Jürgen Köhler

Electronic structure and lattice dynamics of NaFeAs

The similarity of the electronic structures of NaFeAs and other Fe pnictides has been demonstrated on the basis of first-principle calculations. The global double-degeneracy of electronic bands along X-M and R-A direction indicates the instability of Fe pnictides and is explained on the basis of a tight-binding model. The de Haas-van Alphen parameters for the Fermi surface (FS) of NaFeAs have been calculated. A

Strong One‐Dimensional Antiferromagnetic Interactions of Silver(II) Ions in Silver Sulfate

\mathbf{Q}_{M}=(1/2,1/2,0)

The coherent state and its relation to the flat/steep band model

spin density wave (SDW) instead of a charge density wave (CDW) ground state is predicted based on the calculated generalized susceptibility

Lone Pairs, Bipolarons and Superconductivity in Tellurium

\chi(\mathbf{q})

Transition-metal anions in solids and their implications on bonding.

and a criterion derived from a restricted Hatree-Fock model. The strongest electron-phonon (e-p) coupling has been found to involve only As, Na z-direction vibration with linear-response calculations. A possible enhancement mechanism for e-p coupling due to correlation is suggested.

Late transition metal anions acting as p-metal elements

The search for new compounds that contain elements in unusual oxidation states is worthwhile, since these compounds often exhibit novel and unconventional chemical and physical properties, for example, those observed in the superconducting Cu oxides. The dominant oxidation state of Ag is + 1 and only a few compounds with Ag in higher oxidation states have been synthesized to date. The situation is different for the other coinage metals, in that Cu, the lighter homologue of Ag, can easily adopt the oxidation state + 2, whereas the heavier homologue Au is mainly found with the oxidation state + 3. Ag forms stable complexes with a variety of nitrogen donor ligands, such as pyridine or pyrazine (pyz). Other examples of Ag compounds are the binary fluoride AgF2 and its complex salts, for example, Cs2AgF4, [4] which has recently attracted great interest since it orders ferromagnetically at low temperatures. Among fluorides, even higher oxidation states of Ag are known, for example, Ag and Ag in KAgF4 [6] and Cs2AgF6, [7] respectively. Oxides containing Ag in an oxidation state higher than + 1 are very rare. The binary compound AgO is not a Ag compound, but the diamagnetic mixed-valent compound AgAgO2. [8] Ag3O4 is also a mixed-valent compound as indicated by the formula AgAg2O4, [9] whereas Ag2O3 contains only Ag ions. Malinowski et al. recently reported the synthesis of black AgSO4, which has been obtained by a methathetic reaction of Ag(SbF6)2 + K2SO4 in anhydrous HF at around 80 8C or by the reaction of AgF2 with H2SO4 at 35 8C. According to thermoanalytical studies, the product is metastable and decomposes above 120 8C with evolution of O2. The triclinic crystal structure of AgSO4 contains planar rectangular AgO4 units that are connected by SO4 tetrahedra to form a three-dimensional network (Figure 1). The Ag–O distances within the AgO4 units range from 2.09 to 2.20 , as expected for a four-coordinate Ag ion, thus suggesting that AgSO4 is indeed a complex Ag II oxide. The two Ag(1)O4 and Ag(2)O4 units are significantly elongated along the twofold axes with O–O distances of 2.62 and 2.70 for the short edges and 3.30 and 3.34 for the long edges, respectively. The structural chemistry of the 4d ion Ag (4d, S = 1/2) differs considerably from that of its 3d and 5d analogues Cu (3d, S = 1/2) and Au (5d, S = 1/2). This difference is most obvious when the sulfates are compared. The Ag ions in AgSO4 display a rectangular coordination of O atoms, whereas the smaller Cu ions in CuSO4 exhibit a distorted octahedral coordination; it is clear that the Jahn– Teller effect is stronger for Ag than for Cu. In the structure of AuSO4 , diamagnetic Au–Au dumbbells with short Au–Au distances of 249 pm are found, thus reflecting the increasing tendency for the occurrence of metal–metal bonding down a group of transition metals. AgSO4 is also interesting because of its distinct physical properties. IR spectra show that AgSO4 has a band gap of 0.18 eV, and spin-polarized DFT calculations result in nearly the same value for one kind of spin. In contrast, the band gap of CuSO4 calculated in a comparable way is more than ten times larger, namely 2.3 eV. Measurements of the temperature-dependent magnetic susceptibility indicate that the Ag ions (4d, S = =2) in AgSO4 are strongly antiferromagnetically coupled to the chains within the 3D structure. A fit Figure 1. Perspective view of the crystal structure of AgSO4. Ag, S, and O atoms are represented as red, yellow, and gray spheres, respectively. The coordination spheres around Ag and S are graphically emphasized. The path for the 1D antiferromagnetic interaction (AF) in AgSO4 along one distinct diagonal of the triclinic unit cell is indicated by dotted lines.

On the Origin of a Band Gap in Compounds of Diamond-like Structures

Abstract In this report, we give a concise explanation to the connection between the BCS-type (Bardeen, Cooper, Schrieffer) superconducting state and the generalized coherent state in the framework of the flat/steep band model. It is also shown that the findings in the newly discovered Fe-based superconductors support the flat/steep band model.

Unusual Lone Pairs in Tellurium and Their Relevance for Superconductivity

We have characterized a new type of bipolaron in the ambient pressure modification of tellurium, Te-I, which originates from Coulomb interactions instead of electron-phonon coupling as for the conventional Anderson bipolaron. The studies at the Hartree-Fock level and the constrained LDA calculations give an estimate (∼) for the stability of the bipolaron. A 3D-tight binding model has been proposed to explain the electronic structure obtained from first-principle calculations. The possible relevance of such bipolarons with superconductivity in the high pressure phase Te-II is discussed.