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When we lose something, its memory can stay with us, granting the object an ephemeral existence. We know this psychologically and even physiologically – for instance, in the phenomenon of phantom pain from an amputated limb. But in the material world of atoms and electrons, the void created by a thing’s disappearance can be every bit as real as the thing itself.
Take, for instance, electrons in a semiconductor material. When light hits them, they may undergo optical excitation that causes them to jump to a higher energy level, leaving behind a hole. That hole, however, is not a lack of substance but rather an opposite: It is a physical entity in all respects, with a positive charge, as opposed to the electron’s negative one. Sometimes the electron, as it jumps to the higher energy level, briefly orbits the hole – something like an atom with its positively charged nucleus – and the electron-hole system now becomes an “exciton.”
An exciton, in contrast to a natural atom, is a fleeting flash in the pan. Within mere nanoseconds, the electron has dropped down to its former energy state, and the hole disappears. As the electron is reabsorbed, a photon – a particle of light – is emitted. That emission, tiny but measurable, has become a tool much used by researchers to understand the lifestyle of electrons, as well as having many practical applications.
When it comes to excitons, scientists ask: Is it possible to produce complex structures with large numbers of excitons? Could one create, for example, a crystal or a liquid from these atom-like objects? Prof. Israel Bar-Joseph of the Condensed Matter Physics Department, together with Drs. Michael Stern and Vladimir Umansky took up this challenge; they succeeded in observing, for the first time, the formation of an exciton liquid. The results of their research were recently published in Science.
The primary obstacle to creating such complex structures has been the excitons’ short lifespan. They vanish the instant they are formed, making it difficult to amass enough of them in a sufficiently dense pack to flow as a liquid. The idea then, was to extend the survival of excitons. To do this, the researchers used multilayered materials composed of sheets just a few atoms thick. Growing such finely layered materials is one of the specialties of the submicron center at the Weizmann Institute. Within the layers, the scientists created “quantum wells” to trap the electrons, along with their holes.
By designing the structure of the material so that the quantum wells were very close together and then applying an electric field, the researchers managed to produce a situation in which the electrons were trapped in one layer while their holes were stuck in the next layer. Though the mutual attraction was still there, and thus an exciton still formed, the separation of the layers made it much harder for the electron to reunite with its hole. The scientists found that they could use the intensity of the electric field to control the life span of the excitons.
When the system reached very low temperatures (necessary for certain quantum phenomena to occur) and the exciton density rose to a critical level, the researchers observed a sharp transition. The system was now separated into two areas: The first area resembled a gas, in which the excitons moved randomly with no relation to the others; in the second the movement was organized, as in a liquid.
Although the excitons flowed in liquid fashion, the underlying physics was actually the opposite of the forces that produce a liquid state in ordinary substances. Both liquids and crystals form through natural attraction between atoms over small distances. But excitons repel one another; in the thin layers, the repelling force between electron and electron, or hole and hole was just a tad stronger than the attracting force acting on the hole and its electron in the next layer over. The weak residual repulsion caused the now long-lived excitons to array themselves in formation – each equidistant from its neighbors – to produce an organized, dynamic structure resembling a liquid state.
Prof. Israel Bar-Joseph’s research is supported by the Carolito Stiftung; and the estate of Toni Fox. Prof. Bar-Joseph is the incumbent of the Jane and Otto Morningstar Professorial Chair of Physics.
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