X-Ray Scattering Reveals Electronic Properties

Steve Miller

An x-ray technique for studying the fundamental properties of matter could help identify new materials for semiconductor lasers.

Inelastic x-ray scattering uses the high-energy x-rays produced by particles accelerated to nearly the speed of light in a synchrotron to directly probe the quantum nature of materials. When an x-ray deflects, it loses some of its energy to electrons in the target. The change in energy can help determine the structure of the material, such as the unoccupied bands that affect electrical properties.

Unlike conventional techniques such as spectroscopy, inelastic x-ray scattering also provides momentum-resolved information about unoccupied electronic states, which may enable researchers to judge the suitability of a semiconductor for a particular application.

Researchers from universities in the US and Japan and from Lucent Technologies Inc.'s Bell Labs in Murray Hill, N.J., recently used the National Synchrotron Light Source at Brookhaven National Laboratory in Upton, N.Y., as a source of 9-keV x-rays. They looked at Ca₂CuO₂Cl₂ -- a Mott insulator in a class of materials that would be expected to be electrical conductors but instead are insulators that become high-temperature superconductors upon doping.

The work confirmed that the technique could reveal material structure, allowing the researchers to observe the anisotropic propagation of particle-hole excitations.

Applications in photonics could include studies of electronic excitations across the energy gap between occupied and unoccupied states in semiconductors. M. Zahid Hasan, a graduate student at Stanford University and co-author of a June 9 Science report detailing this research, explained that these band gaps determine the semiconductor's optical properties.

Inelastic x-ray scattering cannot predict the behavior of new semiconductors because their energy gaps are small compared with the current experimental capabilities. Higher-energy synchrotron facilities should be able to study the gaps in more materials and screen for those that may be applied to optoelectronics.