obituaries

it was to be a classic.” moved to Rutgers Unihe was appointed State Professor of Surface Sci-

of the Laboratory for

. Under his leaderbecame internationally rec- surface-science research. pioneered the production of -crystal metal surfaces their application as models catalysis research. photon-stimulated delaboratory investigations of production of sodium Moon and Mercury. With and Thom Orlando he memorable paper “Far-out Radiation-induced in the Solar System,”

published in 2002 in Surface Science (volume 500, page 838). While at Rutgers, Ted became interested in the surface chemistry induced by photons in the extreme UV region and in the radiation-induced contamination of first-surface mirrors used in highresolution lithography for semiconductor device fabrication.

Ted was an outstanding member of the scientific community who served actively on many US and international advisory boards. From 1990 to 1995, he was on the American Institute of Physics’s PHYSICS TODAY advisory committee. In the American Vacuum Society, he held several leadership positions, including its presidency in 1981, and was known as Mr. AVS. From 1992 to 1995, he also was president of the International Union for Vacuum Science, Technique, and Application. His great interest in helping develop surface science internationally was rooted in his Polish ancestry, and in 2004 the University of Wrocław recognized his long scientific connection to Poland. I was able to attend the ceremony; it was especially moving to witness and to hear the orchestra play the Academic Festival Overture during the colorful procession: Johannes Brahms had composed the music as a thank-you for receiving the same honorary doctorate in 1879.

Ted published more than 400 papers covering a broad range of surface phenomena. He was an extraordinary scientist with the highest standards in measurement and interpretation. His deep scientific insight, accompanied by his personal warmth, kindness, and humor, will be missed by a great many friends and colleagues fortunate to have known and worked with him.

John T. Yates Jr

University of Virginia Charlottesville

Julius Erich Wess

Julius Erich Wess, a leading figure of modern theoretical physics, died unexpectedly from a stroke on 8 August 2007 in Hamburg, Germany. Mentor to a generation of particle theorists, Wess was best known as a father of supersymmetry, a concept that revolutionized particle physics.

Wess was born on 5 December 1934 in Oberwölz, Austria. He grew up with his two brothers during World War II, hiking and climbing whenever he could. He studied at the University of Vienna with Hans Thirring and Walter Thirring. It was also there that he met his lifelong friend and collaborator Bruno Zumino.

After receiving his PhD in 1957 with a thesis on Compton scattering on vector particles, Wess took a series of postdoctoral positions, first at the nascent CERN laboratory in Geneva, Switzerland; then back in Vienna; and later in Seattle, Washington. In 1966 he was appointed associate professor in mathematical physics at the Courant Institute of Mathematical Sciences at New York University. In 1968 he returned to Europe to take the chair in theoretical physics at the University of Karlsruhe in Germany. Throughout his career, Wess stressed how fortunate and grateful he was, as a rising young physicist, to have had the opportunity to travel to the US and join the international scientific community at a time when the devastations of war were still fresh in everyone’s mind.

Wess was a pioneer in the generation of physicists who harnessed the mathematical power of group theory and applied it to physics. Previously, physicists developed a theory and used symmetries to help find and characterize its solutions. Wess and his peers stood that on its head. They identified the symmetries of systems, then used those symmetries to find the underlying theories. It was precisely that approach that led to the enormously successful standard model of particle physics.

His early work centered on effective field theories for hadrons, especially the interactions connecting pions and kaons with protons and neutrons. His 1969 papers with Sidney Coleman, Curtis Callan, and Zumino detailed the mathematical structure of theories with spontaneously broken symmetries. The papers laid much of the foundation for phenomenological hadron physics, but they have had even wider application. They are still being cited today.

Wess’s most highly cited work is the

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THILO MECHAU

Julius Erich Wess

1971 paper with Zumino on anomalies in effective field theories. Anomalies occur when quantum effects violate classical symmetries, giving rise to physical phenomena such as the decay of a neutral pion into two photons. Wess and Zumino showed that anomalous terms in effective Lagrangians must obey certain consistency relations. Those conditions are so important that the terms are now named after them.

Despite the fame of that early work, Wess will always be known for the 1974 papers in which he and Zumino constructed the first renormalizable supersymmetric quantum field theory in four dimensions and exhibited its nonrenormalization properties at one loop. Their work ignited an explosion of interest in supersymmetry, a concept that has come to dominate much of modern theoretical physics. His textbook on supersymmetry, with one of us (Bagger), is still a standard reference after 25 years.

Supersymmetry is a new type of symmetry that transforms bosons into fermions and vice versa. Deeply embedded in string theory, it predicts that spacetime has new fermionic dimensions that manifest themselves as particles. The nonrenormalization properties of supersymmetric theories have powerful implications for Higgs physics, making the discovery of supersymmetry one of the most sought prizes at the CERN Large Hadron Collider. It is a tragedy that Wess did not live to see the outcome of those experiments.

In 1990 Wess moved to Munich, Germany, where he became a director of the Max Planck Institute for Physics and professor at the Ludwig–Maximilians University. After his official retirement in 2002, Wess moved to DESY, the Ger-

man Electron Synchrotron in Hamburg, where he worked until his death. In his later years, he focused his research on field-theoretic applications of noncommutative geometry, which involves a new type of symmetry beyond Lorentz invariance and even supersymmetry. He also became increasingly active in advisory roles. From 1993 to 1996, he served as chair of the DESY Scientific Council; he was also an influential member of the High Energy Particle Physics Board of the European Physical Society.

For his many contributions, Wess was awarded the 1988 Dannie Heineman Prize for Mathematical Physics of the American Physical Society; the 1987 Max Planck Medal, the highest distinction of the German Physical Society; and the 1986 Gottfried Wilhelm Leibniz Prize from the German Research Foundation. He was a member of the Bavarian Academy of Sciences and Humanities, the German Academy of Sciences Leopoldina, and the Austrian Academy of Sciences. But perhaps his greatest tribute is the number of results that have entered physics textbooks under the label Wess–Zumino.

Over the years Wess supported scientific dialog with Eastern bloc countries and the Soviet Union and strengthening of German–Israeli scientific ties. At the time of his death, he was working to increase scientific opportunities for young physicists in the Balkans. Drawing on his own experiences, he was a firm believer in the international community of scientists.

Despite the importance of his scientific work, Wess was modest and unpretentious. He had a passion for the outdoors and often climbed straight up or down a mountain, irrespective of where the path led. His great warmth and subtle humor inspired a generation of students and postdocs. To the end, he remained a thoughtful man of Viennese charm and grace. He will be greatly missed.

Jonathan Bagger

Johns Hopkins University Baltimore, Maryland

Hermann Nicolai

Max Planck Institute for Gravitational Physics Potsdam, Germany

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January 2009 Physics Today 67

Mechanical oscillators have a long history of assisting scientific pursuit. For example, a desktop torsional balance was a key tool used by Charles Augustin de Coulomb in his late-18th-century studies of electrostatics. Henry Cavendish accomplished his contemporary quantitative measurements of the gravitational force with a mechanically resonant version of a similar torsion balance. Today, mechanically resonant systems range in scale from the very large, such as the 4-km detectors developed for the Laser Interferometer Gravitational-Wave Observatory (see the article by Barry Barish and Rainer Weiss, PHYSICS TODAY, October 1999, page 44), to the very small, including devices that can detect the magnetic force from a single electron spin—perhaps ultimately from a single nuclear spin.

The basic mechanical element for the smallest systems is often a cantilever, a beam that is clamped at one end. To understand how it behaves when fabricated at nanometer dimensions, first consider a more familiar case: a 2-meter long poolside diving board made of fiberglass-coated metal. When a 50-kg diver stands on the free end of the board, the end dips by about 5 cm. Evidently, the diving board has an effective spring constant k of something like 104 N/m. After the diver leaps from the end into the water, the diving board vibrates with its mechanical resonance frequency ν, a few hertz. The cantilever’s effective mass meff, defined by the simple harmonic oscillator formula meff ≡ k/(2πν)2, is thus about 100 kg.

Now imagine scaling all the dimensions of the diving board down by a million, to something like 2 μm long (L), maybe 20 nm thick (t), and 200 nm wide. The effective mass, proportional to the volume, would become a factor of 1018 smaller, about 10−16 kg. As dictated by the Euler–Bernoulli thin-beam theory, the mechanical resonance frequency, proportional to t/L2, would increase to a few million hertz.

The spring constant k = meff(2πν)2 would be around 10−2 N/m. The interest in scaling cantilevers to smaller dimensions im-

mediately becomes apparent: Nanoscale cantilevers have high resonance frequencies, minuscule masses, and small spring constants that yield much larger displacements for a given force.

A look at the atom

A submillimeter-scale cantilever is at the heart of the atomic force microscope, a common analytic instrument that generates images of surfaces with single-atom resolution (see the

article by Daniel Rugar and Paul Hansma, PHYSICS TODAY, October 1990, page 23). To achieve that resolution, the AFM’s roughly 100-μm-long cantilever must have an extremely sharp tip attached to the free end, as shown in the figure. The experimenter drags the tip over a surface; the resulting minute deflections of the cantilever end can be detected with the help of a laser beam. In another mode of operation, the tip is lifted off the surface. Nonetheless, the tip is still acted on by surface forces—either electrostatic forces from surface charges or van der Waals forces from fluctuations in local dipoles on the surface. Those forces effectively change the spring constant of the cantilever and thus its mechanical resonance frequency. By gently shaking the cantilever while watching its motion, an experimenter can monitor the resonance frequency and thus measure and map out surface forces as a function of tip position. Such force maps allow for surface images that still achieve single-atom resolution.

Electrostatic and van der Waals forces are not the only forces that can be probed with tiny cantilevers. Researchers are developing versions of cantilevers that can be used in magnetic resonance force microscopes, instruments that measure and map out forces from electron or nuclear spins on a surface. If a very small magnet is included in the cantilever construction, the cantilever will respond to the magnetic fields generated by the magnetic moments associated with individual surface spins. In practice, one needs cantilevers with extremely small spring constants; that’s achieved by making the cantilever very long and very thin. By optimizing the measurement of cantilever motion, an experimenter can detect the extraordinarily small magnetic moments of a single electron or a few nuclei and then generate an image of the spin density. For example, in a tour de force experiment, Rugar and colleagues from IBM Research detected a single electron spin, achieving a resolution of 25 nm in the scanning direction. Ultimately, the magnetic resonance force microscope may lead to surface maps with single-atom magnetic resolution. Not only would one know the location of an atom, one could also identify the atom through its magnetic moment. Thus, for example, proteins or strands of DNA might be sequenced by direct measurement.

Nanometer-size cantilevers, with their small effective mass, are good tools for inertially “weighing” minute amounts of material. To do that, one adds mass to the cantilever’s free end and monitors the resulting change in the resonance frequency. In that manner, Kenneth Jensen, Alex Zettl,

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50 μm

and colleagues at the University of California, Berkeley, measured the mass of a single gold atom with a cantilever made from a carbon nanotube.

In a second popular type of nanomechanical resonator, a beam is fixed at both ends, rather than at just one as for a cantilever. According to the Euler–Bernoulli thin-beam theory, a doubly clamped beam has a resonance frequency approximately six times that of a cantilever with the same geometry and material; the increased frequency is useful for applications such as timing elements for integrated circuits and filters for telecommunications. An additional reason for the popularity of the doubly clamped beam is that it can be easier to fabricate and measure than a comparable cantilevered beam.

Doubly clamped beams are sometimes referred to as violin strings. After all, their appearance and the shape of their resonant modes closely resemble those of a stretched wire. However, the mechanical response of a beam is dominated by the beam’s material stiffness, whereas that of a violin string is determined by the external tension. As a result, the mechanics of the two resonators are quite different. Indeed, as with the cantilever, the resonance frequency for the doubly clamped beam is proportional to t/L2. By contrast, the resonance frequency of a violin string is proportional to 1/L.

A look at quantum physics

The AFM can measure extremely weak forces. In one particularly interesting experiment, a surface and the tip of an AFM cantilever are coated with metal, kept at the same electrostatic potential, and brought close to one another. At sufficiently small distances, an unusual interaction called the Casimir force becomes detectable. That force is due to the quantum mechanical zero-point energy of the electromagnetic modes localized between the tip and the surface (see PHYSICS TODAY, February 2007, page 40, and May 2007, page 16). Measurements of the Casimir force appear to be in reasonable agreement with those predicted by quantum

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Cantilevers and carbon. A relatively large cantilever such as the one that dominates the foreground is the key component of an atomic force microscope, which is capable of achieving atomicscale resolution. Indeed, the background photograph is an AFM image of the graphite lattice. To the left in the foreground is an ultralow-mass cantilever about 1/10 the size of those usually used in AFMs. Its high resonance frequency— 9 MHz as compared with the 300 kHz of the conventional cantilever—allows for faster imaging and reduces the impact of thermal noise. (Courtesy of Asylum Research, Santa Barbara, CA.)

mechanics, although interesting questions remain.

Researchers are trying to develop other ways in which quantum mechanical effects can be measured in mechanical systems; nanomechanical resonators may provide a means for doing that. The signatures one might look for include the superposition and the interference of the resonator’s quantum states. How-

ever, a number of challenges must be met if those signatures are to be observed. Thermal effects, which cause the resonator to occupy quantum levels with energies up to a few times the thermal energy (kBT, with kB being Boltzmann’s constant and T, the temperature), will “smear out” most quantum effects. They can be minimized if hν kBT (h is Planck’s constant), which means one wants to work with the highestfrequency resonators and at the lowest-possible temperatures. By building resonators with frequencies greater than a gigahertz, or by working at temperatures below a millikelvin, experimenters can satisfy that energy inequality.

Another consideration is that the resonator must retain its energy and its quantum coherence—the relative complex amplitude between two quantum states—long enough that a measurement can be performed. The energy lifetime may be long enough in some mechanical systems, but the coherence time has yet to be measured for any nanomechanical resonator. Perhaps that quest will resonate with some young researcher.

Additional resources

Euler–Bernoulli beam equation, http://en.wikipedia.org/ wiki/Euler-Bernoulli_beam_equation.

A. N. Cleland, Foundations of Nanomechanics: From SolidState Theory to Device Applications, Springer, New York (2003), sec. 9.7.

A. Cho, “Researchers Race to Put the Quantum into Mechanics,” Science 299, 36 (2003).

D. Rugar et al., “Single Spin Detection by Magnetic Resonance Force Microscopy,” Nature 430, 329 (2004).

S. Chao et al., “Nanometer-scale Magnetic Resonance Imaging,” Rev. Sci. Instrum. 75, 1175 (2004).

K. L. Ekinci, X. M. H. Huang, M. L. Roukes, “Ultrasensi-

tive Nanoelectromechanical Mass Detection,” Appl. Phys. Lett. 84, 4469 (2004).

K. Jensen, K. Kim, A. Zettl, “An Atomic-Resolution Nanomechanical Mass Sensor,” Nature Nanotech. 3, 533 (2008).

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