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Stopped cold by a hint of disorder

13.06.2008

With tiny modifications, such as the introduction of impurities or defects, some conducting materials suddenly become insulating. For Philip Anderson, 1977 Physics Nobel Prize winner, the minor disorder introduced by impurities is enough to completely stop electron movement inside a solid.

Anderson's hypothesis had been proved indirectly, but the phenomenon had never been directly observed with particles such as atoms or electrons until recently, when it was witnessed by CNRS researchers Alain Aspect (1) and Philippe Bouyer and their team at the Institut d'Optique (2).

They have, for the first time, shown atoms subjected to minor disorder coming to a complete stop. Published in the June 12, 2008 issue of the journal Nature, these results will make it possible to better understand the role of disorder in the electrical properties of certain materials.

Introducing disorder to certain conducting materials is sometimes enough to make them suddenly become insulating. On our scale, that would be like saying that a few blades of grass scattered haphazardly over a golf course could stop a full-speed golf ball in its tracks. Admittedly, this would a surprising situation, and at our macroscopic scale, small perturbations can slow the movement of material objects, but can never stop them. But this is different at a microscopic level, where matter can also behave like a wave.

In a perfectly ordered solid, an electron moves freely without being disturbed by the underlying regular crystal structure. In disordered solids, however, any flaw will diffuse the matter wave in multiple directions. Combining all these disorder-generated waves can lead to a wave that does not propagate and remains frozen in the crystal.

The electrons (or the atoms) stop their movement, which, in the case of electrons, turns the material into an insulator. Envisioned by Anderson in 1958, this scenario emphasizes the fundamental role of disorder as well as the relevance of studying the electrical properties of disordered materials like amorphous silicon.

In light of the fundamental discoveries made in the 1930s about semi-conductors that led to the invention of the transistor and then to integrated circuits, Anderson's model created strong interest among physicists. While theoretical physicists strived to understand its underlying nature and its significance, experimental physicists tried to observe the phenomenon. Even though convincing experiments existed, direct observation of particle matter located in a weak disorder remained an unattainable goal.

First direct evidence of the Anderson scenario

French researchers at LCFIO took on the challenge by constructing a simple model of the situation that could lead to this phenomenon, called "Anderson localization." In their experiment, ultra-cold (3) atoms play the role of electrons, while the disordered environment is replaced by a perfectly controlled disorder created by light from a laser beam. With the help of a waveguide, the atoms are limited to unidirectional movement. Without disorder, the atoms propagate freely, but when disorder is introduced, all atomic movement stops within a fraction of a second. The researchers then observed the atomic density profile. Its exponential form is characteristic of the scenario envisioned by Anderson (see figure below). By varying the experimental parameters, the researchers were also able to test the theoretical model developed by Laurent Sanchez-Palencia's team at the atomic optics group.

Armed with results obtained from a radically simplified scenario, the physicists at the Institut d'optique now plan on addressing more complex situations in which atoms can move in a plane, or even in the three directions of space. For these conditions approaching those of real materials, theory can not currently precisely predict all situations; experiments alone constitute a type of quantum simulator that can provide part of the answer. Maybe then, by transferring these results to electrons, it will be possible to better define the behavior of these particles in disordered environments. Such results could, in the long run, improve amorphous silicon-based electronic devices, for example.

Used notably in TFT-LCD screens and in some photovoltaic cells, amorphous silicon is significantly less expensive to produce, but currently less effective than the crystalline silicon that forms the base of high performance electronic devices.

(1) CNRS gold medal, 2005.

(2) A team at the atomic optics group which is part of the Laboratoire
Charles Fabry de l'Institut d'optique (LCFIO, CNRS / Université Paris
11/Institut Optique graduate school).

(3) These ultra-cold atoms are in the form of a diluted Bose-Einstein
condensate, formed from several thousand atoms described by the same
wave function, making it possible to observe the atomic density profile.