Site-specific phonons

In many substances, slight differences in the vibrations of just a few constituent atoms can have important effects on macroscopic material properties. For example, the vibrational motion of impurities can determine whether a material is useful as a conductor or semiconductor. Now, a group of researchers in Japan has developed a method to determine the phonon density of states for different sites in a material, even when the difference involves only dissimilar states or environments of otherwise identical atoms. The physicists used a refinement of nuclear resonant inelastic scattering to measure not only phonon energy spectra but also the hyperfine interactions between nuclei and the surrounding electronic states. As a demonstration at Japan’s SPring-8 synchrotron source, the group studied iron atoms in magnetite; two-thirds of those atoms are surrounded by six oxygen atoms and the remaining iron atoms are surrounded by four oxygens. Oscillations in the gamma radiation signal clearly showed the ratio of iron atoms in the two different atomic environments. The researchers say that the new method has many potential applications, for example to study the differences in the dynamical properties of atoms in complex condensed matter systems and in large biological molecules. (M. Seto et al. , Phys. Rev. Lett. 91, 185505, 2003.)