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Neutron and x-ray spectroscopy
B. Keimer
Max-Planck-Institute for Solid State Research
1. self-contained introduction
• neutron scattering and spectroscopy
• x-ray scattering and spectroscopy
2. application to correlated-electron materials
• bulk
• interfaces
outline
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Neutron scattering
strong (nuclear) interactionelastic lattice structureinelastic lattice dynamics
magnetic (dipole-dipole) interactionelastic magnetic structureinelastic magnetic excitations
neutron
excitation: E= E2 -E1
q=q2 -q1
interaction
E1 q1
E2 q2
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Neutron sources
FRM-II Garching, Germany
example research reactor
neutron flux
Maxwellian profile
energy~ 30 meV
spectrum
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Elastic neutron scattering
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Elastic neutron scattering
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Elastic nuclear neutron scattering
Bragg peaksat reciprocal lattice vectors K
scattering length b ~ size of nucleus ~ 10-15 m
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Neutron radiography
mass attenuationcoefficient
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Neutron radiography
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Elastic magnetic neutron scattering
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Elastic magnetic neutron scattering
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Elastic magnetic neutron scattering
non-spin-flip
“classical electron radius”
one electron
σz → σx , σy spin-flip (not possible for nuclear scattering)
average for
unpolarized beam
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Elastic magnetic neutron scattering
one atomapproximated as magnetized sphere, magnetization density M(r)
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Elastic magnetic neutron scattering
polarization factor magnetic structure factor
magnetic reciprocal lattice vectors
generalization for collinear magnets
Bragg peaks
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Neutron diffractometer
powder single crystal
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Example one-dimensional ferromagnet
use interference between nuclear and magnetic scatteringto create spin-polarized neutrons
ηησ ˆˆ~ 22 bbdd
++Ω
(up to prefactors)
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Example one-dimensional antiferromagnet
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Example vortex lattice in type-II superconductor
small-angle neutron scattering
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Example vortex lattice in type-II superconductor
structural phase transition in vortex lattice
H ~ Hc1 : electrodynamic interaction between flux lines
H ~ Hc2 : vortex cores overlap
structure depends on superconducting coherence length
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Inelastic neutron scattering
elastic cross sectionflux) (incidenttime) (unit
into scattered neutrons of #•
=dΩ
dΩdσ
inelastic cross section (energy)flux) (incidenttime) (unit into scattered neutrons of #
••=
dΩdEdΩ
dσ
inelastic nuclear neutron scattering
initial, final state of sample
partition function
energy of excitation created by neutron in sample
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Inelastic nuclear neutron scattering
thermal average
characterized by population ns of phonons of energy in branch s)(ksωλ
K) K)
Debye-Waller factor due to thermal lattice vibrations
phonon creationneutron energy loss
phonon annihilaionneutron energy gain
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Triple-axis spectrometer
monochromator sample detector
analyzer
if
if
kkq
kkm−=
−= )(2
222
ω
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Triple-axis spectrometer
TRISP at FRM-II
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Example C60
typical inelastic nuclear scattering scans
with a triple-axis spectrometer
lattice structure
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Example C60
fcc lattice at room temperature
molecules rotate freely
molecules “lock in“ at low temperatures
unit cell becomes larger
new optical phonon modes appear
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Inelastic magnetic neutron scattering
polarization factor
spin-spin correlation function
Heisenberg antiferromagnet, magnon creation
Km ) Km ,q, Kma
= 0, 1
ηQ
itinerant electrons → next lecture
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Example molecular magnetism
Mn12 acetate molecule
Mn atoms
energy levels inelastic magnetic neutronscattering intensity
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X-ray sources: tube
setup
spectrum
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X-ray sources: synchrotron
insertion devices:
wiggler, undulator
synchrotron
primary spectrum
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X-ray sources: synchrotron
ESRFGrenoble, France
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Interaction of x-rays with matter
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Thompson scattering – one electron
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Thompson scattering – one electron
differential cross section: one electron
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Thompson scattering – one atom
approximated as charged sphere, charge density ρ(r)
atomic form factor
× (polarization factor)
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Thompson scattering – crystal lattice
thermal vibrationsequilibrium positions
Debye-Wallerfactor
Bragg reflectionsat reciprocal lattice vectors K
expansion of ... + …
thermal diffuse scattering
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Inelastic x-ray scattering
photon energy ~ 10 keV
phonon energy ~ 10 meV→ resolution ΔE/E < 10-7
required
triple-axis spectrometer
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Inelastic x-ray scattering
ID-16 ESRF Grenoble, France
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Example MgB2
IXS data
B vibration
- modulates Fermi surface
- drives superconductivity (Tc = 39 K)
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X-ray absorption
scatteringphoton number conserved
absorptionphoton annihilated
interaction Hamiltonian
absorption cross section
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X-ray absorption
mass absorptioncoefficient
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X-ray absorption edges
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Example K-edge
transitions into continuum
absorption cross section
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Example Fe L-edge
valence state
example Fe thin film
chemical analysis
example TbFeCo alloy
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X-ray radiography
dual-energy
x-ray radiography
discriminate between
carbohydrates and metals
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X-ray absorption fine structure
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Example K-edge
transition into unoccupied excited state
electric dipole matrix element
selection rules
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Example vanadium K-edge
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Magnetic circular dichroism
example single atom
electric dipole selection rules
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Magnetic circular dichroism
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Magnetic circular dichroism
classical calculation for bound electron
analogous to Thompson scattering
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Magnetic circular dichroism
circular dichroism
´´f∝μabsorption coefficient
circular birefringence
´fn ∝index of refraction
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X-ray magnetic circular dichroism (XMCD)
cobalt L-edge
L-absorption edge
of ferromagnet
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Example GMR device
Kortright et al., JMMM 207, 7 (1999)
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Example GMR device
XMCD amplitude
Ni
Co
1 2 3
Ni0.8 Fe0.2
CuCo
1
2
3
Bonfim et al., PRL 86, 3646 (2001)
H
highresistance
lowresistance
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Resonant x-ray scattering
form factor