Vladimir Cvetkovic

Multiband magnetism and superconductivity in Fe-based compounds

Recent discovery of superconductivity in Fe-based layered compounds may have opened a new pathway to the room temperature superconductivity. A model Hamiltonian describing FeAs layers is introduced, highlighting the crucial role of puckering of As atoms in promoting d electron itinerancy and warding off large local-moment magnetism of Fe ions, the main enemy of superconductivity. Quantum many-particle effects in charge, spin and multiband channels are explored and a nesting-induced spin density-wave order is found in the parent compund. We argue that this largely itinerant antiferromagnetism and high Tc itself are essentially tied to the multiband nature of the Fermi surface.

Valley density-wave and multiband superconductivity in iron-based pnictide superconductors

In this paper, we introduce a notable element into the theoretical debate by considering a unified model of spin density-wave, orbital density-wave, structural deformation, and superconductivity in Fe pnictides. The model is simple but it contains the necessary physical features. The essential ingredients are electron and hole pockets valleys of the quasi-two-dimensional 2D multiply connected Fermi surface FS. 5–7 To extract the basic physics we consider spinless electrons first and only a single electron and a single hole band with identical band parameters. We then show that this model can be related to a 2D negative U Hubbard model, the ground state of which is known exactly—it is a superconductor. 8 In real FeAs materials, this fictitious superconductivity translates into a fully gapped valley densitywave VDW, a unified state representing a combination of spin, charge, and orbital density-waves SDW/CDW/ODW at the commensurate wave vector M connecting the two valleys. Next, we introduce two different fictitious “chemical potentials,” eh for the electron and the hole valleys, as measured from the bottom and the top of the bands, respectively—this describes the effect of doping the parent iron-pnictide compounds and corresponds to the external Zeeman splitting in our fictitious attractive Hubbard model. As = e − h increases, so does this Zeeman splitting, and eventually our fictitious superconducting state approaches to and exceeds the “Chandrasekhar-Clogston” limit, giving way to a nonuniform Fulde-Ferrell-Larkin-Ovchinikov FFLO ground state at an incommensurate IC wave vector q, where q is set by kF = k F − k F , and thus by doping x. This

Electronic multicriticality in bilayer graphene

We map out the possible ordered states in bilayer graphene at the neutrality point by extending the previous renormalization group treatment of many-body instabilities to finite temperature, trigonal warping and externally applied perpendicular electric field. We were able to analytically determine all outcomes of the RG flow equations for the 9 four-fermion coupling constants. While the full phase diagram exhibits a rich structure, we confirm that when forward scattering dominates, the only ordering tendency with divergent susceptibility at finite temperature is the nematic. At finite temperature this result is stable with respect to small back and layer imbalance scattering; further increasing their strength leads to the layer antiferromagnet. We also determine conditions for other ordered states to appear and compare our results to the special cases of attractive and repulsive Hubbard models where exact results are available.