Transcript RREPS11_Collins

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
X-ray interactions with matter: Key techniques
Absorption/transmission
Elastic scattering
Inelastic scattering
(resonant, non-resonant)
Photoelectron spectroscopy
Max von Laue
1914 Nobel Laureate in Physics
for his discovery of the
diffraction of X-rays by crystals.
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)
Polarization by scattering
x
(εˆ εˆ )  1
(εˆ εˆ )  1
E
E’
2
y
(εˆ εˆ )  0
(εˆ  εˆ )  cos 2
z
(εˆ εˆ )  1
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?
1.02
1.00
Relative transmittance
0.56
0.54
0.52
a=90o
a=0
-1
m (cm )
0.50
0.48
0.98
0.96
0.94
b=-45
0.92
0.90
b=+45
0.88
o
0.86
0.84
0.82
0.46
0
HN22
0.42
33.15
33.20
30
60
90 120 150 180 210 240 270 300 330 360
Polarizer angle a (degrees)
0.44
0.40
33.10
o
33.25
Energy (keV)
33.30
33.35
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…
Fluorescence
Strontium titanate SrTiO3
A
B
B
A
C
C
So what was left for the Bragg’s to
do?
Sir William
Henry Bragg
(1862-1942)
Sir William
Lawrence Bragg
(1890-1971)
1915 Nobel prize for physics "for
their services in the analysis of
crystal structure by means of Xrays".
• 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 Bragg’s
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…
Magnetic forces on electron: Magnetic scattering
Electron
E
Electromagnetic
wave
S
H
There are several other
magnetic terms, each
having different
polarization dependence.
They are all very weak.
Ratio of Zeeman force to
electric force:
Forces:
electric
magnetic
(Zeeman)
f = -eE
f = -2m (SH)
f Z 1 
2

~ 10
2
f e 2 me c
I mag
B
I charge
~ 10 6
or less!
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
N
S
S
N
N
S
A circular dichroism measurement
Magnet
poles
I=Ioe
Ferromagnetic
sample
Circularly
polarized
beam
Magnetizing
field
I=Io
-(mm)t
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 Xray 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 xray 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.