By Peter Weiss
As pinches of spice enliven a dish, sprinkles of disorder can perk up otherwise perfect crystals.
The presence of a few odd atoms, for instance, can stretch or scrunch a crystal’s lattice, introducing desirable electronic, optical, or magnetic traits.
A new technique developed at Brookhaven National Laboratory in Upton, N.Y., now permits scientists to discern lattice shifts as small as a hundredth of an atom’s width. That’s about a tenfold improvement over previous techniques.
The ability to better measure those shifts may lead to new insights about physical properties that emerge from the displacements. Among such properties is high-temperature superconductivity, or the ability to conduct electricity resistance-free at temperatures well above absolute zero (SN: 11/18/00, p. 330: Little Big Wire). More detailed data about crystalline imperfections also may help scientists develop new materials, says Brookhaven’s Yimei Zhu.
As if doing an X-ray diffraction analysis, Zhu and his colleagues placed a thin crystalline sample under a small, bright radiation source. Instead of X rays, however, they used electrons from an electron microscope. When the electrons passed through the crystal, they careened off the lattice, producing interference patterns at detectors on the far side.
The key to the new diffraction method, which is a type of electron holography, is its so-called coherent electron source, notes Stephen J. Pennycook of Oak Ridge (Tenn.) National Laboratory. “That’s definitely a nice, new twist,” he remarks.
Like photons of visible light, electrons behave as waves. When waves come from a coherent source such as a laser, all their peaks and troughs line up exactly. To make optical holograms—for example, those incorporated into credit cards—manufacturers exploit interference effects between laser beams.
In the new crystal-measuring method, the coherence of the electron waves permits even electrons that pass through the crystal by widely different paths to interfere and thus contribute to the measurement. “It’s really because there’s interference over a large distance that [the Brookhaven technique is] more sensitive to smaller displacements” than previous approaches, Pennycook explains.
A report on the new method is scheduled to appear in the Dec. 11 Physical Review Letters. The approach is based on a 1948 proposal by Dennis Gabor, the inventor of holography.
The technique gauges displacements to within a trillionth of a meter. It’s been tried so far only on “stacking faults,” in which an extra atomic plane or portion of one is squeezed into a lattice or a plane that’s normally present is missing.
Stacking faults are common, however. In high-temperature superconductors, their presence can increase a material’s commercial value and boost the current it can carry.
