326 research outputs found
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 LiMoO
The purple bronze LiMoO 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
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