14 DAY TRIAL // A soliton, short for "solitary wave", is an entity made from an electron and a bunch of phonons. A phonon is the particle associated to the vibration occurring in a crystal. It might seem weird to think of vibrations in terms of particles, but in quantum mechanics one has the so-called wave-particle dualism - any wave can also be seen in terms of particles. The reason why quantum mechanics associates particles to waves is that the energy of the waves cannot take any value - the energy can take only certain, discrete values. The vibrational energy of atoms or molecules in a crystal is no different - it too can take only certain, discrete values and thus it is often much more useful to speak in terms of particles - the phonons - than in terms of vibrational waves. Just as a photon is a particle of light energy, a phonon is a particle of vibrational energy.
Solitons are not called "solitary waves" for nothing. Unlike other waves, they remain as a single entity for a long time. "It's like when you make a ripple in water -- it quickly spreads and disappears. But a soliton is a strange kind of object. Once it is made, it maintains its character for a long time," Li explained.
In fiber optics, normal light waves gradually flatten out; unless the signal is boosted periodically, it disappears. In contrast, solitonic light waves retain their structure and keep going without assistance. Some telecommunication companies have exploited that fact by using solitons to cheaply send signals over long distances. But before solitons can be fully exploited in a wider range of applications, scientists must learn more about their basic properties.
Since the 1980s, scientists have known that solitons can carry an electrical charge when traveling through certain organic polymers. A new study now suggests that solitons have intricate internal structures.
Li explained that each soliton is made up of an electron surrounded by other phonons. Now scientists have discovered that the electron inside a soliton can attain different energy states, just like the electron in a hydrogen atom.
"While we know that such internal electronic structures exist in all atoms, this is the first time anyone has shown that such structures exist in a soliton," Li said.
The soliton's quantum mechanical properties -- including these newly discovered energy states -- are important because they affect how the particle carries a charge through organic materials such as conducting polymers at the molecular level.
"These extra electronic states will have an effect -- we just don't know right now if it will be for better or worse," he said.
How solitons travel
Li is especially interested in how solitons carry a charge through conducting polymers, which consist of long, skinny chains of molecules. The tiny chains are practically one-dimensional, and this calls some strange physics into play, Li said.
In their PNAS paper, Li and MIT colleagues Xi Lin, Clemens F?rst, and Sidney Yip describe a detailed calculation of what happens to solitons at a quantum-mechanical level as they travel along a chain of the organic polymer polyacetylene. Their work was mainly funded by Honda R&D Co., Ltd., with computing resources provided by the Ohio Supercomputer Center.
Their mathematical model builds upon a 1979 model called the Su-Schrieffer-Heeger (SSH) model. Alan Heeger, a University of California, Santa Barbara physicist who discovered solitons, won the Nobel Prize in 2000 for his pioneering work on conducting polymers.
Li said the new work extends the SSH model by including the full flexibility of the polymer chain, as well as interactions between electrons.
Possible applications
The finding will likely affect the development of molecular electronics -- devices built from individual molecules.
Because polymer chains tend to bend and twist as solitons pass through them, scientists have wondered whether solitons could be used to power artificial muscles for high-tech robots and devices to aid human mobility. Such muscles would be made of organic polymers, and flex in response to light or electrochemical stimulation.
"If fully understood, solitons may also be harnessed to drive molecular motors in nanotechnology," Li said.
Image courtesy of Ohio State University: A solitary electron wave (ripples of red and blue, center) travels along a polymer chain, causing the chain to bend in the middle.