The transport of energy and information in semiconductors is limited by scattering between electronic carriers and lattice phonons, resulting in diffusive and lossy transport that curtails all semiconductor technologies.

A team of chemists at Columbia University describes the fastest and most efficient semiconductor yet: a superatomic material called Re6Se8Cl2. Re6Se8Cl2 is a superatomic semiconductor created in the lab.

Excitons in Re6Se8Cl2 interact with phonons by scattering and binding with them to form new quasiparticles known as acoustic exciton-polarons. While polarons are present in a wide range of materials, Re6Se8Cl2’s polarons are unique in that they can flow in a ballistic or scatter-free manner. One day, these ballistic behaviors may lead to speedier and more effective technologies.

The scientists observed that acoustic exciton-polarons in Re6Se8Cl2 traveled twice as fast as electrons in silicon, crossing multiple microns of the material in less than a nanosecond. The team believes that the exciton-polarons cover more than 25 micrometers at a time since polarons can endure for approximately 11 nanoseconds.

Furthermore, processing speeds in theoretical devices could approach femtoseconds, which is six orders of magnitude faster than the nanoseconds available in contemporary Gigahertz electronics. This is because all of these quasiparticles are controlled by light, not by an electrical current and gating, at room temperature.

In terms of energy transport, Re6Se8Cl2 is the best semiconductor.

Re6Se8Cl2 was initially brought into the lab by Jack Tulyag, a Ph.D. student- not in quest of a better semiconductor, but rather to test the resolution of the lab’s microscopes using a material that should have conducted little.

Chemistry professor Milan Delor said, “It was the opposite of what we expected. Instead of the slow movement we expected, we saw the fastest thing we’ve ever seen.”

Scientists were keen to know the mechanism behind why Re6Se8Cl2 showed such remarkable behavior. They developed an advanced microscope with extreme spatial and temporal resolution that can directly image polarons as they form and move through the material.

Petra Shih, a PhD candidate in theoretical chemistry at Timothy Berkelbach’s lab, also created a quantum mechanical model that explains the observations. 

Contrary to popular belief, the new quasiparticles achieve their speed by pacing themselves, which is similar to the tortoise and the hare tale.

The fact that electrons can travel through silicon so quickly makes it a desired semiconductor; nevertheless, like the proverbial hare, they bounce around too much and don’t travel very soon. Re6Se8Cl2 excitons move relatively slowly, and because of this, they can couple up with other acoustic phonons that move at a similar sluggish speed.

Like the tortoise, the ensuing quasiparticles are “heavy” and move forward gradually but steadily. Acoustic exciton-polarons in Re6Se8Cl2 eventually travel faster than electrons in silicon, unhindered by other phonons.

Re6Se8Cl2 may be peeled into atom-thin sheets, just like many other new quantum materials being investigated at Columbia. This property implies that the material may be mixed with other comparable materials to search for further unique qualities. Because the first element in the molecule, rhenium, is one of the rarest elements on earth and, therefore, very expensive, it seems unlikely that Re6Se8Cl2 will ever find its way into a commercial product.

However, armed with the new theory from the Berkelbach group and the sophisticated imaging method that Tulyag and the Delor group created to directly monitor the formation and motion of polarons in the first place, the team is prepared to investigate whether any other superatomic contenders can surpass the speed record set by Re6Se8Cl2.