Disorder can greatly affect how waves travel, sometimes even causing them to stop in their tracks. A lone trumpeter on stage has no trouble projecting to the entire audience, since the sound waves from his horn travel freely in every direction. If a small amount of disorder is added, such as balloons, the sound waves can still fill the room, but if too many balloons surround the trumpeter, the reflected waves will perfectly cancel everywhere, and the music is 'localised' at the trumpet - lost in the forest.
Credit: L. Brian Stauffer of student Aaron Romm from the University of Illinois School of Music
GOSFORD: New insight into how waves spread in different kinds of artificial materials could shed light on how disorder affects quantum materials such as superconductors.
Since waves are used in all kinds of applications, from medical imaging to electronics, the physics behind disorder is fundamental to the understanding of how imperfections in the materials that compose these technologies affect wave behaviour.
"While disorder and imperfections are impossible to avoid in materials, there is much we do not understand about how disorder affects their properties," said co-author Brian DeMarco from the University of Illinois in the U.S., of the paper published in Science today.
Testing a 50-year-old theory
In 1958, Nobel laureate Phil Anderson developed a theory known as Anderson Localisation (AL), which is used to explain how any wave, including light, sound or quantum matter, could be completely localised by disorder, or 'stuck' in place.
DeMarco and his colleagues created artificial materials using atom gases cooled to just billionths of a degree above absolute zero temperature to test this theory for the first time in three dimensions.
The atoms played the role of the electrons in a material, and laser light was used to trap them. The goal was to use these analogues to shed light on the way imperfections in materials affect wave behaviour.
A concert hall scenario
What Anderson and now DeMarco's team observed can be compared to what happens to music played in a well-designed concert hall.
When trumpeter sounds his instrument in a concert hall; the sound waves propagate outward in every direction, eventually reaching your ears. Excellent concert halls are designed so that there is little reflection of those sound waves. If they did reflect off of obstructions, the reflected sound waves can overlap and cancel out, which is referred to as 'destructive interference', leading to dead spots in the concert hall.
If you fill up the concert hall with randomly placed or disordered barriers that reflect the waves, AL means that eventually the waves will get stuck at the trumpet. The result would be perfect silence everywhere in the concert hall - as the trumpeter sounds his instrument, the sound waves would never leave the trumpet.
This is what Anderson discovered - that under certain circumstances, destructive interference can be perfect everywhere the wave is not, and the wave gets 'localised.'
Confirming the theory
Although the researchers used quantum matter waves instead of sound waves, and barriers created using a speckled green laser beam, they show that in some materials waves can get stuck even if there is a clear path through the barriers.
"Anderson's work is based on rigorous results, but they apply directly to one dimensional (1-D) systems," commented Yuri S. Kivshar, head of the nonlinear physics centre at Australian National University in Canberra.
"While localisation in 2-D systems is somewhat similar to that in 1-D systems, localisation in 3-D systems is unique because it depends on the strength of disorder (so we may not have localisation at all). In that sense, this paper presents the first solid experimental result that confirms this theoretical knowledge."
AL is used to understand wave propagation in disordered materials, which is important in the field of emerging technologies. Waves are used in all kinds of applications: ultra-sonic waves in medical imaging, lasers for imaging and sensing and electron waves for electronics. As an interest in nano-electronics expands and plays more of a role in technology, the impact of disorder and the role of AL on electronic materials will need to be better understood.
