A Better Look at Atomic Vibrations

The atoms that make up liquids, gases, and even solids are constantly in motion. And in many substances, slight differences in the vibrations of the constituent atoms may have important effects on macroscopic material properties. For example, the motions of impurity atoms can determine whether a material is a useful semiconductor, and measuring the motions of atoms is critical to understanding high-temperature superconductivity, colossal magnetoresistance (which has led to new, high capacity hard drives), and numerous other important effects. Recently, a group of researchers from Kyoto University, the Japan Synchrotron Radiation Research Institute, the Japan Atomic Energy Research Institute, and Osaka University of Education developed a new method that reveals differences in the quantum oscillations of atoms that have, until now, been beyond detection by conventional measurement techniques. The new approach is a refinement of nuclear resonant inelastic scattering, which relies on x-ray radiation from particle beam machines known as synchrotrons to excite atoms, which in turn emit characteristic gamma radiation. Although previous techniques could identify various elements in a material, they were unable to distinguish between identical atoms bound in different atomic configurations.

The researchers have found that by exciting atoms with a pulse of synchrotron radiation and observing oscillations, or quantum beats, in the time spectrum of radiation that the atoms emitted, they could measure the ratio of atoms in various environments in a material. Specifically, the group studied iron atoms in a common iron oxide known as magnetite. Two thirds of the atoms in the magnetite sample are surrounded by six oxygen atoms, and the remaining third are surrounded by only four oxygen atoms. The quantum beats in the gamma radiation signal, which was emitted as a result of the nuclear resonant inelastic scattering, clearly revealed the ratio of iron atoms in the two different atomic environments. The researchers (Makoto Seto, +81-724-51-2445) explain that the new method can be extensively applied to studying the differences in the dynamical properties of atoms in complex condensed matter and large biological molecules, among other substances, leading to a better understanding of the characteristics of such materials. (M. Seto et al., Physical Review Letters, 31 October 2003)