectra at Bf as a
function of filling factor and temperature.
BNB0-Bf . (BN Overhauser shift)
?xx
(3) Detect NMR here
(2) Down-sweep
ESR position
(1) Up-sweep
B(T)
BN
B0
Bf
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Outline

• Description of samples
• Fabrication
• Transport properties
• Existing NMR methods
• optically pumped NMR
• thermally polarized NMR
• Proposed DNP-NMR Method
• Instrumentation Development
• Our Preliminary Results
• Summary

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Instrumentation and Preliminary Results
Helium-3 fridge
10 Tesla Oxford superconducting magnet system
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region immersed In liquid helium-3
heater sensor
coax
rotation stage sample
rotator shaft
QW sample
Microwave antenna
Variable Angle 3He Transport/ENDOR Probe
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20-72 GHz Electrically Detected ENDOR Spectrometer
YIG Oscillator
10-18 GHz
Heliox 3He Probe
Doubling amp
Hall bar sample on goniometer
20-36 GHz
coax
modulator
Slide 24
rf coil
H0
20-36 GHz,12 Hz.
Doubling amp
40-72GHz,12Hz
10MW
coax
RF Synth.
Spectrometer
50 MHz
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Electrically Detected ESR Data
Transport detected ESR data near ?1, T2.4K
S.A. Vitkalov et al., Phys. Rev. B. 61, 5447
(2000)
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Electrically Detected ENDOR Spectra
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The NMR spectrometer scheme
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The circuit layout in the NMR spectrometer
Our home-made spectrometer
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Outline

• Description of samples
• Fabrication
• Transport properties
• Existing NMR methods
• optically pumped NMR
• thermally polarized NMR
• Proposed DNP-NMR Method
• Instrumentation Development
• Our Preliminary Results
• Summary

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The NMR software DATA ACQUISITION Free
induction decay (FID) of Gallium-71 from the bulk
GaAs sample at thermal equilibrium (T4K)
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The NMR software DATA PROCESSING FID and the
spectrum of Gallium-71 from the bulk GaAs sample
at thermal equilibrium (T4K)
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90 pulse calibration for Gallium-71 spectra
(pulse length1,2,3,4,5,6,7,8,9,10,12,14,16,18,20
?s) 90 pulse9?s. All the spectra are detected
after saturation (10 ?s pulse? 50) and delay 20
min.
amplitude
absorption
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Gallium-71 Spectrum from the bulk GaAs sample at
thermal equilibrium (T4K)
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OPNMR signals from the bulk GaAs sample
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Temperature dependence of OPNMR signal from the
71Ga of the bulk GaAs sample.
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Why is it more difficult to get the OPNMR signals
from GaAs quantum wells than from the bulk GaAs
sample? Lets find out how many nuclei in our QW
sample(EA124)
S. Melintes work The total number of QW (Ga
or As) nuclei in the sample ? 3?1018 ?10.6 times
that of our sample.
S. Barretts work (PRL(72) 1994) The total
number of 71Ga nuclei in QWs ? 2.13?1017 ?twice
that of our sample.
Ga
As
71Ga NMR spectra at ?1, ?0?, B5.7T
n1.4?1011cm-2 (S. Melinte 2001)
Solis State Physics (Ashcroft/Mermin, 1976)
Unit cell of diamond lattice
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Sensitivity Analysis
?923 times that of our EA124 QWs sample
The signal amplitude from gallium-71 in the bulk
GaAs sample at thermal equilibrium (T4K) is
6.961 with the signal-to-noise ratio ? 28 . In
terms of the number of 71Ga nuclei, the signal
amplitude from the GaAs quantum wells at thermal
equilibrium sample is supposed to be 0.0075, with
the signal-to noise-ratio ? 0.03!
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Summary

• Describe the fabrication and the structure,
  along with the
• transport properties of our GaAs quantum
  well sample.
• ? Review some NMR techniques
• optically pumped NMR
• thermally polarized NMR
• Propose the newly developed technique by DNP-NMR
  method
• and discuss the advantages.
• ? Introduce the accomplished instrumentation
  development.
• Demonstrate our preliminary results on bulk GaAs
  sample, and
• discuss the possible ways to approach on the
  GaAs quantum
• wells sample.

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Acknowledgement
Dr. Bowers Bowers group Alexey, Bhavin, Anil,
Josh Thank you very much for your participation!

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Sample geometryThe van der Pauw Method
(MIT, NIST website)
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Sheet resistance
The sheet resistance, RS, which yields the
mobility ?e1/RSnee, can be calculated from the
van der Pauw equation numerically exp(-?RA/RS)ex
p(-?RB/RS)1
(MIT, NIST website)
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DNP-NMR Experimental setup
Frequency control
YIG Oscillator
Heliox 3He Probe
Power supply
10-18 GHz
coax
Doubling amp
van der Pauw geometry sample
20-36 GHz
rf coil
H0
I(?0)
V(?0)?Rxx
coax
Spectrometer
RF Synth.
NMR Spectrometer