Exploration of the generality of the recently discovered reaction whereby certain organic molecules can be inserted between the metallic layers of the superconductors tantalum disulfide and niobium disulfide revealed that a large variety of organic and inorganic molecules can penetrate between the crystalline layers of a number of transition metal dichalcogenides and that the resulting complexes are superconducting if the layered chalcogenide from which they are formed is superconducting. The critical temperatures of the 50 new superconductors we report depend on the nature of the intercalate but are insensitive to a separation of the superconducting planes of up to 57 angstroms.

The ferroelectric degenerate semiconductor Sn 1−δ Te exhibits superconductivity with critical temperatures, Tc, of up to 0.3 K for hole densities of order 10 21 cm −3 . When doped on the tin site with greater than xc = 1.7(3)% indium atoms, however, superconductivity is observed up to 2 K, though the carrier density does not change significantly. We present specific heat data showing that a stronger pairing interaction is present for x > xc than for x < xc. By examining the effect of In dopant atoms on both Tc and the temperature of the ferroelectric structural phase transition, TSP T , we show that phonon modes related to this transition are not responsible for this Tc enhancement, and discuss a plausible candidate based on the unique properties of the indium impurities.

Superconductivity persists in several, layered, transition metal dichalcogenide superconductors when the layers are spread apart to accommodate organic molecules between them. These materials are of interest not only because of their two-dimensional character but also because they may provide a means for examining hypotheses regarding organic molecules and superconductivity.

We report a comparison study of LaOFeP and LaOFeAs, two parent compounds of recently discovered iron-pnictide superconductors, using angle-resolved photoemission spectroscopy. Both systems exhibit some common features that are very different from well-studied cuprates. In addition, important differences have also been observed between these two ferrooxypnictides. For LaOFeP, quantitative agreement can be found between our photoemission data and the LDA band structure calculations, suggesting that a weak coupling approach based on an itinerant ground state may be more appropriate for understanding this new superconducting compound. In contrast, the agreement between LDA calculations and experiments in LaOFeAs is relatively poor, as highlighted by the unexpected Fermi surface topology around (π,π). Further investigations are required for a comprehensive understanding of the electronic structure of LaOFeAs and related compounds. : 74.25.Jb, PACS