Unconventional Fermi surface instabilities in the Kagome Hubbard Model

We investigate the competing Fermi surface instabilities in the Kagome tight-binding model. Specifically, we consider onsite and short-range Hubbard interactions in the vicinity of van Hove filling of the dispersive Kagome bands where the Fermiology promotes the joint effect of enlarged density of states and nesting. The sublattice interference mechanism [Kiesel and Thomale, Phys. Rev. B Rapid Comm., in press.] allows us to explain the intricate interplay between ferromagnetic fluctuations and other ordering tendencies. On the basis of functional renormalization group used to obtain an adequate low-energy theory description, we discover finite angular momentum spin and charge density wave order, a two-fold degenerate d-wave Pomeranchuk instability, and f-wave superconductivity away from van Hove filling. Together, this makes the Kagome Hubbard model the prototypical scenario for several unconventional Fermi surface instabilities.Comment: 4+e pages, 5 figure

Anisotropic chiral d+id superconductivity in NaxCoO2 yH2O

Since its discovery, the superconducting phase in water-intercalated sodium cobaltates NaxCoO2 yH2O (x~0.3, y~1.3) has posed fundamental challenges in terms of experimental investigation and theoretical understanding. By a combined dynamical mean-field and renormalization group approach, we find an anisotropic chiral d+id wave state as a consequence of multi-orbital effects, Fermi surface topology, and magnetic fluctuations. It naturally explains the singlet property and close-to-nodal gap features of the superconducting phase as indicated by experiments.Comment: 4 pages plus references, 5 figure

Renormalization group analysis of competing quantum phases in the J1-J2 Heisenberg model on the kagome lattice

Recent discoveries in neutron scattering experiments for Kapellasite and Herbertsmithite as well as theoretical calculations of possible spin liquid phases have revived interest in magnetic phenomena on the kagome lattice. We study the quantum phase diagram of the S=1/2 Heisenberg kagome model as a function of nearest neighbor coupling J1 and second neighbor coupling J2. Employing the pseudofermion functional renormalization group, we find four types of magnetic quantum order (q=0 order, cuboc order, ferromagnetic order, and Sqrt{3}x\Sqrt{3} order) as well as extended magnetically disordered regions by which we specify the possible parameter regime for Kapellasite. In the disordered regime J2/J1<<1, the flatness of the magnetic susceptibility at the zone boundary which is observed for Herbertsmithite can be reconciled with the presence of small J2>0 coupling. In particular, we analyze the dimer susceptibilities related to different valence bond crystal (VBC) patterns, which are strongly inhomogeneous indicating the rejection of VBC order in the RG flow.Comment: 4+e pages, 3 figures; 2 pages of supplementary materia

Spin-triplet superconductivity in a weak-coupling Hubbard model for the quasi-one-dimensional compound Li0.9_{0.9}Mo6_6O17_{17}

The purple bronze Li$_{0.9}$Mo$_6$O$_{17}$ is of interest due to its quasi-one-dimensional electronic structure and the possible Luttinger liquid behavior resulting from it. For sufficiently low temperatures, it is a superconductor with a pairing symmetry that is still to be determined. To shed light on this issue, we analyze a minimal Hubbard model for this material involving four molybdenum orbitals per unit cell near quarter filling, using asymptotically exact perturbative renormalization group methods. We find that spin triplet odd-parity superconductivity is the dominant instability. Approximate nesting properties of the two quasi-one-dimensional Fermi surfaces enhance certain second-order processes, which play crucial roles in determining the structure of the pairing gap. Notably, we find that the gap has accidental nodes, i.e. it has more sign changes than required by the point-group symmetry.Comment: Update

Mechanism for a Pairing State with Time-Reversal Symmetry Breaking in Iron-Based Superconductors

The multipocket Fermi surfaces of iron-based superconductors promote pairing states with both s_{+-}-wave and d_{x^2-y^2}-wave symmetry. We argue that the competition between these two order parameters could lead to a time-reversal-symmetry breaking state with s+id-pairing symmetry in the iron-based superconductors, and propose serveral scenarios in which this phase may be found. To understand the emergence of such a pairing state on a more rigorous footing, we start from a microscopic 5-orbital description representative for the pnictides. Using a combined approach of functional renormalization group and mean-field analysis, we identify the microscopic parameters of the s+id-pairing state. There, we find the most promising region for s+id-pairing in the electron doped regime with an enhanced pnictogen height