Sr2_{2}VO3_{3}FeAs as Compared to Other Fe-based Superconductors

One of the most popular scenarios for the superconductivity in Fe-based superconductors (FeBSC) posits that the bosons responsible for electronic pairing are spin-fluctuations with a wave vector spanning the hole Fermi surfaces (FSs) near $\Gamma$ and the electron FSs near M points. So far all FeBSC for which neutron data are available do demonstrate such excitations, and the band structure calculations so far were finding quasi-nested FSs in all FeBSC, providing for a peak in the spin susceptibility at the desired wave vectors. However, the newest addition to the family, Sr$_{2}$VO$_{3}$FeAs, has been calculated to have a very complex FS with no visible quasi-nesting features. It was argued therefore that this material does not fall under the existing paradigm and calls for revisiting our current ideas about what is the likely cause of superconductivity in FeBSC. In this paper, I show that the visible complexity of the FS is entirely due to the V-derived electronic states. Assuming that superconductivity in Sr$_{2}$VO$_{3}$FeAs, as in the other FeBSC, originates in the FeAs layers, and the superconducting electrons are sensitive to the susceptibility of the FeAs electronic subsystem, I recalculate the bare susceptibility, weighting the electronic states with their Fe character, and obtain a susceptibility that fully supports the existing quasi-nesting model.Comment: Journal reference adde

Suppression and restoration of superconductivity in PrBa2Cu3O7

I review the principal experimental findings about superconductivity, suppression thereof, and related properties of RE_{1-x}Pr_xBa_2Cu3O7 in a decision-tree manner, eliminating the models substantially incompatible with established experimental facts, eventually focussing on the pf\sigma hybridization models and listing on a verbal level the interpretations of the existing experiments as they emerge from such models.Comment: Talk given at the HTS-99 conference, Miami, 1999. Requires aipproc.sty, epsfig. V.3, correcte

Unresolved problems in superconductivity of CaC6

We discuss the current status of the theory of the "high-temperature" superconductivity in intercalated graphites YbC6 and CaC6. We emphasize that while the general picture of conventional, phonon-driven superconductivity has already emerged and is generally accepted, there are still interesting problems with this picture, such as weak-coupling regime inferred from specific heat suggesting coupling exclusively with high-energy carbon phonons coming in direct contradiction with the isotope effect measurements suggesting coupling exclusively with the low-energy intercalant modes. At the same time, the first principle calculations, while explaining Tc, contradict both of the experiments above by predicting equal coupling with both groups of phonons.Comment: Contribution to the Proceedings of the M2S Conference in Dresden, 200

Rational Parking Functions and LLT Polynomials

We prove that the combinatorial side of the "Rational Shuffle Conjecture" provides a Schur-positive symmetric polynomial. Furthermore, we prove that the contribution of a given rational Dyck path can be computed as a certain skew LLT polynomial, thus generalizing the result of Haglund, Haiman, Loehr, Remmel and Ulyanov. The corresponding skew diagram is described explicitly in terms of a certain (m,n)-core.Comment: 14 pages, 8 figure