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Hans C. Siegmann
H. C. Siegmann on April 13, 2006, at the Final Focus Test Beam Facility (FFTB) close to the location where the experiments on ultrafast magnetic switching (PRL 94, 197603, 2005 and Nature 428, 831, 2004) had been done. FFTB is presently being reconstructed to accept the hard x-ray laser LCLS. The left side of the photo shows the apparatus with precision manipulator which has been in place to expose magnetic samples to the field pulse of the electron bunches.
Short description of my accomplishments and life as a scientist
Since Dec. 2000, I am at the Stanford Linear Accelerator Center, after having served 33 years as Professor of Physics at the Swiss Federal Institute of Technology in Zurich (ETHZ).
1. Spin Polarized Electrons and Magnetism: Revival of an Old Topic through Technical InnovationsHistorical Context:
The quantum mechanical concept of the electron spin was developed mostly by Pauli, Dirac and Heisenberg in the time period 1925 – 1928. It is based on the famous experiment of direction quantization by Stern and Gerlach as well as on Goudsmith’s and Uhlenbeck’s interpretation of atomic spectra. Although it was clear from the beginning that the electron spin had to be at the root of magnetic phenomena, it was not obvious how the quantum physics of the spin would be useful for describing and understanding magnetism in solids. But in the 1930s the role of the spin in establishing the electronic and magnetic structure of magnetic materials was explained and cast into theoretical concepts by people like Mott, Stoner, Slater, Van Vleck and Néel. After fifty more years of theoretical (e.g. density functional methods by Kohn) and experimental (e.g. neutron scattering by Brockhouse and Shull) developments, the field of magnetism had become rather stagnant by the late 1980s. The magnetic structure of bulk materials appeared reasonably well understood.
However, today the electron spin has entered everyday life and is modifying the entire human civilization through its utilization in magnetic devices such as computer hard drives and high density advanced magnetic memories without which the internet and many other modern commodities could not exist. It turns out that layered magnetic materials and their interfaces are the building blocks of advanced information technology as we know it today. Some fundamental discoveries emerging from the art of producing such structures with well controlled interfaces as well as new spin-based spectroscopies initiated a paradigm shift in our thinking and a technological revolution where the electron spin is now the sensor and as well as the carrier of information.
I had the privilege to coauthor a book describing these amazing developments:
The book particularly distinguishes itself from other books on “magnetism” in its thorough history-oriented discussion of the ground laying concepts and the associated experiments, the treatment of spin polarized electron physics, the discussion of spin polarized transport, the novel technique of x-ray dichroism for the study of magnetic materials, and the emphasis on the dynamics of magnetic phenomena down to the sub-picosecond time scale. By and large, the book avoids complex mathematics and is suitable as a text book for advanced undergraduate and graduate classes.
My own research: Up to the sixties, it appeared impossible to extract spin polarized electrons from ferromagnetic metals into vacuum by field emission, secondary emission or photoemission, despite numerous attempts. Yet, together with my collaborators at ETH Zürich I was able to show in 1969 that spin polarized electrons with sizeable spin polarization are emitted from ferromagnets in photoemission. It only requires that the surface layers of the material, or the interface with the vacuum, be magnetically active. Today, the emission of spin polarized electrons following photon or electron beam excitation is practiced, e.g., in spin polarized photoemission, spin polarized Auger electron spectroscopy and spin polarized scanning electron microscopy (SEMPA), in many cases by my former collaborators and students.
It provides the tool for the study of magnetic materials and surfaces. Over the last thirty-plus years spin polarized spectroscopy has provided the most basic information on the magnetic structure of surfaces and solids. Spin polarized photoemission directly yields the spin dependent electronic structure. This has served as a crucial test of theory, confirming many aspects of spin polarized density functional band theory for the description of magnetism, yet also exposing the limits of the theoretical treatment of materials at finite temperature and with strong electron correlation. Spin polarized electron spectroscopy in its various forms has become an invaluable tool of condensed matter physics, particularly relevant today in combination with pulsed photon sources for the study of ultrafast electron dynamics.
Photoemission of polarized electrons thus pointed the way to the detection of the spin-polarized band-structure of ferromagnetic surfaces. Our original observations with threshold photoelectrons generated widespread interest and prompted comments by Walter Gerlach, N.F. Mott, Peter Wohlfarth, P.W. Anderson [Phil. Mag. 24, 203(1971)], S. Doniach [AIP Conf. Proc. 5,549 (1972)], Martin Gutzwiller [AIP Conf.Proc.5,549 (1972)], and many others. Difficulties with the interpretation of the experiments arose because threshold photoemission was in fact the first manifestation of the unexpectedly strong preferential scattering of minority spins in ferromagnetic metals, often called “the spin filter effect”. At interfaces between ferromagnetic and non-magnetic metals, a similar scattering generates giant magneto-resistance.
Spin polarized photoelectrons may be extracted from non-magnetic materials, as well. This was first demonstrated in 1974 in my laboratory at ETH Zürich as documented by our invention of the GaAs photocathode. The spin can simply be switched from “up” to “down” by switching from right- to left-circularly polarized light in the excitation of the electrons. Under the leadership of Sidney D. Drell, the Stanford Linear Accelerator Center (SLAC) decided to use our technique as a source of polarized electrons for the accelerator. In 1978, the spin polarized GaAs gun was essential in the important 1978 experiment at SLAC by C. Prescott et al. (Phys. Lett. B 77, 347 (1978); ibid 84, 524 (1979)), directly revealing parity violation in electromagnetic interaction of high energy electrons. Today, the GaAs source is a crucial feature in many high energy electron accelerators.
GaAs-type cathodes are now the most common sources of polarized electrons with numerous applications in solid state and high energy physics. Besides being useful to construct the best source of polarized electrons, the underlying concept of optical spin orientation in connection with photoelectron emission is also employed in the study of the electronic structure of atoms, molecules and non-magnetic solids as it depends on the spin-orbit coupling.
In magnetism research, the GaAs source is the basis for spin polarized inverse photoemission spectroscopy revealing the unoccupied band structure of magnetic solids at energies above the Fermi energy. Spin polarized low energy electron diffraction (SPLEED) and spin polarized low energy electron microscopy (SPLEEM) rely on the GaAs source to produce high resolution images of magnetic structures in nanoscopic elements. For their success in determining the magnetic structure of surfaces, spin polarized electrons have been called “surface neutrons”. They have been as important in surface magnetism as neutrons have been for the study of bulk magnetism
In my laboratory at ETH Zürich, I remained focused on low energy electrons with kinetic energies of a few electron volts from photoelectric threshold using the original cathode design of Fig.1 in which the electron spin polarization is perpendicular to the surface. This makes it possible to apply strong magnetic fields to the specimen. Very often, we even lowered the photoelectric threshold by application of the metal cesium. We could then use inexpensive and/or powerful sources of energy for the excitation of the electrons such as excimer lamps, pulsed lasers, or primary electron beams. My approach to electron spectroscopy was simple, close to applications, and, with the advent of pulsed lasers, opened up phenomena taking place on the shortest time scales. Most of the results are in the field of surface and 2D magnetism, or alternatively on the fast dynamics of ferromagnetic spins. Two-photon photoemission for example with time delayed laser pulses enabled us to investigate the spin dependence of electron-electron scattering for the first time in the time domain.
The GaAs source has opened the door for elegant reflection and transmission beam experiments that uniquely elucidate the interaction of low-energy spin-polarized electrons with magnetic materials, thereby revealing the elusive elastic and inelastic quantum mechanical exchange interaction as it depends on electron energy and linear momentum.
We showed that transmission and reflection of very low energy spin polarized electron beams from ferromagnetic films depends on the relative direction of spin polarization and magnetization. The motion of the electron spin, observed either in transmission or reflection, directly measures the “spin-torque” exercised on the magnetization. This recently has become a central issue in the excitation of the magnetization by spin currents.
In my 1994/95 sabbatical leave from ETH, I had pioneered the fastest technique of magnetic switching today labeled “ballistic” or “precessional” switching, using the powerful electromagnetic field pulses generated in solids by passing through relativistic bunches of electrons from the Final Focus Test Beam Facility at SLAC, schematically illustrated in Fig.3.
Continuation of this work in the group of Joachim Stöhr after being professor emeritus at ETH revealed exciting additional features of switching with ultrafast electromagnetic field pulses:
2. Work on Air Pollution: Exposing a Tragic Ineptness of
My work on air pollution took its beginning when I was a student of physics at the LMU Munich. Walter Gerlach asked us to measure the radioactivity of dust collected from the roof of the laboratory. Sometimes the dust was very radioactive and contained radioactive isotopes that do not occur naturally. I remember specifically one day when the radioactivity of the dust was so high that the Geiger counters at our disposal simply remained silent because the rate of radioactive decay was above their maximum capacity. The high radioactivity turned out to be due to a US hydrogen bomb test in the South Pacific. Apparently, the high altitude jet stream had carried the dust around half the globe within a few weeks to Munich where it reached the ground in a strong summer thunderstorm.