An x-ray hologram shows the positions of cobalt atoms in a sample of cobalt oxide with an accuracy of 0.5 x. A new method of x-ray holography, developed by Miklos Tegze and Gyula Faigel of the Research Institute for Solid State Physics (Budapest, Hungary) and adapted to synchrotron radiation with colleagues at the Commissariat ... l'Energie Atomique (Grenoble, France), gives a clearer picture of the location of atoms than previous work of this type.
There are two earlier versions of x-ray holography. In the first, the atoms of the sample serve as point sources of x-rays, producing a hologram by the interference of a reference wave with scattered waves from neighboring atoms. In the other earlier method, the interference of the incident and scattered radiation produce an electromagnetic field in the sample; the atoms act as detectors, giving a fluorescent signal proportional to the field. The problem with both methods is that for practical reasons, only part of the hologram could be measured, which makes it difficult to construct a three-dimensional image. The more-developed field of electron holography has the same problem; the resolution in one direction is seriously degraded.
The research team carried out some of their experiments at the European Synchrotron Radiation Facility (Grenoble, France). They used an avalanche photodiode as a detector, with a curved pyrolytic graphite monochromator to suppress unwanted radiation. Because the energy from the synchrotron beam was so high, the researchers could get a full set of holographic data in three hours instead of the 20 days it would take to do it in a normal laboratory setup. The detector was attached to a goniometer. Another goniometer with a horizontal axis held the sample. This setup allowed the researchers to move the sample and detector together in a circle, relative to the x-ray source, and to rotate the sample while keeping the detector still.1
By filtering out unwanted signals, such as x-ray standing waves, and by combining their various measurements, the researchers were able to produce the images shown. The bar at the right of the image shows color coding for the intensity of the fluorescence.
"Our technique is practical only in the sense that now we can reconstruct the environments of atoms in large, flat, single crystals," Tegze said. "Obviously, x-ray diffraction can do the same job, and I don't think that x-ray holography will ever compete with diffraction for the structure determination of bulk crystals."
The advantage that x-ray holography has over diffraction, he added, is that it allows researchers to create an image of the neighborhood of the atom they are interested in, making it possible to study the sample's impurities or buried interfaces. Tegze said it will take a lot of work to accomplish that goal. The problems are mainly technical, however, and are a result of the method's very low signal-to-background ratio, which is approximately 0.001.
- M. Tegze et al., Phys. Rev. Lett. 82, 4847 (14 June 1999)