Physicists find unexpected electron crystals in new ultra-thin materials

MIT physicists report the unexpected discovery of electrons forming crystal structures in a material that is a billionth of thickness. This piece will be added to a gold mine of discoveries derived from materials discovered by the same team about three years ago.

In a paper published on January 22, the team explains how some of the electrons in the manufactured device of a new material become solid, or form, crystallized by changing the voltage applied to the device when it is held at a temperature similar to that of outer space. Under the same conditions, they also showed the emergence of two new electronic states last year that they added to the work they reported indicating that electrons can be split into their own fractions.

Physicists were able to discover thanks to new custom-made filters for better insulation in the equipment involved in the work. These allowed the device to cool down to temperatures several orders of magnitude colder than previous results achieved.

The team also observed all of these phenomena using two slightly different “versions” of the new material, consisting of five layers of atomic thin carbon. Another consisting of four layers. This “indicatingly shows that there are families with materials that can get this kind of behavior,” says Long Ju, an assistant professor at the MIT School of Physics. JU is also affiliated with MIT’s Materials Research Laboratory and Research Labol of Electronics.

Referring to a new material known as the pentagonal graphene in Rombohedral, JU says, “We find the gold mines and every scoop reveals something new.”

New materials

Rhombohedral pentagonal graphene is essentially a special form of pencil lead. Pencil lead, or graphite, is made up of graphene, a single layer of carbon atoms placed on hexagon, similar to the honeycomb structure. Rhombohedral Pentalayer graphene consists of five layers of graphene stacked in a specific overlap order.

Ju and his colleagues discovered the material, so they tinkered with it by adding another layer of material that they thought might highlight the properties of graphene and generate new phenomena. For example, in 2023, I created a sandwich of lombohedral pentalayer graphene with “bread” made from nitrogen nitride. By applying different voltages or amounts of electricity to the sandwich, they discovered three important properties that they have never seen before in natural graphite.

Last year, Ju and his colleagues reported an even more important and even surprising phenomenon. The electrons became a fraction of their own when they applied current to a new device consisting of rhombohedral pentagonal graphene and hexagonal nitride.

This is important. This is because this “fractional quantum Hall effect” is usually seen in very high magnetic fields only in some systems. JU’s work showed that this phenomenon can occur with fairly simple materials without magnetic fields. As a result, it is called the “fractional quantum abnormal Hall effect” (anomaly indicates that no magnetic field is required).

New Results

In the current study, the JU team reports even more unexpected phenomena from a typical rhombohedral graphene/boron nitride system when cooled to 30 milliKelvin (1 milliKelvin equals -459.668 degrees Fahrenheit). In last year’s paper, JU and his colleagues reported the electronic states of six fractions. The current work reports two more findings of these fractional states.

They also discovered another anomalous electronic phenomenon: integer quantum abnormal Hall effects in a wide range of electron densities. The Hall effect of fractional quantum anomalies was understood to appear in the electron “liquid” phase similar to water. In contrast, the new states the team is currently observing can be interpreted as the “solid” phase of electrons that soften the formation of “ice” in electrons. This can also coexist with a fragmentary quantum anomaly hall state when the system voltage is carefully tuned at ultra-low temperatures.

One way to think about the relationship between integer and fractional states is to imagine a map created by adjusting the voltage. By tuning the system with different voltages, you can create a “landscape” that resembles a river (represents a fractional state like a liquid) to a glacier (represents an integer effect like a solid).

Ju notes that his team observed all of these phenomena not only in Pentalayer Rhombohedral Graphene, but also in Rhombohedral graphene, which consists of four layers. This creates a family of materials and indicates that there may be other “relatives” there.

“This piece shows how abundant this material is when displaying exotic phenomena. It added more flavour to this very interesting material,” says Zhengguang Lu, co-author of the paper. Lu, who worked as a postdoc at MIT, is currently a faculty member at Florida State University.

This story has been republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and education.