Physics and chemistry have rewritten the fable of the hare and the tortoise in the quantum realm more than 2,500 years after it was created by Aesop. Just as the slowest animal, thanks to its endurance and strategy, defeated the fast hare in a race, a research group from Columbia University (New York, USA) accidentally found a superatomic material called Re₆Se₈Cl₂ (a compound of rhenium, selenium and chlorine), which served as a semiconductor, allowing electrons to travel through micrometers in less than a nanosecond in experiments. “Theoretically, they have the potential to reach femtoseconds, or six orders of magnitude [10⁶] faster than the speed that can be achieved in current gigahertz electronics and at room temperature,” explain the researchers.
The discovery, published in Science, was accidental and thanks to student Jack Tulyag, who is working on his doctoral thesis with Milan Delor, a chemistry professor at Columbia University. The first brought Re₆Se₈Cl₂ into the laboratory, believing it to be a material without high conductivity, to test high-resolution microscopes that can detect particles moving at ultrafast and ultramicroscopic scales. “It was the opposite of what we expected. Instead of the slow movement we expected, we saw the fastest movement we had ever seen,” says Delor.
According to the researcher, silicon-based semiconductors enable rapid electron movement, which was not expected in the superatmic material. But the experiment made it possible to discover that in Re₆Se₈Cl₂ the exciton (a quantum state formed by electrons that have absorbed energy, and the hole formed when the particle jumps to a higher energy state) with the phonon, a Quasi-particle carrying energy pairs of fundamental importance for electrical conductivity. This association creates a new quasiparticle called the acoustic exciton polaron, which is heavier but, paradoxically, has been shown to be faster.
Representative diagram of the behavior of particles in various semiconductors, reminiscent of Aesop’s fable. Jack Tulyag/Columbia University
Delor draws on Aesop’s fable to explain it. In silicon, electrons can move through it very quickly, but like the rabbit, confident in its ability, “they bounce around too much and end up not going very far very quickly.” In contrast, in superatomic material, excitons pair with phonons to surround themselves like the turtle to move “slowly but steadily” in “a ballistic or dispersion-free flow.” This behavior is similar to that of a fluid flowing through a pipe without friction and therefore losing no kinetic energy.
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“Unhindered, the acoustic exciton polaron in Re₆Se₈Cl₂ ultimately moves faster than electrons in silicon,” the researcher summarizes.
In the experiments, acoustic exciton polarons reached several micrometers of the sample in Re₆Se₈Cl₂ in less than a nanosecond. This speed, and the fact that they remain stable for about 11 nanoseconds and can be controlled with light rather than electricity, allows the researchers to calculate that they could theoretically “travel more than 25 micrometers in femtoseconds.”
Milan Delor, last right, and Jack Tulyag, second to last, in a picture of members of the Columbia University lab. Delor Labs
This theoretical potential means a speed a million times greater than that of the electron in silicon, a ratio similar to that of the speed of light to the speed of sound or the speed of an airplane at 900 kilometers per hour. Today’s computer processors are also 10⁶ faster than those of computers from 20 years ago. “As far as energy transport is concerned, Re₆Se₈Cl₂ is, at least so far, the best semiconductor we know,” says Delor.
José Luis Salmerón, outside research and director of the Data Science Lab at the University of Cunef, explains the significance of the result: “The transfer of energy and information in semiconductors is limited by the scattering between electronic carriers and phonons of the network that results. “ to losses that limit all semiconductor technologies. Using a superatomic semiconductor such as Re₆Se₈Cl₂, the authors demonstrate the formation of acoustic exciton polarons that are protected from phonon scattering.”
Salmerón, included in the latest list of most cited scientists by Elsevier and Stanford University, emphasizes that the new semiconductor has a structure organized into layers connected by van der Waals forces: “ There are attractive forces that act between atoms and molecules. due to temporal fluctuations in electronic charge distributions. This particular arrangement gives it semiconductor properties, meaning it can conduct electricity differently than traditional conductors and insulators. “What sets Re₆Se₈Cl₂ apart as a superatmic semiconductor is its ability to exhibit extraordinary electronic properties that go beyond the individual properties of its constituent atoms.”
The application of this potential in commercial processors is limited because the discovered semiconductor contains rhenium, a rare chemical element on Earth that is used in nickel-based superalloys or along with molybdenum and tungsten in aviation engines, and in chemical and petrochemical catalysts for corrosion-resistant coatings.
Columbia University Professor of Chemistry, Milan Delor, Columbia University
However, after two years of work, the research team believes they can use the combination of other elements to find semiconductors with capacities similar to Re₆Se₈Cl₂. “This is the only material where sustained ballistic exciton transport has been observed at room temperature. But now we can begin to predict what other materials might be capable of this behavior that we simply hadn’t thought of before. “There is a whole family of superatomic semiconductor materials and others with favorable properties for the formation of acoustic polarons,” says Delor.
Salmerón, research associate at Autonomous Chile and senior data scientist at Capgemini, agrees: “This discovery offers new perspectives in the search for materials with revolutionary applications in electronics and semiconductor technology. “This discovery not only expands our understanding of superatomic semiconductors, but also opens up new opportunities for the development of more efficient and advanced technologies in electronics and computer science.”
“In the specific case of Re₆Se₈Cl₂,” adds the Spanish researcher, “protected polaron transport was observed, which means that these quasiparticles can move more efficiently and are less affected by interactions with the lattice vibrations.” “This can have a significant impact on efficiency and have speed in semiconductor and electronics applications.”
“This is a major advance because the possibility of producing ballistic semiconductors at room temperature represents a significant step towards improving electronic technology in terms of efficiency, speed and versatility of possible applications,” concludes Salmerón.
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