Visualizing an enigma

To further explore the weird world of quasicrystals, Rotenberg's international team probes their electronic properties. They chose an aluminum-nickel-cobalt alloy, which is composed of stacked planes of atoms arranged in an aperiodic, tenfold pattern along two of its three dimensions. They aim soft x-rays at these aperiodic planes, and measure the emission angles and the kinetic energy of the electrons that are knocked out. This, in turn, is used to create an energy-momentum map of the alloy's valence electrons.

Remarkably, they found that at least some of these electrons roam in bandlike patterns, much like electrons in normal metals. This debunks the longstanding assumption that quasicrystal electrons only orbit around small clusters of atomic nuclei. "You can count on your fingers why the electrons shouldn't move as in a normal metal, but they do," Rotenberg says.

Furthermore, the team determined that the alloy's electronic states reveal both periodic and aperiodic patterns. To visualize these phenomena, Rotenberg uses computer programs to render mountains of data into three-dimensional images. His team can fire a beam of photons at the alloy, and within minutes develop computer representations of the electrons' momentum and energy.

"I had this vision years ago that we were going to have graphical interfaces between experimental research and people," Rotenberg says. "We can now take so much data and analyze it because we have robust visualization techniques."

Despite Rotenberg's research, fundamental questions remain. Quasicrystals are poor conductors, yet they're often composed of highly conductive elements. The more perfect the quasicrystal, the more resistive it becomes. Why? And are there localized electrons in addition to the delocalized electrons Rotenberg discovered?

More about the electronic structure of quasicrystals.

A bright light

It's not called the Advanced Light Source for nothing: electrons are accelerated to nearly the speed of light and are forced to bend in circular path by powerful magnets. This, in turn, emits ultraviolet and x-ray light. Researchers use this super-bright light to study everything from computer chips to malaria to advanced polymers.

The light source is big. The storage ring has a diameter two-thirds the length of a football field. And it's busy. It runs round-the-clock, seven days a week for most of the year except during scheduled maintenance downtime.

The underlying rationale behind the ALS is that it is impossible to see phenomena that are smaller than the wavelength of the light you are using. So, in order to peer into atomic structures, the ALS produces light that has wavelengths roughly the size of atoms, molecules, and chemical bonds. This light is directed along 27 different beamlines toward experimental workstations, giving a wide range of researchers almost simultaneous access to the light source. Down at beamline 7.0, Rotenberg's team examined the quasicrystal alloy samples using a technique called angle-resolved photoemission.

More about the ALS.