Obituary of Hans Christoph Siegmann

After a research career spanning half a century, Hans Christoph Siegmann passed away on June 19, 2009 in his home in San Francisco. To the last days of his life, he 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 in high energy physics which relied on polarized electrons. 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 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.

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 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 polarized electrons in GaAs by pulsed lasers is used for the study of spin transport, a key area of spintronics. In high energy physics, the spin-polarized GaAs source was essential in the 1978 experiment at SLAC showing parity violation in spin scattering of high energy electrons, confirming the Weinberg-Salam gauge theory of the weak and electromagnetic interactions.

HC’s work on air pollution began in 1958 when Otto Hahn and Walther Gerlach asked him to investigate whether the radioactivity of the Munich air, in particular the contribution from atomic bomb tests, could induce changes of the electrical conductivity of the air. 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 with an ionization chamber. When he tried to disseminate the alarming results on air pollution revealed by his study 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 for the environment in Switzerland.

Beyond his professional life, HC had diverse hobbies. He played the violin with his wife Katrina, and he was an enthusiastic sailor and windsurfer. Those of us who had the privilege of working with him will remember him for his deep physical insight, his inspiring lectures and his dedicated mentorship, encouraging and promoting his students far beyond the end of their thesis work.

We have lost a motivating charismatic personality, full of ideas, with a deep desire to explore the secrets of Nature. We will miss him as a colleague, mentor and friend.

Joachim Stöhr

Stanford, August 10, 2009