"In the real world we are wrapped by surfaces, and therefore we live between two and three dimensions."
--J. G. Dash
"The study of two-dimensional matter would seem to be a completely theoretical branch of physics, were it not that such matter actually exists," states UW physicist Greg Dash.
There are two-dimensional gases, liquids, and solids. In fact much of the rich variety we see in bulk, or three-dimensional, matter can be created in two-dimensional matter. The field is the subject of intense study in many laboratories around the world, thanks at least in part to the pioneering efforts of Dash and colleagues in the UW physics department.
Real two-dimensional materials are made of very thin films that form on the surfaces of solids. Very thin. A single layer of atoms, in fact.
The study of thin films dates back to the previous century and has its origins in such practical goals as improving paints and coatings, developing new catalysts, and separating and purifying gases. Over the decades, scientists began using the concept of one- and two-dimensional systems as a tool to aid conceptual understanding of more complex systems. What can be understood in first one and then two dimensions can be applied to the more complex reality of 3-D. It's an approach most science students would find familiar.
Early in this century, scientists began to study how dimensionality--whether an object is one-, two-, or three-dimensional--affects its fundamental properties. Theorists predicted that certain ordered states, such as crystals, magnets, and superconductors, could not exist in two-dimensional matter. They understood that heat motion can disrupt the regularity of atomic positions required for those properties, but they did not realize that another kind of connectedness can preserve long-range order. This realization was the signal achievement of David Thouless, a UW physics professor (then at the University of Birmingham), and his colleagues in the 1970s. The new insight "stimulated a great increase in theoretical activity and in experimental efforts to realize model two-dimensional systems," notes Dash, who with UW colleagues Oscar Vilches and Michael Schick played a major role in the evolution of this study.
Thouless shared the prestigious Wolf Prize in physics in 1990 with Pierre-Gilles de Gennes of College de France, Paris. The award recognized their pioneering contributions to the understanding of the organization of complex condensed-matter systems.
Dash set out in the 1960s to create two-dimensional helium materials. After several frustrated attempts to find a suitable support for the helium layer, he and his colleagues hit on the use of a type of graphite called Grafoil (the use of graphite for thin film studies had been pioneered years earlier by UW chemistry professor George Halsey). With the Grafoil support, "many of the elusive phases and details [of two-dimensional matter] became apparent, plus several that had not been expected."
Dash and colleagues studied how helium atoms move on the graphite surface. Graphite, which is composed of carbon atoms, has a regular patterned surface, like a tiled floor. As the helium gas is cooled, atoms begin to stick to the surface, and then skim about, occasionally colliding with one another. As they are cooled further, the atoms tend to slow down and linger at the indentations in the patterned "floor." In the case of heavier gases, further cooling would cause the atoms to stick at the crevices. But helium atoms are sufficiently light and weakly interacting that they can remain mobile even near the absolute zero of temperature. Nevertheless, if enough helium atoms are put on the surface they will organize themselves into regular arrangements in the dimples on the graphite surface. Out of the tug-of-war between the helium atoms and the surface atoms there arises a set of novel and complex systems called "two-dimensional superlattices." The floor of the lobby of the UW's new physics and astronomy building has a tiled pattern representing a helium superlattice on graphite.
Under some circumstances, a film may strongly resist the force that the surface tries to impress on it. It then becomes a real two-dimensional solid, floating free from the graphite surface. Several examples have been observed, including those formed of the rare gases helium, neon, argon, and xenon. Dash and colleagues have characterized some of the unique properties of these special forms of matter. Improving our understanding of these exotic, two-dimensional materials may ultimately help physicists better understand our everyday, 3-D world.