2,946 research outputs found
Competing electronic orders on Kagome lattices at van Hove filling
The electronic orders in Hubbard models on a Kagome lattice at van Hove
filling are of intense current interest and debate. We study this issue using
the singular-mode functional renormalization group theory. We discover a rich
variety of electronic instabilities under short range interactions. With
increasing on-site repulsion , the system develops successively
ferromagnetism, intra unit-cell antiferromagnetism, and charge bond order. With
nearest-neighbor Coulomb interaction alone (U=0), the system develops
intra-unit-cell charge density wave order for small , s-wave
superconductivity for moderate , and the charge density wave order appears
again for even larger . With both and , we also find spin bond order
and chiral superconductivity in some particular
regimes of the phase diagram. We find that the s-wave superconductivity is a
result of charge density wave fluctuations and the squared logarithmic
divergence in the pairing susceptibility. On the other hand, the d-wave
superconductivity follows from bond order fluctuations that avoid the matrix
element effect. The phase diagram is vastly different from that in honeycomb
lattices because of the geometrical frustration in the Kagome lattice.Comment: 8 pages with 9 color figure
W=0 Pairing in Carbon Nanotubes away from Half Filling
We use the Hubbard Hamiltonian on the honeycomb lattice to represent the
valence bands of carbon single-wall nanotubes. A detailed symmetry
analysis shows that the model allows W=0 pairs which we define as two-body
singlet eigenstates of with vanishing on-site repulsion. By means of a
non-perturbative canonical transformation we calculate the effective
interaction between the electrons of a W=0 pair added to the interacting ground
state. We show that the dressed W=0 pair is a bound state for resonable
parameter values away from half filling. Exact diagonalization results for the
(1,1) nanotube confirm the expectations. For nanotubes of length ,
the binding energy of the pair depends strongly on the filling and decreases
towards a small but nonzero value as . We observe the existence
of an optimal doping when the number of electrons per C atom is in the range
1.21.3, and the binding energy is of the order of 0.1 1 meV.Comment: 16 pages, 6 figure
Chiral d-wave superconductivity in doped graphene
A highly unconventional superconducting state with a spin-singlet
-wave, or chiral d-wave, symmetry has recently been
proposed to emerge from electron-electron interactions in doped graphene.
Especially graphene doped to the van Hove singularity at 1/4 doping, where the
density of states diverges, has been argued to likely be a chiral d-wave
superconductor. In this review we summarize the currently mounting theoretical
evidence for the existence of a chiral d-wave superconducting state in
graphene, obtained with methods ranging from mean-field studies of effective
Hamiltonians to angle-resolved renormalization group calculations. We further
discuss multiple distinctive properties of the chiral d-wave superconducting
state in graphene, as well as its stability in the presence of disorder. We
also review means of enhancing the chiral d-wave state using proximity-induced
superconductivity. The appearance of chiral d-wave superconductivity is
intimately linked to the hexagonal crystal lattice and we also offer a brief
overview of other materials which have also been proposed to be chiral d-wave
superconductors.Comment: 51 pages, 8 figures. Invited topical review in J. Phys.:Condens.
Matte
On-Site Repulsion as the Source of Pairing in Carbon Nanotubes and Intercalated Graphite
We show that different non-conventional superconductors have one fundamental
feature in common: pair eigenstates of the Hamiltonian are repulsion-free, the
W=0 pairs. In extended Hubbard models, pairing can occur for resonable
parameter values. For nanotubes the binding energy of the pair depends
strongly on the filling and decreases towards a reduced but nonzero value for
the graphite sheet .Comment: 4 pages, 2 figure
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