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Coherent Raman spectroscopy for the detection of electron spin resonance

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Title: Coherent Raman spectroscopy for the detection of electron spin resonance


1
Coherent Raman spectroscopy for thedetection of
electron spin resonance
  • Daniel Wolverson
  • Department of Physics
  • University of Bath

2
Spintronics motivation
  • Magnetic semiconductors from well-known to highly
    controversial...
  • CdMnTe (Mg)
  • ZnMnSe (Be,S)
  • GaMnAs (Al)
  • GaMnN
  • Other transition metals besides Mn
  • Cr, Co, Fe...

dilute magnetic semiconductor (DMS)
  • M Oestreich et al, Appl. Phys. Letts 74 (1999)
    1251 (Marburg / Hull)
  • R Fiederling et al, Nature 402 (1999) 787
    (Wuerzburg)
  • D Ferrand et al., Sol. St. Commun. 119 (2001) 237
    (Grenoble / Wuerzburg / Warsaw)

3
Optical measurement of g factors
Electron Spin Resonance
(resolution)
(sensitivity and resolution)
(sensitivity)
4
SFRS experimental system
  • resolution limited either by spectrometer or
    laser
  • corresponds to 0.01 in the g-factor (which is of
    order 1 to 2 typically)
  • Fabry-Perot detection and single-frequency dye
    laser might be a conventional next step.

5
Example electron SFRS of cubic CdSe
6
Spin flip Raman introduction
  • Inelastic light scattering via spin flip
  • Sidebands are produced shifted up and down in
    energy with respect to the laser line a Zeeman
    splitting is measured.
  • Spin relaxation lifetimes, band non-parabolicity
    effects, constraints on momentum transfer of free
    carriers, and even collective 2DEG excitations in
    QWs can all modify SFRS energy or linewidth
  • SFRS is strongly resonant at free or bound
    exciton transition energies.

7
Spin flip Raman spectra
  • Electron SFRS seen in modulation-doped 15nm
    layers of ZnMnSe
  • Transitions between Mn2 3d levels (PMR) also
    seen (g2.0)
  • Electron SFRS Raman shift follows Brillouin
    function of field
  • Doping levels from 109 to 3 x 1011 cm-2 were
    studied.
  • Bulk-like layers with 1015 lt n lt 1019 cm-3 were
    also studied.

8
Electron SFRS in Cd1-xMnxTe
  • Nominal x 0.005 (0.5).
  • SFRS measures CB splitting only (in contrast to
    PL, PLE, reflectivity). No VB or diamagnetic
    effects.
  • CB splitting here follows a Brillouin function.
  • Fitting yields effective x, electron temperature
    (and, in general, bound magnetic polaron energy).
  • Mn2 3d5 has pure spin S5/2, g-factor 2.00
    Dms1 signal also well-known in SFRS.

9
What is CRESR?
  • At spin resonance, the microwave field B1 induces
    the precession of the magnetization about the
    static magnetic field B0
  • The component of the magnetization along the
    laser beam direction oscillates in sign
  • The circular dichroism (circularly polarized
    absorption) oscillates
  • The transmitted beam is modulated at the
    precession frequency (in the microwave region).

Magnetic field B0
Microwave magnetic field B1
Laser, RCP / LCP
10
Experimental setup for CRESR
  • Laser beam passes through or reflects from sample
    and onto a fast photodiode
  • At resonance, microwaves induce coherence between
    the two spin states
  • The Raman scattered beam propagates co-linearly
    with laser beam
  • These mix on the photodiode to produce a
    microwave signal optical heterodyne detection.

Magnetic Field
Specimen
Raman
Laser
Fast photo-detector
Microwaves
dc
Microwave mixer
Microwave source
11
Why optical heterodyne detection?
  • High (near single photon) sensitivity for
    coherent optical signals
  • Blind to broadband incoherent backgrounds (i.e.
    luminescence)
  • Allows both amplitude and phase measurements of
    the optical signal
  • Highly efficient detectors with bandwidths of
    several hundred GHz have now been developed.

12
Our first semiconductor CRESR
  • ZnSe single resonance seen near 0.88 Tesla
  • g-factor is 1.1162
  • Precision ? 0.0001
  • Accuracy ? 0.001
  • Comparison of energy scales between CRESR and
    SFRS (inset) shows CRESR has very much higher
    resolution.

S. J. Bingham, J. J. Davies and D. Wolverson,
Phys. Rev. B 65 155301 (2002)
13
CRESR dependence on excitation energy
  • Excitation profiles of SFRS and CRESR are
    similar
  • PL spectra of the donor-bound exciton show the
    effects of changing the strain state
  • a strained to substrate
  • b free-standing ZnSe
  • c ZnSe attached to silica.

SFRS
CRESR
a
b
c
14
CRESR applied to heterostructures
  • ZnSe on GaAs (reflection geometry substrate now
    not removed)
  • Complex lineshape may indicate more than one
    shallow donor species
  • ZnSe quantum well in ZnBeMgSe barriers
  • Detectable signal from a single quantum well

15
CRESR of bulk Cd1-xMnxTe (i)
  • First application to a magnetic semiconductor
  • Archetypical CdMnTe chosen
  • Magnetic field swept through microwave (spin)
    resonance condition for a laser energy in the
    exciton region
  • Internal Dms1 transition of Mn2 ions is seen,
    as noted in our SFRS spectra earlier, at B
    0.48T for 13.7GHz
  • Lineshape results from hyperfine interaction with
    Mn I5/2 nucleus (bars on figure)
  • Dispersion- and absorption-like components shown.

16
CB and VB splittings, Cd0.995Mn0.005Te
  • Vertical meV
  • Horiz. Tesla
  • Dots CB
  • Lines VB
  • T0 0.3K
  • Effective x is 0.0048

17
Excitation energy dependence
  • Laser energy swept through optical resonance
    condition for a set of magnetic fields near the
    g2.00 spin resonance field (0.49T)
  • Compare structure seen to the predicted exciton
    energies (which are also measured via PLE)
  • lh, hh degenerate in bulk, therefore ? 8
    transitions (S1/2 ? J3/2)
  • Exciton electron-hole exchange and diamagnetic
    shift taken into account.

18
Simulation of CRESR
  • On wider magnetic field scale, see a second broad
    signal
  • Is it CB electron spin flip?
  • Can simulate data with
  • set of usual ESR lineshapes at Mn2 g2.00 line
    (including hyperfine interaction with Mn nucleus)
    and
  • one line at the position of the broad signal.
  • So far, consistent with electron spin flip Raman.

19
CRESR of bulk Cd1-xMnxTe (ii)
  • Magnetic field swept through microwave (spin)
    resonance condition for a set of laser energies
    in exciton region
  • Internal Dms1 transition of Mn2 ions seen at B
    0.5T (centre of figure), independent of optical
    excitation energy
  • Broad signal has g-factor dependent on
    excitation energy (!)
  • CB electron SFRS or optical detuning effect?

20
Excitation-energy dependent broad peak
  • Positions of broad signal marked by ?
  • Do not observe a signal at field corresponding to
    spin resonance condition for CB electrons
  • The microwave frequency (13.7 GHz) is too low for
    this (35 GHz available)
  • Symmetrical shift to higher field with detuning
    from resonance follows the outer exciton energy
    levels (red).

21
Summary
  • First application of CRESR technique to a dilute
    magnetic semiconductor demonstrates feasibility,
    high sensitivity, high resolution and shows
    expected signals for a simple bulk sample
  • New features also seen Mn hyperfine structure,
    complex resonance behaviour
  • Extension to multiple Mn2 signals with Dmsgt1
    planned
  • First results on GaMnAs also obtained (not
    presented) and are completely different

Funding EPSRC, DFG, INTAS
22
Team and collaborators
  • Lowenna Glover
  • Dr. Stephen Bingham
  • Prof. J. John Davies
  • Shanshan Zeng
  • Dr. Gazi Aliev
  • Dr David Richards, KCL.
  • Prof. Jean Geurts and Prof. Laurens Molenkamp,
    Würzburg.
  • Dr. Richard Harley, Southampton.
  • U. Bremen
  • CEA-Grenoble
  • Heriot-Watt U.
  • A.F. Ioffe Institute
  • U. Lecce
  • Philipps-U. Marburg
  • U. Nottingham
  • Polish Acad. Sci., Warsaw
  • U. Würzburg
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