The use of angle-resolved photo-emission has enabled a team of US and German researchers to observe unexpected similarities in the charge-conduction processes in quasicrystals and in ordinary metals.
The material they observed was a quasicrystalline aluminum nickel cobalt (AlNiCo) alloy that was grown by the Czochralski method at the Ludwig-Maxmillians-University (Munich, Germany). Samples with 10-fold surfaces were then cut and polished at the Free University of Berlin (Berlin, Germany). Researchers at the Advanced Light Source in the Lawrence Berkeley National Laboratory (Berkeley, CA) then prepared the sample surfaces and performed the characterization and examination using low-energy electron diffraction (LEED) and angle-resolved photo-emission.1
"Before quasicrystals were discovered by Dan Schechtman and his colleagues in 1984, most people would have said they were structurally impossible," said Eli Rotenberg, a staff scientist at the Advanced Light Source. The complex geometry of quasicrystals includes five-fold and other "forbidden" symmetries that would seem to prevent the long-range structural order that they possess. Although the structural order of quasicrystals is not periodic, as in the case of metals, it is still ordered, as one might find in the linear example of a Fibonnaci series (see Figure). In addition, the recent experiments showed that electrons moving through quasicrystals tended to move along "bands" that could be described by functions analogous to the Bloch functions that describe the movement of electrons through metal conductors. The researchers also found that the electron momenta and energies were correlated with the quasicrystal structure.
The researchers measured the emission angles and kinetic energy of electrons scattered from the near surface of the material using soft x-rays. "These are the valence electrons, [which are] not as tightly bound as electrons near the atomic cores," Rotenberg said.
The AlNiCo alloy that was examined consisted of stacked planes of atoms exhibiting 10-fold symmetry. The researchers observed the effects of the quasicrystalline ordering by looking at the behaviors of electrons in the plane, and they observed the effects of the periodic, crystalline-like ordering of the stack by looking perpendicular to the planes.
The researchers rotated the sample to get a complete distribution of electron angles and energies, and eventually obtained a plot of the electronic states of the valence electrons in momentum space.
"Our principal findings were that the distribution of the electronic states in momentum space correlates with the electron diffraction pattern, just like an ordinary crystal," Rotenberg said. "The electrons aren't localized to clusters; instead, they [respond to] the long-range quasicrystal potential."
Potentially useful properties of quasicrystals include their durability, stability at high temperatures, ability to store hydrogen at high density, and facility for making non-stick coatings.
In a commentary on the quasicrystal work, Patricia A. Thiel and Jean Marie Dubois note the researchers did not rule out the possibility of critical states co-existing in the quasicrystal alongside the extended or band-like states that were observed.2 They added that the metallic behavior of decagonal AlNiCo can be partly explained by its being one of the most metallic of quasicrystals, and that the next step will be to look for band-like conduction in other alloys such as the very nonmetallic icosahedral aluminum palladium rhenium AlPdRe.
REFERENCES
- E. Rotenberg, et al., Nature, 406, 602, Aug. 10, 2000.
- P. A. Thiel and J. M. Dubois, Nature 406, 570, Aug., 10, 2000.