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.
- Magnetic imaging: Micro-flowers increase the local magnetic fieldMaterials with magnetic nanostructures have many potential applications such as in spintronics. To explore such materials, nanoscale magnetic-sensitive imaging techniques are very useful, but up to now only weak magnetic fields could be applied during the imaging process. Now an international collaboration led by Dr. Sergio Valencia, HZB, has developed an approach that overcomes this limitation. The team designed tiny magnetic flux concentrators (MFCs), into which the sample is placed. The geometry of the MFCs resembles a flower with a number of petals which focus the applied magnetic field into its center. This greatly expands the magnetic field range available during imaging, and so the range of magnetic systems that can be investigated. The micro-flowers, enhancing magnetic fields locally, can find application in different nanometric magnetic microscopy techniques.
- CIGS-perovskite tandem cell achieves record efficiency of 25.5 %A Berlin-based team from HZB and Center for the Science of Materials Berlin (CSMB) at the Humboldt-Universität zu Berlin has set a new record for a tandem solar cell. Using a combination of a CIGS semiconductor layer and perovskite, along with several optimised intermediate layers, they were able to convert 25.5% of sunlight into electrical energy. The previous record for this combination of materials and this size of cell stood at 24.6%. The new record has been certified and is visible in the prestigious Solar Cell Efficiency Tables (the "Green Tables"), which serve as the definitive ledger for the global photovoltaic community.
- Disorder creates new properties in compound semiconductorsAn international research team has demonstrated that the intrinsic disorder of the compound semiconductor CuInSnS₄ can be exploited to influence its optical properties. While the atomic vibrations also sense the local disorder, their response is averaged over many different local environments and therefore appear isotropic, as expected for a cubic crystal. In contrast, the optical excitations, known as excitons, are much more sensitive to the local arrangement of atoms. Surprisingly, they show a direction-dependent optical response even though the average crystal structure is cubic. These findings shed new light on the relationship between disorder and material properties, opening up new options for targeted 'disorder engineering' in optoelectronic and photocatalytic devices.