In Memory of H' C' Siegmann

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In Memory of H. C. Siegmann - the father of
modern spin physics Joachim Stöhr SLAC
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• Ph.D. in 1961, LMU Munich - last student of
  Walther Gerlach
• Full professor at ETH Zurich in 1974
• 33 years at ETH supervised 120 Diploma and 62
  PhD dissertations
• Robert Wichard Pohl Prize 1992 from the German
  Physical Society
• After retirement from ETH in 2000, guest
  Professor at SLAC
• Co-authored textbook on magnetism and helped
  supervise 15 PhD dissertations

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Hans role in magnetism research a
historical perspective
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Magnetic structure of matter atomic moments
B. N. Brockhouse
C. G. Shull
Bulk atomic and magnetic structure
e.g. NiO
Prize came at the end of an era in
magnetism intensive work in 1950 - 60 by 1970s
much was known about bulk magnetic
structure Modern magnetism is different the new
era started in 1970s
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The pioneers of modern spin physics

• 1969 - 76 H. C. Siegmann
• Spin polarized photoemission -
• spin dependent electronic structure of matter
• Invention of the GaAs spin polarized source
• spin polarized electron beams
• 1986 - 88 P. Grünberg and A. Fert
• Discovery of the GMR effect
• Atomic engineering of novel materials

These discoveries have moved the quantum
mechanical concept of the electron spin from its
scientific discovery in the 1920s to a
cornerstone of modern technology. They have
triggered the spintronics revolution.
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for the discovery of giant magnetoresistance
A. Fert
P. Grünberg

• Discovery combined two concepts
• atomic engineering of sandwiches with
  different magnetization directions
• current becomes spin polarized and flows in two
  separate (up and down)
• spin channels with different resistivity
  Motts two current model

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Beams of spin polarized atoms
Walther Gerlach
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The Stern-Gerlach experiment 1921 - beginning of
transient spins
Postcard by Walther Gerlach to Niels Bohr, Feb.
8, 1922
m
Note neutral atoms (no charge) with spin
sz
-sz
QM wavefunction
cos q a2 b2
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1969 a key breakthrough
the discovery of spin-polarized
photoemission or from spin polarized
atoms to spin polarized electrons

• Importance
• electron spin polarization present in material
  is preserved
• when electrons are liberated with photons
• spin polarized electrons can reveal spin
  structure of materials !

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Discovery was prominently acknowledged
Postcard from Walther Gerlach sent March
28, 1969
Letter from Sir Neville Mott sent March
268, 1971
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Spin polarized photoemission has made important
contributions Electronic structure of the
oldest magnetic material magnetite Fe3O4
130 K metallic state

• At room temperature, magnetite is a half metal
• Conduction by minority spins only explains
  Motts model of Verwey transition

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Spin polarized photoemission has revealed
theoretical limitations electronic
correlations can only be approximately described
Ni(110) spin polarized bands

• Observed exchange splitting smaller than
  calculated
• Observed splitting depends on position in
  Brillouin zone
• Observed bandwidth is narrower than calculated
• Observed temperature dependence cannot be
  calculated

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The beginning of spin-polarized electron beams -
1973
Garwin, Pierce, Siegmann, Helv. Phys. Acta, 47,
29 (1974) and
The GaAs spin-polarized gun
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Spin polarized electron beams in high
energy physics
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Invitation to SLAC - 1973
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Beginnings of The Standard Model - The
Electroweak Force
SLAC test of a
theoretical prediction Unification of the
electromagnetic and weak forces into one
electroweak force
Abdus Salam
Steven Weinberg
Sheldon Glashow
Proposed in the 1960s by Weinberg, Glashow,
and Salam In 1967 Weinberg publishes A Model
of Leptons which met all theoretical goals
The model produces a major controversy The
proposed electroweak model violates parity
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SLAC 1978 Test of Weinbergs model
If parity is violated, oppositely polarized
electrons will scatter with different
probabilities
GaAs Source Characteristics High intensity up
to 5x1011 electrons per pulse at 120 Hz Good
polarization 40 Fast reversal of
polarization
The SLAC experiment produced the first
observation of parity violation in a neutral
current interaction!
C. Y. Prescott et. al., Physics Letters 77B, 347
(1978) C. Y. Prescott et. al., Physics Letters
84B, 524 (1979) Physics Today 9, 17 (1978)
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Spin polarized electron beams in
magnetism
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Spin dependent transport in materials
1936 Motts two-currrent model explains
Gerlachs resisitivity of Ni
The current flows independently in two spin
channels -- no spin flips !
Motts two-currrent model
explains the GMR effect
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An ingenious test of the two current model
probability of spin-conserving and
spin-flip transitions
Energy
spin detector
beam
ballistic transport
EV
transport
diffusive transport
no suitable spin polarization detector
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Test of the two current model with spin polarized
beams
Siegmann, Meier, Erbudak, Landolt, Adv. El. and
El. Phys. 62, 1- 99 (1984)
No spin flips are detected !
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The end