Christopher L. Henley died June 29, 2015 in Ithaca, New York, after a year’s struggle with brain cancer. He was 59 years old. Chris was Professor of Physics at Cornell University where he worked in theoretical condensed matter physics, in the Laboratory of Atomic and Solid State Physics. His mind remained clear and he remained scientifically active almost to the end.
Chris had a gift for mathematics, winning in the Putnam competition while an undergraduate at Caltech. He seemed to be headed for a career in mathematics when, after graduating in 1977, he studied functional equations at Silesian University in Katowice, Poland. However, memories of Feynman’s Physics-X lectures at Caltech in the end proved decisive, and Chris entered the Harvard Physics graduate program the following year. At Harvard he worked under Bertrand Halperin, writing a thesis on the low temperature properties of vector spin glasses.
During his two postdocs (1983-1987), first at Bell Labs and then Cornell, Chris first took up the scientific challenge that would define the first half of his career. Quasicrystals were discovered in 1984, turning crystallographic orthodoxy on its head. Choosing not to devote his prodigious mathematical skills to the finer points of the quasicrystal formalism, Chris instead dove headlong into the problem of unraveling these novel structures at an atomic level of detail. This meant sifting through dozens of known crystal structures of similar composition, with the hope of discovering shared structural principles that might flesh out the “tile” shapes that served as cartoons of the novel form of order. The intense period of library research payed off, when Chris observed that the quasicrystal structure could be systematically approximated by a series of ever larger unit cell crystals — and identified with known structures — in much the same way that the golden mean is approximated by ever larger ratios of Fibonacci numbers (1/1, 2/1, 3/2, etc.). The known crystal structures led Chris to the first proposals for the atomic structures of quasicrystals.
Although other topics in condensed matter and statistical physics made increasing demands on his time in subsequent years, Chris never turned his back on important quasicrystal questions that remained unresolved, even when the popularity of the subject diminished. The most notable of these is the stabilizing mechanism: is it the configurational entropy of tile-rearrangement degrees of freedom, or the energetics of “tile matching rules” and how these are implemented by actual atoms. He was named a Fellow of the American Physics Society in 1996 for “theoretical contributions to the understanding of the structure and physics of quasicrystals and related crystalline structures.”
The role of geometry is a common theme in Chris’ research, not just on quasicrystals but also on magnetic systems and biology. He understood that there would always be problems that could not be reduced to spherical cows, and felt he was uniquely qualified to take charge in those situations. Examples include his discovery of semi-classical interference effects associated with “weathervane modes” of spins in kagome magnets, and his models of retro-virus capsid self-assembly. Chris’ geometrical toolbox was immense, and a rich source of metaphor, as his students grew to appreciate.
Old-fashioned in his ways (he never owned a laptop or cell phone), Chris throughout his life tried to preserve the tradition of informal face-to-face discussion at a blackboard, and plain-talk when expressing ideas. To his students he was somebody who could be counted on for help at odd hours and weekends. Many of these students went on to distinguished careers in physics.
Chris was a deeply ethical man, and combined a disarming honesty with a sharp wit. He was quiet about his private life. It is known that reading the Harry Potter series was among his few indulgences. He lived spartanly, donating generously to charities that supported the environment and fought disease and poverty. He ran, swam, and bicycled in the beautiful Ithaca countryside.
Chris is survived by his mother, Nancy, and a son, Christopher King. He will be very much missed by them, and by his many academic colleagues at Cornell and around the world.
Veit Elser and N. David Mermin
