Inside an extremely thin piece of semiconductor material, too thin to be seen with the naked eye, a research team observed unusual electron behavior. They found that at very low temperatures, within a single layer of a material called tungsten diselenide, the electrons ceased to move as individual particles and instead arranged themselves in a regular, crystal-like pattern.
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This phenomenon is known as a "Wiganer crystal," named after the physicist Eugen Wigner, who predicted nearly a century ago that electrons could form a lattice if the repulsive energy between them became greater than their kinetic energy. The idea is based on the fact that electrons carry the same negative charge, so they repel each other; if their motion slows down sufficiently, this repulsion can cause them to settle into relatively stable positions, like tiny spheres arranged neatly on a flat surface.
However, due to the nature of the formation of these crystals, seeing them is not easy; they are not made up of atoms that can be photographed directly, but rather of electrons within a material.
the researchers used a remarkable optical method, in which they sent light to a very thin layer of semiconductor, then read the subtle changes in the reflected light, as if they were listening to the echo of electrons from within the material.
Tomasz Smolenski, a physics professor at the University of Basel and the study's lead author, says the work's significance lies not only in confirming the existence of electronic order, but also in observing how this crystal interacts with light. The optical excitation within the material doesn't merely pass over the crystal as a fleeting visitor; rather, it can merge with its collective vibrations and movements, transforming into a new hybrid entity. This entity is what the study calls a "Wiganr polaron."
This undated image provided by the Lawrence Livermore National Laboratory shows a deuterium and tritium capsule, sphere in window at center, inside a cylindrical hohlraum container about 0.4 inches tall. In research reported Wednesday, Feb. 12, 2014 by the journal Nature, scientists say they've taken a key step towards harnessing nuclear fusion as a new way to generate power, an idea that has been pursued for decades. In tests, 192 laser beams briefly fired into the small gold cylinder which held the two types of hydrogen. The energy from the lasers kicked off a process that compressed the ball by an amount akin to squeezing a basketball down to the size of a pea, said Debbie Callahan, an author of the paper.
The team conducted their experiments on two single-layer tungsten diselenide devices, surrounded by insulating layers, where the number of electrons could be controlled by an electrical voltage. The samples were placed in a cooling device at 1.6 Kelvin, very close to absolute zero. Under these conditions, the movement of electrons becomes slow enough for the strong repulsive forces between them to become clearly visible.
Tomasz explains that at low electron densities, previously known signals indicative of a Wigner crystal appeared in the reflection spectra. These were then accompanied by new, weaker, but distinct signals. It was these signals that the researchers interpreted as Wigner crystal polarons; that is, traces of light interacting with the electron crystal, not simply ordinary reflections from the material.
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