Polarization Dependence in X-ray Spectroscopy and Scattering S P

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  • Polarization Dependence in

    X-ray Spectroscopy and

    Scattering

    S P Collins et al

    Diamond Light Source

    UK

  • Overview of talk

    1. Experimental techniques at Diamond: why we

    care about x-ray polarization

    2. How polarized x-rays are generated

    3. Future directions

  • Photoelectron spectroscopy

    Inelastic scattering

    (resonant, non-resonant)

    Elastic scattering

    X-ray interactions with matter: Key techniques

    Absorption/transmission

  • 1914 Nobel Laureate in Physics

    for his discovery of the

    diffraction of X-rays by crystals.

    Max von Laue

    Laue predicted that if x-rays were a form

    of short-wavelength electromagnetic

    radiation then they should produce

    diffraction effects as they pass through

    crystals

    This idea was put to the test by Friedrich

    The field of X-ray diffraction and

    crystallography was born

  • X-ray Diffraction

    and

    Crystallography

    (and why we

    need

    synchrotrons)

  • x

    E

    E

    z

    y

    Polarization by scattering

    1)(

    1)(

    1)(

    2cos)( 0)(

    2

  • Polarization of Synchrotron Radiation

    Intense beams of linearly

    polarized x-rays

  • X-ray Diffraction & Scattering: Why do we care

    about polarization?

    Because the scattering depends strongly on linear

    polarization; scattering can become very weak in the

    horizontal plane; data must be corrected for

    polarization.

    But the polarization dependence tells us nothing about

    the sample, it just reminds us that light is a transverse

    wave.

    Bragg scattering can be used as a polarization analyser.

  • Absorption/transmission

    Is polarization important in absorption?

  • Polarizing glasses are very

    cool

    Linear dichroism and

    birefringence gives information

    about internal polarization of

    materials.

    Does it work with x-rays?

  • 0.40

    0.42

    0.44

    0.46

    0.48

    0.50

    0.52

    0.54

    0.56

    33.10 33.15 33.20 33.25 33.30 33.35

    Energy (keV)

    m(c

    m-1

    )

    a=90o

    a=0

    HN22

    0.82

    0.84

    0.86

    0.88

    0.90

    0.92

    0.94

    0.96

    0.98

    1.00

    1.02

    0 30 60 90 120 150 180 210 240 270 300 330 360

    Polarizer angle a (degrees)

    Re

    lati

    ve

    tra

    ns

    mit

    tan

    ce

    b=-45o

    b=+45o

  • X-ray Absorption: Why do we care about

    polarization?

    Because absorption from anisotropic systems depends

    on linear polarization.

    This effect can give rise to x-ray dichroism and

    birefringence at particular photon energies

    One could construct polarizing devices or study, for

    example, orientations of chemical bonds.

    And going beyond the electric dipole approximation

    one can observe more exotic high-order atomic

    multipoles such as hexadecapoles in cubic systems

  • A B

    C Flu

    ore

    scen

    ce

    A B

    C

    Strontium titanate SrTiO3

  • Sir William

    Lawrence Bragg

    (1890-1971)

    Sir William

    Henry Bragg

    (1862-1942)

    1915 Nobel prize for physics "for

    their services in the analysis of

    crystal structure by means of X-

    rays".

    So what was left for the Braggs to

    do?

    The father and son team carried out

    their own experiments and, in analogy

    with optical diffraction, worked out a

    formula for the wavelength of the

    diffracted wave: the famous Braggs

    Law

  • Resonant forbidden scattering: Why do we care

    about polarization?

    Because the polarization breaks the symmetry that

    normally causes an exact cancellation of the scattering

    at these positions

    The residual scattering is extremely interesting as it

    provides direct information about very weak processes

    that are normally hidden, e.g. exotic electronic

    polarization effects, magnetism

  • E

    H

    SElectromagneticwave

    Electron

    Forces:electric

    magnetic

    = -e

    = -2 ( )

    f E

    f S H mB

    Magnetic forces on electron: Magnetic scattering

    There are several other

    magnetic terms, each

    having different

    polarization dependence.

    They are all very weak.

    Ratio of Zeeman force to

    electric force:

    or less!

    6

    charge

    mag

    2

    2

    10~

    10~2

    1

    I

    I

    cmf

    f

    ee

    Z

    (Zeeman)

  • FeBO3: A weak ferromagnet

    studied by x-ray diffraction

    (Diamond I16)

  • Magnetic x-ray scattering: Why do we care about

    polarization?

    Because the magnetic x-ray scattering has a very

    different polarization dependence from change

    scattering

    This enables it to be identified as magnetic

    It also allows us uniquely to obtain information about

    the distribution of spin and orbital magnetic moments

    in the material

    S

    N

    N

    S

    N

    S

  • Magnetpoles

    Circularlypolarizedbeam

    I=Io

    I=I eo- t( )mm

    Ferromagneticsample

    Magnetizingfield

    A circular dichroism measurement

  • X-ray absorption and orbital polarization

  • Beamline I06 - Nanoscience

    A polarised soft x-ray beamline for microscopy and spectroscopy

    PEEM images recorded using X-Ray Magnetic Circular Dichroism (left) and X-ray Magnetic Linear Dichroism (right) showing ferromagnetic and antiferromagnetic domains, respectively, in Co thin films grown on NiO.

  • Magnetic Circular Dichroism: Why do we care

    about polarization?

    Because the angular momentum of the photon circular

    polarization couples directly to the angular momentum

    of electronic states to give a huge sensitivity to

    magnetism.

    Synchrotron radiation is now one of the major tools for

    studying magnetic materials

    This process also forms the basis of novel microscopy

    techniques allowing magnetic domains and dynamics

    to be studied 10 nm resolution

    There are similar effects in resonant scattering.

  • Diamond

    Beamline

    I16

  • Tellurium results from I16: 001

    and 002 forbidden reflections

  • Studies of Chiral Systems: Why do we care about

    polarization?

    Because circular polarization breaks the mirrors

    symmetry of the photon beam, allowing studies of

    chiral samples

    These are of fundamental importance to chemistry and

    biology (nature is chiral)

    These effects play an important role in contemporary

    condensed matter physics, i.e. the magnetoelectric

    effect, chiral magnetic structures

  • X-ray birefringence imaging - Dynamical Diffraction

    Transmission

    image through

    diamond -

    horizontal

    polarization

    Vertical

    polarization

  • Polarization of Synchrotron Radiation

    Intense beams of linearly

    polarized x-rays

    Quarter-wave

    phase plate

  • The Future:

    Production of linear and circular beams: already very efficient,

    especially linear polarization

    Reversible circular polarizers to pick out very small changes that

    couple to photon helicity: still challenging. The state-of-the-art is

    sensitivity at 10-5 level but this if very difficult. 10-3 is more

    typical; 10-7 would certainly provide new techniques such as x-

    ray natural circular dichroism in chiral liquids.

    Polarization analysers and polarization sensitive detector: very

    challenging. The efficiency and complexity of current devices is

    perhaps the main limiting factor is synchrotron techniques such

    as magnetic scattering.