„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.
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. We carry out experimental studies of charge transfer processes, charge transfer states and charge transfer excitations in order to determine many relevant properties and to provide a comprehensive knowledge of these issues.
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.
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.
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
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).
T. Li, V.K. Bandari, M. Hantusch, J. Xin, R. Kuhrt, R. Ravishankar, L. Xu, J. Zhang, M. Knupfer, F. Zhu, D. Yan and O.G. Schmidt
Fully integrated high-frequency molecular scale rectifiers based on organic nanostructure heterojunctions
Nature Communications (2020), im Druck
O. Kataeva,K. Ivshin, K. Metlushka, S. Latypov, K. Nikitina, D. Zakharychev, A. Laskin, V. Alfonsov, O. Sinyashin, E. Mgeladze, A. Jäger, Y. Krupskaya, B. Büchner, and M. Knupfer
Charge transfer complexes of linear acenes with a new acceptor perfluoroanthraquinone. The interplay of charge transfer and F···F interactions
C. Habenicht, J. Simon, M. Richter, R. Schuster, M. Knupfer and B. Büchner
Potassium-intercalated bulk HfS2 and HfSe2: Phase stability, structure, and electronic structure
R. Schuster, C. Habenicht, M. Ahmad, M. Knupfer and B. Büchner
Direct observation of the lowest indirect exciton state in the bulk of hexagonal boron nitride
Nonlocal dielectric function and nested dark excitons in MoS2, A. Koitzsch, A.-S. Pawlik, C. Habenicht, T. Klaproth, R. Schuster, B. Büchner, and M. Knupfer, npj 2D Materials and Applications 3, 41 (2019)
Thickness dependent electronic structure of exfoliated mono- and few-layer 1T′−MoTe2 , A.-S. Pawlik, S. Aswartham, I. Morozov, M. Knupfer, B. Büchner, D. V. Efremov, and A. Koitzsch, Phys. Rev. Materials 2, 104004 (2018)
Nearest-neighbor Kitaev exchange blocked by charge order in electron-doped alpha-RuCl3, A. Koitzsch, C. Habenicht, E. Müller, M. Knupfer, B. Büchner, S. Kretschmer, M. Richter, J. van den Brink, F. Börrnert, D. Nowak, A. Isaeva, and Th. Doert, Phys. Rev. Materials 1, 052001 (2017)
Polarization driven conductance variations at charged ferroelectric domain walls, A.-S. Pawlik, T. Kämpfe, A. Haussmann, T. Woike, U. Treske, M. Knupfer, B. Büchner, E. Soergel, R. Streubel, A. Koitzsch, L. M. Eng, Nanoscale 9, 10933 (2017)
Dr. Axel Lubk (group leader)
Phone: +49 351 4659 302
Dr. Daniel Wolf (Senior Scientist)
Phone: +49 351 4659 302
Dr. Pavel Potapov (PostDoc)
Phone:+49 351 4659 832
Johannes Schultz (PhD student)
Phone: +49 351 4659 1117
Arsha Thampi (PhD student)
Phone: +49 351 4659 1117
Subakti Subakti (PhD student)
Phone: +49 351 4659 659
Holographic vector field electron tomography of three-dimensional nanomagnets Communications Physics 2, 87
This Independent Junior Research Group lead by Dr. Aliaksei Charnukha is funded by an Emmy Noether Starting Grant from the German Research Foundation (DFG). The group investigates equilibrium and dynamic emergent phenomena in complex correlated-electron materials by means of terahertz near-field microscopy and spectroscopy at cryogenic temperatures.
Strongly correlated electron systems and the diverse emergent phenomena they host are one of the cornerstones of contemporary condensed-matter physics research. The intimate interplay of lattice, orbital, spin, and charge degrees of freedom emphasized by strong electronic interactions has been found to give rise to an enormous complexity of correlated phases and physical properties. Due to the strong coupling between the aforementioned degrees of freedom, it is possible to navigate between different phases by means of easily accessible external parameters such as static or transient lattice strain, static or transient electric/magnetic fields, and temperature. In many cases the transition between phases is of first order, entailing a phase separation in real space and the occurrence of substantial hysteresis in physical properties. The latter is of substantial technological importance because it provides persistent electronic states that can be switched at will: an essential property of the next generation transistor. Such an implementation is particularly appealing as it minimizes leakage current, which limits further miniaturization and reduction in power consumption of modern electronic devices. It is, therefore, of both fundamental and technological importance to achieve a good understanding of the formation, dynamics, and interplay between various correlated phases. Given that the phase separation typically occurs on the nano-/mesoscale and that the phases often exhibit dramatically different transport properties (such as a metal-insulator transition in VO2), a spectroscopic, low-energy and -temperature, ultrafast optical probe with nanometer spatial resolution is indispensable. Our group is currently constructing such an instrument based on the unique combination of already existing and well-established techniques: scattering-type near-field optical spectroscopy affording spectroscopic investigation of quasiparticle optical response at sub-10 nm spatial resolution, coupled with ultrafast time-domain terahertz spectroscopy providing access to the static and transient lowest-energy material optical response, all at cryogenic temperatures down to 1.6 K required for state-of-the-art condensed-matter research.