5,954 research outputs found
Singlet and triplet bipolarons on the triangular lattice
We study the Coulomb-Fr\"ohlich model on a triangular lattice, looking in
particular at states with angular momentum. We examine a simplified model of
crab bipolarons with angular momentum by projecting onto the low energy
subspace of the Coulomb-Fr\"ohlich model with large phonon frequency. Such a
projection is consistent with large long-range electron-phonon coupling and
large repulsive Hubbard . Significant differences are found between the band
structure of singlet and triplet states: The triplet state (which has a flat
band) is found to be significantly heavier than the singlet state (which has
mass similar to the polaron). We test whether the heavier triplet states
persist to lower electron-phonon coupling using continuous time quantum Monte
Carlo (QMC) simulation. The triplet state is both heavier and larger,
demonstrating that the heavier mass is due to quantum interference effects on
the motion. We also find that retardation effects reduce the differences
between singlet and triplet states, since they reintroduce second order terms
in the hopping into the inverse effective mass.Comment: Proceedings of SNS 200
Recommended from our members
Between the waves: currents in contemporary feminist thought
With a continuing focus on liberal feminism, Marxist feminism and essentialism, one would be forgiven for
thinking that feminist theory is unable to break free from the āsecond waveā. This is not the case. This article reviews three books which take on these feminist issues and offer new readings on the questions at the heart of feminism. Each provides clear links to feminism of the past but also connects to present debate and makes
suggestions for future directions for feminism. There is plenty of literature which bemoans the end of feminism and some which triumphantly hails our era as post-feminist: no longer in need of feminist theory. Contrary to such claims, each book tackles the problem of womenās oppression from a different perspective, each presents different solutions and in so doing they demonstrate that feminism is alive and well
Polarons in highly doped atomically thin graphitic materials
Polaron spectral functions are computed for highly doped
graphene-on-substrate and other atomically thin graphitic systems using the
diagrammatic Monte Carlo technique. The specific aim is to investigate the
effects of interaction on spectral functions when the symmetry between
sub-lattices of a honeycomb lattice has been broken by the substrate or
ionicity, inducing a band gap. Introduction of electron-phonon coupling leads
to several polaronic features, such as band-flattening and changes in particle
lifetimes. At the K point, differences between energies on each sub-lattice
increase with electron-phonon coupling, indicating an augmented transport gap,
while the spectral gap decreases slightly. Effects of phonon dispersion and
long-range interactions are investigated, and found to lead to only
quantitative changes in spectra
Electron and phonon dispersions of the two dimensional Holstein model: Effects of vertex and non-local corrections
I apply the newly developed dynamical cluster approximation (DCA) to the
calculation of the electron and phonon dispersions in the two dimensional
Holstein model. In contrast to previous work, the DCA enables the effects of
spatial fluctuations (non-local corrections) to be examined. Approximations
neglecting and incorporating lowest-order vertex corrections are investigated.
I calculate the phonon density of states, the renormalised phonon dispersion,
the electron dispersion and electron spectral functions. I demonstrate how
vertex corrections stabilise the solution, stopping a catastrophic softening of
the phonon mode. A kink in the electron dispersion is found in the
normal state along the symmetry direction in both the vertex-
and non-vertex-corrected theories for low phonon frequencies, corresponding
directly to the renormalised phonon frequency at the point. This kink
is accompanied by a sudden drop in the quasi-particle lifetime. Vertex and
non-local corrections enhance the effects at large bare phonon frequencies.Comment: I am posting reprints of the final submitted versions of previous
articles to improve access. Here ARPES "kinks" are discussed. Article was
published in 2003. 17 pages, 9 figure
Breakdown of Migdal--Eliashberg theory via catastrophic vertex divergence at low phonon frequency
We investigate the applicability of Migdal--Eliashberg (ME) theory by
revisiting Migdal's analysis within the dynamical mean-field theory framework.
First, we compute spectral functions, the quasi-particle weight, the self
energy, renormalised phonon frequency and resistivity curves of the half-filled
Holstein model. We demonstrate how ME theory has a phase-transition-like
instability at intermediate coupling, and how the Engelsberg--Schrieffer (ES)
picture is complicated by low-energy excitations from higher order diagrams
(demonstrating that ES theory is a very weak coupling approach). Through
consideration of the lowest-order vertex correction, we analyse the
applicability of ME theory close to this transition. We find a breakdown of the
theory in the intermediate coupling adiabatic limit due to a divergence in the
vertex function. The region of applicability is mapped out, and it is found
that ME theory is only reliable in the weak coupling adiabatic limit, raising
questions about the accuracy of recent analyses of cuprate superconductors
which do not include vertex corrections.Comment: 19 pages, 10 figures, accepted for publication in Journal of Low
Temperature Physic
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