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In Memory of Hans-Christoph Siegmann
by Jo Stöhr, July 24, 2009

Hans Portrait
After a research career spanning half a century to the last days of his life, Hans-Christoph Siegmann was fully active in studying ultrafast magnetization phenomena. With a remarkable intuition for physics, he became a pioneer in two diverse fields, spin physics and environmental science.

He pioneered spin-polarized photoemission in 1969, invented the GaAs spin-polarized electron source in 1974, and played a major role in the development of the field of ultrafast magnetization dynamics in the 1990s. His contributions have had profound impact on the field of magnetism and electron spectroscopies, and enabled important experiments with polarized electrons in high energy physics. His work on airborne particles generated in automotive exhaust, starting in 1959, led him to develop detectors for air pollution and to become an early advocate for global environmental control.

Hans Sailing
Learning to sail on Lake Konstantz.

Hans Christoph, or HC for short, was born in 1935 near Lake Konstanz in southern Germany. He received his doctorate in 1961 from the Ludwig Maximilian University (LMU) in Munich as the last student of renowned physicist Walther Gerlach. After spending time at the University of Edinburgh and at LMU, he joined the Swiss Federal Institute of Technology (ETH) in Zürich, Switzerland in 1967 and was promoted to full professor in 1974. During his 33 years at the ETH he supervised 120 Diploma- and 62 PhD dissertations. HC became a Fellow of the American Physical Society in 1989 and received the 1992 Robert-Wichard-Pohl Prize from the German Physical Society for his pioneering work in spectroscopy with spin-polarized electrons. His resume contains over 200 titles and 25 inventor's patents. After his retirement from ETH he continued his research on ultrafast magnetic processes as a guest Professor at the SLAC National Accelerator Center at Stanford University, where he co-authored a textbook on magnetism and was involved in the supervision of 15 PhD dissertations.

Hans violin
Hans playing violin in a physics lecture at ETH.

Until 1969 it appeared impossible to extract spin-polarized electrons from ferromagnetic metals into vacuum by field emission, secondary electron emission or photoemission. This changed when HC and collaborators realized and demonstrated that the problem is overcome through preparation of clean surfaces without magnetic dead layers. Over the last forty years spin-polarized photoemission has revealed the detailed spin-resolved electronic structure of materials and exposed the limits of our theoretical treatments. Today, it is used to explore ultrafast magnetization dynamics, and spin-polarized scanning electron microscopy provides high-resolution images of magnetic nanostructures.

In 1974 HC and co-workers showed that spin-polarized photoelectrons may also be extracted from semiconductors, by illuminating GaAs with circularly polarized light. In magnetism research, the GaAs source is used today for spin-polarized inverse photoemission, reflection and transmission experiments, and nanoscale imaging by means of spin-polarized low-energy electron diffraction and microscopy. Today, the generation of spin-polarized electrons in GaAs by pulsed lasers is used for the study of transport, a key area of spintronics. In high energy physics, the spin-polarized GaAs source was essential in the 1978 experiment at SLAC that confirmed the Weinberg-Salam gauge theory of the weak and electromagnetic interactions.

Hans and Yuri
Hans was apparently amused by this 8 inch ceramic "bellow".

Hans Book
Hans and Jo celebrating the first edition of their book: Magnetism.

The work of HC on air pollution began in 1958 when two famous physicists, Otto Hahn and Walther Gerlach, asked him to investigate whether the radioactivity of the air caused by atomic bomb tests, could induce changes of its electrical conductivity. HC found that the conductivity was instead dominated by very small airborne particles generated in automotive exhaust. He applied for a patent on how to measure air contamination but when he tried to disseminate the alarming results on air pollution through a Munich newspaper, the editors were not interested. At ETH, HC and his students resumed work on air pollution, and over the years developed innovative automatic sensors that today are available world-wide to characterize air pollution, in particular the detection of polycyclic aromatic hydrocarbons, which are known to induce lung cancer. In 1983, the results finally received recognition through front page publication in the press, enhancing awareness and influencing public opinion.

Hans Wind Surfing
Hans wind surfing in Costa Rica

Hans and Katrina
Hans and Katrina at Diamond Head, Hawaii, 2009

Beyond his professional life, HC had diverse hobbies. He played the violin with his wife Katrina, and he was an enthusiastic sailor and windsurfer. We have lost a motivating charismatic personality, full of ideas, with a deep desire to explore the secrets of Nature. He will be remembered by the scientific community for his pioneering work in spin physics, by his colleagues for his deep physical insight and his keen interest to learn and understand, and by his students for his outspoken yet caring personality and his dedicated mentorship. I will miss our daily discussions which have made me a better scientist, his wonderful stories, his great sense of humor and our friendly banter.

Hans Look
Oh that familiar look!


Hans Sigmann's Homepage

Katrinia Krimsky's Homepage

Prof. H. C. Siegmann at ETH Zürich