Physicists Report Promising Approach to Harnessing Alien Electronic Behavior

For about fifty years, scientists have worked to exploit Bloch oscillations, a type of exotic behavior of electrons that could introduce a new field of physics – and important new technologies – just as more conventional electronic behavior has led. everything from smartwatches to sufficiently powerful computers. to bring us to the moon.

Today, physicists at MIT are presenting a new approach for obtaining Bloch oscillations in newly introduced graphene superlattices. Graphene, a material composed of a single layer of carbon atoms arranged in hexagons resembling a honeycomb structure, is an excellent conductor of electricity. Its electronic properties undergo an interesting transformation in the presence of an “electrical mesh” (a periodic potential), resulting in new types of electronic behavior not seen in virgin materials. In a recent issue of Physical examination letters, scientists explain why graphene superlattices can be a game-changer in the pursuit of Bloch oscillations.

Normally, electrons exposed to a constant electric field accelerate in a straight line. However, quantum mechanics predicts that electrons in a crystal, or in a material made up of atoms arranged in an orderly fashion, may behave differently. When exposed to an electric field, they can oscillate in tiny waves, Bloch oscillations. “This surprising behavior is an iconic example of coherent dynamics in quantum multi-body systems,” says Leonid Levitov, professor of physics at MIT and head of current work. Levitov is also affiliated with MIT’s Materials Research Lab.

The other authors are Ali Fahimniya and Zhiyu Dong, both MIT graduate students in physics, and Egor I. Kiselev of Karlsruher Institut fur Technologie.

Towards new applications

It is important to note that Bloch oscillations occur at a frequency value which is the same for all electrons and is tunable by the applied electric field. Additionally, typical frequency values ​​- in the terahertz range, or billions of cycles per second – fall into a range that is difficult to access by conventional means. Today’s electronics and optics operate at frequencies below and above terahertz, respectively. “Terahertz frequencies are something in between, and we don’t benefit from them as much as the rest of the spectrum,” Levitov said. “If we could access it easily, there could be many applications, ranging from better non-invasive airport security scanning to new electronic designs.”

Due to the interesting physics and potential applications of Bloch oscillations, over the years many scientists have attempted to demonstrate the behavior. Bloch oscillations, however, are very sensitive to diffusion processes in the material due to lattice vibrations (phonons) and disorder. As a result, although earlier work to create Bloch oscillations has been extremely important – one approach, relying on semiconductor superlattices, has led to a Nobel Prize and modern solid state lasers. – it has only met with limited success towards its initial objective. “People saw Bloch oscillation signatures in these systems, but not at the level that would be useful for anything practical. There was inevitably a phase shift, which turned out to be quite overwhelming. [for the phenomenon]”says Levitov.

A new material

Enter a new material known as moiré graphene. Launched at MIT by physics professor Pablo Jarillo-Herrero, moiré graphene is made up of two sheets of thin atomic layers of graphene placed on top of each other and turned at a slight angle. “And according to theory, this material should be an ideal candidate for seeing Bloch oscillations,” Levitov says. In the recent article, he and his colleagues analyzed the parameters of the material that impact how electrons move in it and how little mess it presents, and “we show that in all respects, moiré graphene is as good as semiconductor superlattices, if not better “.

In addition, other attractive varieties of superlattices have emerged recently, involving graphene in combination with hexagonal boron nitride or patterned dielectric superlattices. Among the additional advantages, graphene superlattices are much easier to make than the key complicated structures of previous work. “These systems have only been produced by a few highly skilled groups around the world,” says Levitov. Moiré graphene is already manufactured by several groups in the United States alone, and many more around the world.

Finally, according to Levitov and his colleagues, moiré graphene meets another important criterion for making Bloch oscillations practical. While the electrons involved in the oscillations do so at the same terahertz frequency, without a little help, they will do it independently. The key is to persuade them to oscillate in synchrony. “If you can do that, then you’re essentially going from a one-electron phenomenon to macroscopic oscillations which will be easily detectable and very usable because they will become a source of macroscopic current,” says Levitov. Scientists believe that the electrons in moiré graphene should lend themselves to synchronization using standard techniques.

Dmitri Basov, Professor Higgins and holder of the chair of physics at Columbia University, comments: “Like many other predictions of Leonid Levitov and his team, this new result on the oscillations of Bloch will certainly motivate many studies. experimental. I predict it won’t be easy to watch. Bloch oscillations in moire flatband systems, but we’ll definitely try. »Basov did not participate in the work reported in Physical examination letters.

Levitov is excited to continue the work, which will include undergraduates from MIT. “The best part of that will come later when we see experimental results that prove the idea,” he says.