Hans Christoph Siegmann

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).

Born in 1935, I grew up in Germany at the shores of Lake Konstanz in the romantic village Nonnenhorn. There I survived the war as a child. From 1946 -1953 I attended the Gymnasium in Lindau, and marveled each year at the celebrities of physics at the “Nobelpreisträgertagung”. I then studied physics at the Ludwig Maximilian University (LMU) in Munich and obtained my PHD in 1961 as the last student of Walter Gerlach. I acquired the venia legendi (the right to give lectures) at the same place for my thesis “Production and Application of Polarized Electrons”, Munich 1967

1. Spin Polarized Electrons and Magnetism: Revival of an Old Topic through Technical Innovations

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:
Magnetism: From Fundamentals to Nanoscale Dynamics
Springer Series in Solid-State Sciences, Preliminary entry 180
Joachim Stöhr and Hans Christoph Siegmann
June 2006, Approx. 840 p. 400 illus., Hardcover, ISBN: 3-540-30282-4 http://www.springer.com/east/home/generic/search/results?SGWID=5-40109-22-100725344-0

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.

Fig. 1: Photocathode delivering the first spin polarized electron beam from a ferromagnet. (a)Vacuum envelope, (b, c) poles of the electromagnet, (e) electrode to accelerate the photoelectrons,(d) ferromagnetic sample. The blue lines are magnetic field lines

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.

Fig. 2: Motion of the spin polarization P of
an electron beam in ferromagnet with
magnetization M, adapted from Dan Ralph,
Science 291, 999 (2001)

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.

Fig. 3: Schematic of the ultrafast switching experiment using an electron bunch which carries an electric and a magnetic field, as shown. The high-energy electron bunch simply traverses the sample, imprinting a magnetic pattern that depends on the number of electrons in the bunch and the temporal length of the bunch. The pattern shown was generated by one bunch of 5.26 ps duration in a 15 layer thick single crystalline film of Fe grown epitaxially on a GaAs substrate. The film is uniformly magnetized prior to the arrival of the electron bunch.

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:

●Increased damping at large angles between magnetization and magnetic field through spin wave instabilities
●Generation of a new type of magneto-electronic anisotropy (unpubl.)
●Modification of electronic structure and non-linear conduction at high fields (unpubl.)

In other experiments we injected spin polarized electron currents into a small magnetic element. Similar structures are commonly used in current perpendicular to plane (CPP) spin valves serving to read data from high density magnetic memories. We obtained the first time resolved images of the magnetic switching process in a nano-scopic magnetic element, using ultrafast x-ray microscopy. This experiment shows that magnetic switching at the nano-scale through injection of a spin current may be accomplished by the motion of a magnetic vortex across the sample rather than by coherent rotation of the magnetization. The magnetic field produced by the Oersted field of the charge current supports the formation of the vortex, while the spin current simply has to shift the vortex across the sample to complete the switching.

2. Work on Air Pollution: Exposing a Tragic Ineptness of
Human Civilization for Preventive Action

During the summer vacation in August 1958, Otto Hahn, the discoverer of atomic fission, and Walter Gerlach both visited me in the laboratory at the Physics Institute at LMU Munich. Since everybody else was on vacation, they asked me to investigate whether the radioactivity of the air, in particular the contribution from atomic bomb tests, could induce changes of the electrical conductivity of the air. I had a vibrating quartz electrometer for the electrical conductivity, and I managed to measure the radioactivity of the ambient air with an instrument of the type used by Mme Curie. It became obvious that the electrical conductivity of the Munich air, albeit initiated by natural or man made radioactivity was dominated by very small airborne particles generated in automotive exhaust. The positive and negative ions mainly formed by the a-decay of naturally occurring Radon have the tendency to attach themselves to airborne particles thereby becoming immobile which in turn reduces electrical conductivity of the air to practically zero. When I proudly told Gerlach and Hahn that I had solved the problem and that it was actually particles from car exhaust that determined the electrical conductivity of the Munich air, they both lost all interest. I realized that I had to proceed on my own. I applied for a patent on how to measure the concentration of very small airborne dust particles with an ionization chamber. I used a model instrument made of a can and some fly mesh to demonstrate the method, and measured the air pollution in Munich whenever I had spare time.

In 1959, I had to cross the center of Munich by bicycle to reach the Physics Institute at LMU. The inner city was usually blocked with cars billowing exhaust, and sometimes the smoke was so thick that it was difficult to see the car in front. It is hard to imagine today the horrible air quality that we had to put up with then. It was worse than what is observed today in the big centers of the developing world. My instrument gave me insight into what was going on, and I wanted to communicate the alarming results to a broader public in the “Süddeutsche Zeitung”. The editors refused however to publish my article on the Munich air, saying they had already dealt sufficiently with the subject. In my mind, the only remedy was to severely limit the use of cars. Yet at the time, the people in Germany were poised to become motorized, irrespective of the consequences, even if that meant that they and their children had to breathe car exhaust which was even more toxic then because it contained lead. It was well known that lead fumes affect the brain and are particularly dangerous for children. The proposal to strictly regulate the use of a car was probably the reason why the article was considered unsuitable for publication. Naturally, at the time I was very disappointed, but today I have the satisfaction that a pedestrian zone has indeed been created in the center of Munich and many other cities allowing people to enjoy life again

After I had become a full professor at ETHZ, I felt it was time to resume work on air pollution. I had observed that cigarette smoke could be ionized with light of energy of 3.8 eV. Yet solids exposed to air require light of much higher energy to emit photoelectrons. So what substance was responsible for photoemission from cigarette particles? After almost 10 years of research funded by the entrepreneur Branco Weiss and the Swiss National Science Foundation, and thanks to the collaboration of many smart students it turned out that polycyclic aromatic hydrocarbons (PAH) induce the photoelectric emission of electrons and enhance the photoelectric yield if present at the surface of the particles. PAH are produced in any incomplete combustion of organic fuels and occur abundantly in Diesel motor exhaust, cigarette smoke, and fumes from wood fires. Spurious amounts of PAH adsorbed at a particles surface induce a substantial fraction of the photoelectric yield. Thus we had a very simple way to detect them.

PAH from combustion are of concern because they induce lung cancer in humans, and lung cancer was on the rise, scaling with the exponential growth of motorization. The result of this research was published in 1983 on the front page of the “Tagesanzeiger”, the most read newspaper in Switzerland. From there it became a topic in other prominent papers as well and also was dispersed in radio and television interviews. This time, our work on air pollution had certainly reached the public. Yet in the end all of this had no effect in practice as people soon forgot it, self appointed specialists said we had not proven anything, our colleagues thought that more research was needed, and the motor industry said that the tar content of the Diesel fumes would be reduced soon

This sounds like the familiar reaction to the dangers of global warming. Global warming is yet another consequence of the mindless usage of 19th century discoveries that we encounter in motorization. 20 years later, the Swiss government now agrees officially with our initial conjecture that the number of people killed in traffic accidents is about equal to the number of people killed by the traffic fumes!

When my son Konstantin, a chemist, joined us in 1992, we were able to study airborne particles “in situ” at the molecular level in more sophisticated experiments using the monochromatic light emitted by excimer lamps as well as pulsed lasers combined with time of flight mass spectroscopy. From this work, a detailed picture emerged of how:

PAH form in combustion of organic material, see Fig.4. The particles generated in different combustion processes such as Diesel motors, wood fires, and cigarettes have different chemical properties that may be used to distinguish them by combining photoelectric and diffusion charging in handheld, battery operated instruments. We also showed how one can do serious surface science with the airborne particles while the particles are suspended in their natural gaseous environment.

Our innovative automatic sensors are available worldwide, see http://www.ecochem.biz/ and http://www.matter-engineering.com/, and are being used to characterize air pollution in a way that is more relevant to human health than the legal method which is still collecting the particles on a filter and weighing them. For instance, in the book entitled: The Technology-Energy-Environment-Health (TEEH) Chain in China, K.R. Polenske ed., Springer 2006, we made extensive use of the sensors to evaluate the impact of air pollution on human health in the Chinese coke making industry. This book has received the Industry Studies Best Book Award of the Alfred P. Sloan Foundation in 2008. Yet another example of the application of our sensors is shown in Fig.5 where the particle density in the size range of 20-200 nm is automatically registered at a walk through the city of Zürich followed by a boat trip across lake Zürich. The data are automatically inserted into a Google map using a global positioning system (GPS).

To curb air pollution, including greenhouse gases, to a sufficient level, automotive traffic would have to be regulated by law, eliminating all heavy weight passenger cars and allowing the use of light weight economical vehicles only in special circumstances. New ultra-light materials would have to be used for constructing such light vehicles, and the production of energy would have to be by renewable sources as well as nuclear reactors. Technically, the global warming catastrophe can be avoided through available scientific discoveries. Yet it is highly questionable whether the present human race, calling itself “homo sapiens” will be able to realize these technical possibilities in time to save the world’s atmosphere.

Short description of my accomplishments and life as a scientist

1. Spin Polarized Electrons and Magnetism: Revival of an Old Topic through Technical Innovations

2. Work on Air Pollution: Exposing a Tragic Ineptness of Human Civilization for Preventive Action

2. Work on Air Pollution: Exposing a Tragic Ineptness of
Human Civilization for Preventive Action