MIT researchers announce the surprising revelation of electrons creating crystalline formations in a material merely billionths of a meter thick. This research contributes to a treasure trove of findings originating from this material, which the same group uncovered about three years prior.
In a study published on Jan. 22 in Nature, the group details how electrons in devices constructed, in part, from the novel material can solidify, or crystallize, by altering the voltage applied to the devices while maintaining a temperature akin to that found in outer space. Under these identical conditions, they also observed the appearance of two new electronic states that build upon research they reported last year, demonstrating that electrons can divide into fractions of themselves.
The researchers managed to achieve these discoveries due to newly designed custom filters that enhance the insulation of the apparatus used in the experiments. These advancements enabled them to chill their devices to a temperature vastly colder than what they could attain for previous findings.
The team also witnessed all these phenomena utilizing two slightly distinct “versions” of the new material: one consisting of five layers of atomically thin carbon and the other made up of four layers. This suggests “that there’s a family of materials where you can observe this kind of behavior, which is thrilling,” states Long Ju, an assistant professor in the MIT Department of Physics who spearheaded the research. Ju is also associated with MIT’s Materials Research Laboratory and the Research Lab of Electronics.
Referring to the innovative material known as rhombohedral pentalayer graphene, Ju remarks, “We’ve discovered a gold mine, and every scoop unveils something novel.”
New material
Rhombohedral pentalayer graphene essentially represents a unique type of pencil lead. Pencil lead, or graphite, comprises graphene, a single layer of carbon atoms arranged in hexagons resembling a honeycomb pattern. Rhombohedral pentalayer graphene consists of five layers of graphene stacked in a specific overlapping arrangement.
Since Ju and his team found this material, they have experimented with it by introducing layers of another substance they believed could enhance graphene’s properties or even generate new phenomena. For instance, in 2023, they created a composite of rhombohedral pentalayer graphene layered with “buns” made of hexagonal boron nitride. By applying varying voltages, or electrical quantities, to the composite, they unveiled three significant properties never previously observed in natural graphite.
Last year, Ju and his colleagues announced yet another crucial and even more astonishing phenomenon: Electrons became fractions of themselves when current was applied to a new device formed from rhombohedral pentalayer graphene and hexagonal boron nitride. This is vital because this “fractional quantum Hall effect” has only been observed in a limited number of systems, typically under very high magnetic fields. Ju’s work demonstrated that the phenomenon could manifest in a relatively simple material without the need for a magnetic field. Consequently, it has been dubbed the “fractional quantum anomalous Hall effect” (anomalous indicates that no magnetic field is necessary).
New results
In the current study, Ju’s team reveals even more unexpected phenomena from the overall rhombohedral graphene/boron nitride system when cooled to 30 millikelvins (1 millikelvin is equivalent to -459.668 degrees Fahrenheit). In the previous year’s research, Ju and his peers documented six fractional states of electrons. In the present study, they report the discovery of two additional fractional states.
They also identified another peculiar electronic phenomenon: the integer quantum anomalous Hall effect across a broad spectrum of electron densities. The fractional quantum anomalous Hall effect was believed to arise in an electron “liquid” phase, similar to water. In contrast, the new state observed by the team can be interpreted as an electron “solid” phase — akin to the creation of electronic “ice” — that can also coexist with the fractional quantum anomalous Hall states when the system’s voltage is meticulously adjusted at ultra-low temperatures.
One way to conceptualize the connection between the integer and fractional states is to visualize a map created by adjusting electric voltages: By varying the voltages applied to the system, one can create a “landscape” reminiscent of a river (representing the liquid-like fractional states) meandering through glaciers (symbolizing the solid-like integer effect), explains Ju.
Ju emphasizes that his team observed all of these phenomena not exclusively in pentalayer rhombohedral graphene, but also in rhombohedral graphene comprising four layers. This indicates a family of materials and suggests that other “relatives” may be present.
“This research illustrates the richness of this material in showcasing exotic phenomena. We’ve just enriched this already fascinating material with more depth,” states Zhengguang Lu, a co-first author of the paper. Lu, who conducted this research as a postdoc at MIT, currently holds a faculty position at Florida State University.
In addition to Ju and Lu, other principal authors of the Nature paper include Tonghang Han and Yuxuan Yao, both affiliated with MIT. Lu, Han, and Yao are co-first authors of the publication who contributed equally to the work. Other MIT contributors are Jixiang Yang, Junseok Se, Lihan Shi, and Shenyong Ye. Additional team members include Kenji Watanabe and Takashi Taniguchi from the National Institute for Materials Science in Japan.
This research received support from a Sloan Fellowship, a Mathworks Fellowship, the U.S. Department of Energy, the Japan Society for the Promotion of Science KAKENHI, and the World Premier International Research Initiative of Japan. Device fabrication was conducted at the Harvard Center for Nanoscale Systems and MIT.nano.