13,056 research outputs found
Density Fluctuation Mediated Superconductivity
We conpare predictions of the mean-field theory of supercnductivity for
metallic systems on the border of a density instability for cubic and
tetragonal lattices. The calculations are based on a parametrisation of an
effective interaction arising from the exchange of density fluctuations and
assume that a single band is relevant for superconductivity. The results show
that for comparable model parameters, desnity fluctuation mediated pairing is
more robust in quasi-two dimensions than in three dimensions, and that the
robustness of pairing increases gradually as one goes from a cubic to a more
and more anisotropic tetragonal structure. We also find that the robustness of
density fluctuation mediated pairing can depend sensitively on the incipient
ordering wavevector. We discuss the similarities and differences between the
mean-field theories of superconductivity for density and magnetically mediated
pairing
Next nearest neighbour Ising models on random graphs
This paper develops results for the next nearest neighbour Ising model on
random graphs. Besides being an essential ingredient in classic models for
frustrated systems, second neighbour interactions interactions arise naturally
in several applications such as the colour diversity problem and graphical
games. We demonstrate ensembles of random graphs, including regular
connectivity graphs, that have a periodic variation of free energy, with either
the ratio of nearest to next nearest couplings, or the mean number of nearest
neighbours. When the coupling ratio is integer paramagnetic phases can be found
at zero temperature. This is shown to be related to the locked or unlocked
nature of the interactions. For anti-ferromagnetic couplings, spin glass phases
are demonstrated at low temperature. The interaction structure is formulated as
a factor graph, the solution on a tree is developed. The replica symmetric and
energetic one-step replica symmetry breaking solution is developed using the
cavity method. We calculate within these frameworks the phase diagram and
demonstrate the existence of dynamical transitions at zero temperature for
cases of anti-ferromagnetic coupling on regular and inhomogeneous random
graphs.Comment: 55 pages, 15 figures, version 2 with minor revisions, to be published
J. Stat. Mec
Environment Dependence of Disk Morphology of Spiral Galaxies
We analyze the dependence of disk morphology (arm class, Hubble type, bar
type) of nearby spiral galaxies on the galaxy environment by using local
background density (), projected distance (), and tidal
index () as measures of the environment. There is a strong dependence of
arm class and Hubble type on the galaxy environment, while the bar type
exhibits a weak dependence with a high frequency of SB galaxies in high density
regions. Grand design fractions and early-type fractions increase with
increasing , , and , while fractions of flocculent
spirals and late-type spirals decrease. Multiple-arm and intermediate-type
spirals exhibit nearly constant fractions with weak trends similar to grand
design and early-type spirals. While bar types show only a marginal dependence
on , they show a fairly clear dependence on with a high
frequency of SB galaxies at small . The arm class also exhibits a
stronger correlation with than and , whereas the
Hubble type exhibits similar correlations with and . This
suggests that the arm class is mostly affected by the nearest neighbor while
the Hubble type is affected by the local densities contributed by neighboring
galaxies as well as the nearest neighbor.Comment: 13 pages, 14 figure
Bond-ordered states and -wave pairing of spinless fermions on the honeycomb lattice
Spinless fermions on the honeycomb lattice with repulsive nearest-neighbor
interactions are known to harbour a quantum critical point at half-filling,
with critical behaviour in the Gross-Neveu (chiral Ising) universality class.
The critical interaction strength separates a weak-coupling semimetallic regime
from a commensurate charge-density-wave phase. The phase diagram of this basic
model of correlated fermions on the honeycomb lattice beyond half-filling is,
however, less well established. Here, we perform an analysis of its many-body
instabilities using the functional renormalization group method with a basic
Fermi surface patching scheme, which allows us to treat instabilities in
competing channels on equal footing also away from half-filling. Between
half-filling and the van-Hove filling, the free Fermi surface is hole-like and
we again find a charge-density wave instability to be dominant at large
interactions. Moreover, its characteristics are those of the half-filled case.
Directly at the van-Hove filling the nesting property of the free Fermi surface
stabilizes a dimerized bond-order phase. At lower filling the free Fermi
surface becomes electron-like and a superconducting instability with -wave
symmetry is found to emerge from the interplay of intra-unitcell repulsion and
collective fluctuations in the proximity to the charge-density wave
instability. We estimate the extent of the various phases and extract the
corresponding order parameters from the effective low-energy Hamiltonians.Comment: 11 pages, 11 figure
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