Featured Lendület Researcher: Péter Nemes-Incze

In solid-state physics, the study of the behaviour of electrons in crystalline materials is of fundamental importance, as the propagation of electrons in a sample through a material determines many of its properties, such as electrical conductivity and optical characteristics, for example, whether the material is transparent or not.

Péter Nemes-Incze

In the early days of quantum mechanics, physicists were fortunate that certain properties of materials could be understood by studying a single electron (i.e., a non-interacting electron system) – this is the basis for the fundamental insights of solid-state physics. However, in order to understand many other phenomena, it is necessary to examine the electrons as a whole (i.e., interacting electron systems), which is a much more difficult task.

The need for a simpler model

“The most important and interesting current questions in solid-state physics relate to interacting electron systems.

One example is high-temperature superconductivity, in which electrons seem to “hold hands” and move forward together, yet we do not understand exactly how this happens,

” said Nemes-Incze. “In physics, simpler models have always been very important. However, the phenomena of interacting electron systems usually occur in relatively complex materials. Therefore, a simpler model is needed in which these mechanisms can be studied. Rhombohedral graphite could serve as such a model.”

Rhombohedral graphite is a modification of elemental carbon. It is therefore composed exclusively of carbon atoms, but its crystal structure differs from that of conventional hexagonal graphite. In rhombohedral graphite, the hexagonally structured atomic layers (graphene) are stacked with a relative shift. Rhombohedral graphite is rarer than the hexagonal form and was thus studied much less in the past; at the same time, it gives rise to a number of highly interesting physical phenomena. For example, its electrons show strong interaction phenomena.

Simple, therefore wonderful

“The great thing about rhombohedral graphite is that its crystal lattice is very simple, and therefore its study can provide answers to many problems in solid-state physics,” said the research group leader, who has now won his second Momentum grant. “While previously (at the time of graphene’s discovery) the sensation lay in producing ever thinner carbon lattices, down to a single atomic layer, nowadays we are striving to build crystals with specific structures by stacking these lattices on top of one another, possibly with a relative twist.”

One of the special features of rhombohedral graphite is that the more graphene layers it consists of, the stronger the interactions between electrons. In hexagonal graphite, the carbon layers stacked on top of one another are laterally shifted by half a hexagon relative to each other; it is customary to refer to one layer as A and the next as B. In the hexagonal structure, an A-position layer follows B again, meaning that every second layer lies directly above the one below. In other words, successive layers have an ABABABAB stacking sequence. In a rhombohedral structure, the graphene layers can be positioned in three different positions (A, B and C), and the crystal lattice has an ABCABCABC stacking sequence. At the same time, deviations from this structure may occur, either forming randomly or being deliberately introduced; as a result, the physical structure of the material changes. In the coming years, this Momentum group will use these atomic modifications to “tune” the behaviour of interacting electron systems in rhombohedral graphite and study the resulting states.

In his previous Momentum grant, Nemes-Incze and his research group succeeded in modifying the classic adhesive-tape method used to produce graphene, increasing the proportion of rhombohedral graphite from the roughly one percent found in nature to around fifty percent. In addition, they developed a method that uses laser illumination to easily determine the proportion of perfect ABCABC structures present in rhombohedral graphite.

The research group was the first in the world to measure, on an atomic scale, the electronic structural changes caused by strong interactions between electrons in thick (10+ graphene layers) rhombohedral graphite samples.

This demonstrated that this crystal can indeed serve as an excellent model for studying interacting electron systems.

The goal is to change the surface state

“In this project, our main goal is to change, or tune, the surface state of rhombohedral graphite using various methods,” said Nemes-Incze. “For example, we will change the number of interacting electrons and then examine the sample using a scanning tunnelling microscope. As more and more electrons enter the system, the interaction between them becomes stronger.”

In one of the planned experiments, an additional layer of graphene will be placed on the rhombohedral graphite surface, twisted at a predetermined angle. This may create a moiré effect, familiar from old television broadcasts, when a studio guest (ignoring the producers’ requests) appeared on camera wearing a herringbone suit. If the periodicity of the jacket pattern was close to the pixel size of the camera, a wave-like pattern appeared on the screen. In carbon crystals, a similar effect can be achieved by stacking crystals with nearly identical lattice constants on top of one another. By varying the angle between the graphene layers, the properties of the effect can also be tuned.

“In these samples, effects typically emerge that are very difficult to predict using theoretical models. To study them precisely, experiments therefore need to be carried out and the results analysed,” said the physicist. “However, by knowing the laws of physics, we can create experimental conditions that make it likely that we will observe something exciting. In many cases, we are confronted with huge surprises. This is what makes this research so incredibly exciting.”