Physics and chemistry have rewritten the fable of the hare and the tortoise in the quantum realm more than 2,500 years after Aesop created it. Just as the slowest animal beat the fast hare in a race thanks to its perseverance and strategy, a research group from the Columbia University (New York-USA) has found by chance a superatomic material, called Re₆Se₈Cl₂ (composed of rhenium, selenium and chlorine), which has served as a semiconductor so that electrons have traveled through micrometers in experiments in less than a nanosecond. “Theoretically, they have the potential to reach femtoseconds, six orders of magnitude [10⁶] faster than the speed achievable in current gigahertz electronics and at room temperature,” the researchers explain.
The discovery, published in Science, it has been coincidental and thanks to student Jack Tulyag, who is working on his doctorate with Columbia Chemistry professor Milan Delor. The first brought Re₆Se₈Cl₂ to the laboratory, believing it to be a material without high conduction capacity, to test super-resolution microscopes, which can capture 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 we have ever seen,” says Delor.
According to the investigator, silicon-based semiconductors allow rapid movement of electrons that was not anticipated 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 generated when the particle jumps to a higher energy state) pairs with the phonon, a quasiparticle carrying energy and fundamental in electrical conductivity. This association generates a new quasiparticle, called acoustic exciton-polaron, which is heavier, but which, paradoxically, has turned out to be faster.
Delor turns to Aesop’s fable to explain it. In silicon, electrons can move through it very quickly, but like the hare, which lingers confidently in its ability, “they bounce around too much and don’t get very far very fast in the end.” By contrast, in superatom material, excitons pair with phonons to move, like the turtle, “slowly but steadily,” in “a ballistic or dispersion-free flow.” This behavior is similar to that of a fluid that flows without friction through a conduit and, therefore, without losing kinetic energy.
“Unobstructed in the way, the acoustic exciton-polaron ultimately moves faster in Re₆Se₈Cl₂ than electrons in silicon,” summarizes the researcher.
In the experiments, acoustic exciton-polarons reached several micrometers of the sample in Re₆Se₈Cl₂ in less than a nanosecond. This speed, taking into account that they can remain stable for about 11 nanoseconds and be controlled with light instead of electricity, allows the researchers to calculate that, theoretically, “they could cover more than 25 micrometers in femtoseconds.”
This theoretical potential means a speed one million times greater than the electron in silicon, a ratio similar to that of the speed of light versus 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. “In terms of energy transport, Re₆Se₈Cl₂ is the best semiconductor we know of, at least so far,” says Delor.
Jose Luis Salmeron, outside the research and director of the Data Science Lab at Cunef University, explains the importance of the finding: “The transfer of energy and information in semiconductors is limited by the dispersion between electronic carriers and phonons of the lattice, which results in losses that restrict all semiconductor technologies. Using a superatomic semiconductor such as Re₆Se₈Cl₂, the authors demonstrate the formation of acoustic exciton-polarons protected against phonon scattering.”
Salmerón, included by Elsevier and Stanford University in the latest list of the most cited scientists, highlights that the new semiconductor presents a structure organized in layers linked by Van der Waals forces: “They are attractive forces that act between atoms and molecules due to temporary fluctuations in the electronic charge distributions. This peculiar arrangement gives it semiconductor properties, which means it can conduct electricity differently than conventional conductors and insulators. “What distinguishes Re₆Se₈Cl₂ as a superatmic semiconductor is its ability to exhibit exceptional electronic properties that transcend the individual characteristics of its constituent atoms.”
The application of this potential in commercial processors is limited because the discovered semiconductor includes rhenium, a rare chemical element on Earth that is used in nickel-based superalloys or, together with molybdenum and tungsten, in aeronautical engines, in chemical and petrochemical catalysts. or for corrosion resistant coatings.
However, after two years of work, the team of researchers believes that they can use the combination of other elements to find semiconductors with capacities similar to those of Re₆Se₈Cl₂. “This is the only material in which 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 considered 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 principal 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 new possibilities for the development of more efficient and advanced technologies in the field of electronics and computing.”
“In the specific case of Re₆Se₈Cl₂″, adds the Spanish researcher, “a protected polaron transport has been observed, which means that these quasiparticles can move more efficiently and less affected by interactions with the lattice vibrations. “This can have significant implications in terms of efficiency and speed in semiconductor and electronics applications.”
“This is a great advance because the ability to have ballistic semiconductors at room temperature represents a significant step towards the improvement of electronic technology in terms of efficiency, speed and versatility of possible applications,” concludes Salmerón.
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