- Molecular and low-dimensional solids (Martin Knupfer)
- Correlated electrons at interfaces and in f-systems (Andreas Koitzsch)
- Advanced Methods of Electron Microscopy (Axel Lubk)
„Microscopic understanding“ of a phenomenon is a synonym for the ability to describe it by first principles. The notion mirrors the outstanding importance of truly microscopic techniques for the investigation of matter. Here we use electron microscopy in various forms to study novel materials and combine it with laterally resolved and integrated spectroscopy. The range of problems that can be addressed comprises e.g. the investigation of the microstructure and the distribution of electric, magnetic and strain fields of solids, their chemical in homogeneities, the characterization (and even manipulation) of nano-objects, down to the direct observation of the atomic arrangement of a material. This information is naturally complemented by electronic structure investigations for which electron spectroscopy is the leading tool. We focus on photoemission and (in)elastic electron scattering using state-of-the art laboratory equipment and we develop new microscopy instrumentation and methods, e.g., for electron tomography or cryogenic microscopy.
Organic semiconductor interfaces
The fundamental properties and the application of organic materials in electronic devices is an issue of intensive research. Potential devices are anticipated to enable flexible, low cost electronics. The importance of interfaces for the device performance cannot be overestimated as they determine charge injection and charge flow in the devices. Here, we investigate particular organic semiconductor interfaces in order to develop a general understanding for those. Recently, we have discovered an organic heterojunction, which is characterized by charge- and spin-transfer between the constituents. Related publications
Transition metal dichalcogenides
The material class of transition metal dichalcogenides (TMD) comprises a number of fascinating physical phenomena. For instance, various competing ground states – such as superconductivity and charge density waves – can be found in some of its representatives. Moreover, these materials have a two-dimensional crystal structure with weak bonding along the crystal c axis, which is reminiscent of graphene. In this regard, TMD materials are also considered to be representatives for future electronics beyond graphene. We investigate the electronic properties, in particular the dynamic response, which e.g. reveals intriguing dispersion relations for plasmons and excitons in some of these materials. Related publications
Oxide heterostructures and interfaces
Fascinating and counterintuitive phenomena have beenobserved at the interface of certain complex oxides. The mostimportant is the appearance of metallic conductivity betweentwo firm insulators such as SrTiO3 and LaAlO3 reported by Ohtomo and Hwang in 2004. But also superconductivity and magnetism have been found. We investigate the electronic and chemical structure of these new interfaces which hold prospects for both, fruitful fundamental research and the implementation into devices. Related publications
Heavy fermion materials
The properties of heavy fermion materials sharply deviatefrom conventional metals, most notably in the occurrenceof very large effective masses at low temperatures. Thisis often accompanied by rich phase diagrams consisting ofmagnetic order and unconventional superconductivity. The electronic structure is determined by a subtle interplay between f-electrons and conduction electrons which can be analyzed by means of electron spectroscopy. Related publications
Recently, an emergent class of magnetic inhomogeneities and textures, such as skyrmions and spin polarons opened up new avenues to a wide range of fundamental problems in spintronics, eventually enabling novel magneto-electronic applications such as storage devices. Substantial scientific progress on nanoscale magnetic inhomogeneities depends on qualitative improvements of the capabilities and detection limits of the current magnetic imaging and characterisation techniques. Here, we develop new techniques for high resolution imaging of magnetic textures using advanced electron holographic techniques. We particularly elaborate on the tomographic reconstruction of magnetic fields in 3D and the mapping of these fields at cryogenic temperatures under the application of external electric and magnetic fields.
Plasmon resonances are collective excitations of the conduction electrons in solids. In particular at surfaces of, e.g., metallic nanoparticles, spatially confined resonances referred to as localised surface plasmon resonances (SPR) or surface plasmon polaritons (SPP) lead to a range of extraordinary properties, such as strong and locally tunable transient electrical fields, which are very sensitive to nanometer scale environmental changes. Emerging opto-electronic devices exploiting SPRs comprise on-chip light spectrometers and linear accelerators, increased efficiency LED and photovoltaics, and metamaterials with properties such as negative refractive index and slow-light propagation and flat metalenses. Here we develop and apply advanced electron microscopy techniques such as SPR mapping to characterize the resonant modes in terms of energy spectrum and spatial distribution.
Mesocrystals are built from nanocrystals, which self-assemble into a crystalline superlattice, maintaining a specific crystallographic orientation across the individual nanocrystal building blocks. Different non spherical nanocrystals allow generating a variety of complex superlattices depending on the particular type of faceting. Mesocrystals are a new class of materials, allowing to extend properties of nanoparticles, such as superparamagnetism, to mesoscopic and macroscopic length scales. Here, we plan to investigate the structure of such crystals with advanced transmission electron microscopy methods (e.g., holography).