Charge ordering in cationic mixed-valence compounds is of crucial importance for materials science. The prototypic example for a transition from a charge-disordered to a charge-ordered state has been magnetite, Fe3O4, where Evert Verwey observed a sudden jump in resistivity near -150°C. In the journal Science Advances now a research team of scientists from Germany and Slovenia reports a Verwey-type charge-ordering transition in a different class of mixed-valence compounds which is composed of negatively charged dioxygen molecules.
The crystal structure and chemical bonding scenario of the complex intermetallic compound Be21Pt5 was solved in a joint collaboration spearheaded by scientists from the Max-Planck Institute for Chemical Physics of Solids. Be21Pt5 shows the rare case of superconductivity in a complex intermetallic compound with over 400 atoms per unit cell. Real space analysis of the calculated electron density and the electron localizability indicator (ELI) reveal structural units with collective intra-cluster interactions, termed cluster bonding, which are linked in the structure by strongly polar three-center interactions.
SmB6 is under focus because it may be the first topological insulator that is also strongly correlated. It is intermediate valent and a gap opens only at low temperatures due to an intricate interplay between correlation effects and hybridization, yet the surface is always conducting. The topological properties of the surface are determined by the bulk so that it is surprising that the bulk crystal-field ground state of Sm3+ was unknown until now despite many attempts by e.g. neutron scattering. Recently, scientists from the Max-Planck Institute for Chemical Physics of Solids and the University of Cologne clarified this issue by applying a new technique, namely core-level non-resonant inelastic x-ray scattering, see Sundermann et al., Phys. Rev. Letter 120, 016402 (2018). The finding that the ground state is the Γ8 quartet and not the Γ7 doublet, contradicts all existing band structure calculations and illustrates in a sobering manner the difficulties in making reliable predictions for the properties of correlated systems.
The rare earth nickelates RNiO3 with the high Ni3+ oxidation state have continued to attract enormous interest due to the famous metal-insulator transition and unusual charge and spin-order phenomena together with the prediction for multi-ferroicity and even superconductivity in thin film hetero-structures. One of the long standing puzzles is why LaNiO3 seems to be the only RNiO3 that stays metallic and paramagnetic down to lowest temperatures. Recently, the team around Alexander Komarek was able to grow centimeter-sized and impurity-free single crystals of LaNiO3 and then to unveil its true electrical and magnetic properties. The result is that the phase diagram of the RNiO3 system has to be redrawn and that theoretical concepts have to be reconsidered about how to explain the properties and the electronic structure of these high oxidation state materials.
The high-temperature superconductivity in iron-pnictides and –chalcogenides is based on the intricate interplay of crystal structure, electronic and magnetic states. The concerted study by scientists from different departments of the Max Planck Institute for Chemical Physics of Solids and from the Leibniz Institute for Solid State and Materials Research found for iron-telluride the remarkable result that hydrostatic pressure leads to the emergence of ferromagnetic order in a peculiar way.
Ultraclean metals show high conductivity with a high number of charge carriers, whereas semiconductors and semimetals with low charge carriers normally show a low conductivity. This scenario in semimetals can be changed if one can protect the carriers from scattering.
In the new iron-based superconductors (FeSC), superconductivity occurs close to an antiferromagnetic phase. This indicates that correlation effects may be important in these materials. Theoretical calculations predict that different from cuprates, besides the on-site Coulomb interaction also the on-site Hund exchange interaction plays an important role in the correlation effects of FeSCs. This would mean that a completely new state of correlated matter has appeared which has been termed “Hund’s metal”.
Graphite is a model system for the study of electrons in the so-called magnetic quantum limit. This quantum limit is attained when the magnetic field is strong enough to confine the charge carriers to their lowest Landau levels. It is expected that the model of the free electron gas fails beyond this limit since here electron interactions may play a significant role in determining the electronic ground state. Indeed, our thermodynamic studies in high magnetic field revealed that even electron-lattice interaction has to be taken into account in addition to electron-electron interaction.
The simple cubic structure of CeB6 does not change while intricate multipolar phenomena are developed upon a non-magnetic La substitution. However, phase diagrams of Ce1-xLaxB6 have been elusive for the last decades. Recently, detailed phase transitions in Ce1-xLaxB6 have been revealed by scientists of MPI-CPfS, TU-Dresden, and I. M. Frantsevich Institute. In addition, a simple feature which is likely to be observed in a multipolar heavy-fermion system has been found.