Researchers at RIKEN’s Discovery Research Institute in Wako, in collaboration with researchers from National Chiao Tung University, Taiwan, have made the surprise discovery that—under certain circumstances—electrons moving through a metal propagate faster as the number of defects that cause them to scatter is increased.
Intuitively, we know that if a traveling body bounces off an obstacle, its average propagation speed will reduce. As such, attempting to walk against the flow of a large crowd of people can be a futile undertaking, as it is difficult to gain any ground (Fig. 1). Similarly, electrons moving through a metal are expected to slow after scattering off inherent defects: the greater the number of impurities in the metal, the larger the electrical resistance of current flowing through the sample. This means that the time it takes for electrons to lose all prior information on their original state by scattering—the so-called ‘dephasing’ time—is reduced.
Writing in the journal Physical Review Letters1, the researchers report that for extremely impure metallic samples at temperatures close to absolute zero, the electron dephasing time actually increases. “This observation is very surprising indeed, as one would normally expect a decreasing dephasing time with impurity content,” explains Shiu-ming Huang from the RIKEN team.
In other words, the electrons are helped on their way by the ‘scatterers’. This is only possible if the scattering is of a dynamic nature. In terms of walking against a crowd, this would mean that the people one hits on the way through push back and actually promote one’s movement against the crowd. As Huang notes, such dynamic scattering of electrons is only possible “if not only the electron’s state is changed, but also the scatterer is affected by the process.”
One explanation previously proposed by physicists is that magnetic impurities, where electrons bounce off the magnetic field, are generated by magnetic impurities. However, the researchers could rule out this conventional explanation by demonstrating that this behavior is independent of any external magnetic fields.
The origins of this effect therefore remain unknown, and our theoretical understanding of such dynamic scattering is incomplete. According to Huang, what would be needed to understand this behavior “is a series of measurements that not only investigates the temperature behavior of the dephasing time, but also the influence of the materials' properties themselves.” As impurities and defects are present in many nanoscale systems, resolving this issue will be of broad and general relevance.
Reference
1. Huang, S. M., Lee, T. C., Akimoto, H., Kono, K. & Lin, J. J. Observation of strong electron dephasing in highly disordered Cu93Ge4Au3 thin films. Physical Review Letters 99, 046601 (2007).
