Graphene on titanium carbide triggers a novel phase transition
Graphene-induced Lifshitz-transition from a petal-shaped Fermi surface to a gear-shaped hole Fermi surface revealed by comparative full photoemission mapping of the band structures of bare TiC(111) and graphene/TiC(111). © HZB
Researchers have discovered a Lifshitz-transition in TiC, driven by a graphene overlayer, at the photon source BESSY II. Their study sheds light on the exciting potential of 2D materials such as graphene and the effects they can have on neighboring materials through proximity interactions.
Stacking 2D materials has garnered a lot of attention in recent years as it provides a unique opportunity to tailor material properties in a highly controllable manner. However, the influence of 2D materials on the properties of neighboring materials through proximity effects is not yet fully understood. In particular, very sensitive properties such as band gaps in semiconductors and excitonic properties have been observed to be influenced. Fermi surfaces of bulk metals have so far not been among the properties sensitive to a proximity effect.
The Fermi surface of a metal is a mathematical concept to represent the electrons of the highest energy in the material. Only these electrons participate in properties such as electrical conductivity. An important aspect of the Fermi surface is that it represents them in terms of the direction of their movement.
The new study by Andrei Varykhalov and his colleagues at BESSY II shows that a graphene layer can induce a Lifshitz transition in the near-surface region of an underlying metal, TiC: The Fermi surface transforms from a hole-like to an electron-like Fermi surface. The reported change in Fermi surface character is particularly relevant since it changes the orientation of the movement of the electrons and in the presence of a magnetic field it changes the orientation of the macroscopic electric current.
The present finding is an exciting development as it provides a new avenue for controlling and manipulating the electronic properties of materials, which has implications for a range of technological applications, for example designing materials with quantum properties such as high temperature superconductivity.
- The future of corals – what X-rays can tell usThis summer, it was all over the media. Driven by the climate crisis, the oceans have now also passed a critical point, the absorption of CO2 is making the oceans increasingly acidic. The shells of certain sea snails are already showing the first signs of damage. But also the skeleton structures of coral reefs are deteriorating in more acidic conditions. This is especially concerning given that corals are already suffering from marine heatwaves and pollution, which are leading to bleaching and finally to the death of entire reefs worldwide. But how exactly does ocean acidification affect reef structures?
Prof. Dr. Tali Mass, a marine biologist from the University of Haifa, Israel, is an expert on stony corals. Together with Prof. Dr. Paul Zaslansky, X-ray imaging expert from Charité Berlin, she investigated at BESSY II the skeleton formation in baby corals, raised under different pH conditions. Antonia Rötger spoke online with the two experts about the results of their recent study and the future of coral reefs.
- Long-term stability for perovskite solar cells: a big step forwardPerovskite solar cells are inexpensive to produce and generate a high amount of electric power per surface area. However, they are not yet stable enough, losing efficiency more rapidly than the silicon market standard. Now, an international team led by Prof. Dr. Antonio Abate has dramatically increased their stability by applying a novel coating to the interface between the surface of the perovskite and the top contact layer. This has even boosted efficiency to almost 27%, which represents the state-of-the-art. After 1,200 hours of continuous operation under standard illumination, no decrease in efficiency was observed. The study involved research teams from China, Italy, Switzerland and Germany and has been published in Nature Photonics.
- Energy of charge carrier pairs in cuprate compoundsHigh-temperature superconductivity is still not fully understood. Now, an international research team at BESSY II has measured the energy of charge carrier pairs in undoped La₂CuO₄. Their findings revealed that the interaction energies within the potentially superconducting copper oxide layers are significantly lower than those in the insulating lanthanum oxide layers. These results contribute to a better understanding of high-temperature superconductivity and could also be relevant for research into other functional materials.