in memory of h. c. siegmann - the father of modern spin physics joachim stöhr slac
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In Memory of H. C. SiegmannIn Memory of H. C. Siegmann- -
the father of modern spin physics the father of modern spin physics
Joachim Stöhr SLAC
• 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
Magnetic structure of matter: atomic moments
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 differentthe new era started in 1970s…
B. N. Brockhouse C. G. Shull
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
The pioneers of modern spin physics
These discoveries have moved the quantum mechanical concept of the electron spinfrom its scientific discovery in the 1920s to a cornerstone of modern technology.
They have triggered the spintronics revolution.
“for the discovery of giant magnetoresistance”
A. Fert P. Grünberg
nm
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 – Mott’s “two current model”
The Stern-Gerlach experiment 1921 - beginning of transient spins
m
Postcard by Walther Gerlachto Niels Bohr, Feb. 8, 1922
= a + b
cos = a2 – b2
+sz
-sz
Note: neutral atoms (no charge) with spin
QM wavefunction
1969 – a key breakthroughbreakthrough 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 !
Postcard from Walther Gerlach sent March 28, 1969
Discovery was prominently acknowledgedDiscovery was prominently acknowledged
Letter from Sir Neville Mott sent March 268, 1971
• At room temperature, magnetite is a half metal
• Conduction by minority spins only explains Mott’s model of Verwey transition
Spin polarized photoemission has made important contributionsSpin polarized photoemission has made important contributions Electronic structure of the oldest magnetic material: magnetite Fe3O4
130 K metallic state
• 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
Spin polarized photoemission has revealed theoretical limitationsSpin polarized photoemission has revealed theoretical limitations electronic correlations can only be approximately described
Ni(110) spin polarized bands
The beginning of spin-polarized electron beams - 1973The beginning of spin-polarized electron beams - 1973
The GaAs spin-polarized gun
Garwin, Pierce, Siegmann, Helv. Phys. Acta, 47, 29 (1974) and
Spin polarized electron beams Spin polarized electron beams in high energy physicsin high energy physics
Beginnings of The Standard Model - The Electroweak ForceBeginnings of The Standard Model - The Electroweak Force
Steven Weinberg Sheldon Glashow Abdus Salam
• 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
SLAC test of a theoretical predictionSLAC test of a theoretical prediction Unification of the electromagnetic and weak forces into one “electroweak” forceUnification of the electromagnetic and weak forces into one “electroweak” force
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
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)
The SLAC experiment produced the first observation of parity violation in a neutral current interaction!
SLAC 1978: Test of Weinberg’s modelSLAC 1978: Test of Weinberg’s model
The current flows independently in two spin channels -- no spin flips ! Mott’s “two-currrent model” explains the GMR effect
1936: Mott’s “two-currrent model”explains Gerlach’s resisitivity of Ni
Spin dependent transport in materialsSpin dependent transport in materials
An ingenious test of the “two current model” probability of spin-conserving and “spin-flip” transitions
transport
no suitable spin polarization detector
spindetector
beam
EV
diffusivetransport
ballistictransport
Energy
Test of the two current model with spin polarized beamsTest of the two current model with spin polarized beams
No spin flips are detected !
Siegmann, Meier, Erbudak, Landolt, Adv. El. and El. Phys. 62, 1- 99 (1984)
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