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Research Topics

Our research at the interface of solid state chemistry, physics and materials science, is focused on crystal chemistry - physical property relations in inorganic materials. Our special interest concerns materials with highly correlated electrons. Our aim is to discover relations between chemical structures and the physical phenomena occurring in these compounds, among them superconductivity, metal-insulator transitions and all the diverse manifestations of magnetism. Today it is clear that correlated electrons create these outstanding properties, but a coherent theory of correlated materials is still due.

Correlated electrons present some of the deepest challenges in physics and such systems are the subject of many solid state (physics) groups around the world. In contrast to this, the solid state chemistry of correlated materials appears underdeveloped. We believe that the special competence of solid state chemist’s in synthesis, structural characterisation and chemical bonding has a high potential to widen the knowledge about correlated materials by exploring new compounds and structures that exhibit such fascinating properties like superconductivity.

Special focus: superconductivity in iron pnictides

High-Tc superconductivity is one of the biggest challenge in modern material science. More than 25 years after the discovery of superconductivity in copper-oxides by Bednorz & Müller, no coherent theory is able to explain this phenomenon. It remains a big mystery in solid state physics, but on the other hand, the mystery is even bigger in solid state chemistry. While plenty of physical theories at least try to explain the enigma, there is no workable concept regarding the exact chemical ingredients to synthesise a superconductor. Up to now, there is no concrete relation between chemistry and superconductivity known. It appears therefore not surprising, that new superconductors were mostly discovered by serendipity. This happened again in 2008. Hideo Hosono from Japan was searching for new semiconductors, when he discovered superconductivity at 26 K in the iron arsenide-oxide LaO1-xFxFeAs. This heralded a new era of superconductivity research. Chinese groups quickly increased the Tc to 55 K in SmO1-xFxFeAs, and it became clear that the second family of high-Tc superconductors was born. Also in 2008, we discovered another class if iron arsenide superconductors. This was not by chance, but rather by chemical intuition. BaFe2As2 with the long known ThCr2Si2 (actually BaZn2P2)-type structure contains similar FeAs layers as LaOFeAs with the same charge. We could induce superconductivity by potassium substitution up to a maximum Tc of 38 K in Ba0.6K0.4Fe2As2. With respect to the extremely robustness of superconductivity (high critical fields), this compound has a high potential to become a technical material. This and other so-called “122-superconductors” rank among the most investigated solids world-wide. In spite of the enormous progress that has already been made, plenty of questions remain open. This rapidly growing field will certainly remain fascinating for years.

Methods

We use a combination of material synthesis, structure characterisation techniques and state-of-the-art quantum chemical calculations. Our main methods are:

  • high-temperature solid state reactions under inert conditions
  • conversion of solids with reactive gases and reactive flux methods
  • X-ray diffraction (single crystal and powder methods)
  • low temperature structure determination (powder X-ray, 10 K < T < 900 K)
  • Scanning electron microscopy with EDX
  • magnetic measurements using in-house AC- and SQUID-magnetomenters
  • electrical resistance measurements between 4 and 300 K
  • electronic band-structure calculations based on density functional theory (DFT)
  • neutron scattering (at FRM-II Garching)
  • muon spin rotation (at PSI, Villigen Switzerland)

Results

Are published in a series of articles listed here.