15,531 research outputs found
Theory of triangular lattice quasi-one-dimensional charge-transfer solids
Recent investigations of the magnetic properties and the discovery of
superconductivity in quasi-one-dimensional triangular lattice organic
charge-transfer solids have indicated the severe limitations of the effective
1/2-filled band Hubbard model for these and related systems. Our computational
studies of these materials within a 1/4-filled band Hubbard model in which the
organic monomer molecules, and not their dimers, constitute the sites of the
Hamiltonian are able to reproduce the experimental results. We ascribe the spin
gap transition in kappa-(BEDT-TTF)_2B(CN)_4 to the formation of a
two-dimensional paired-electron crystal and make the testable prediction that
the spin gap will be accompanied by charge-ordering and period doubling in two
directions. We find enhancement of the long-range component of superconducting
pairing correlations by the Hubbard repulsive interaction for band parameters
corresponding to kappa-(BEDT-TTF)_2CF_3SO_3. The overall results strongly
support a valence bond theory of superconductivity we have proposed recently.Comment: 8 pages, 7 figure
Bond patterns and charge order amplitude in 1/4-filled charge-transfer solids
Metal-insulator transition accompanied by charge-ordering has been widely
investigated in quasi-one-dimensional conductors, including in particular
organic charge-transfer solids. Among such materials the 1/4-filled band
charge-transfer solids are of strong interest, because of the commensurate
nature of the charge-ordering in these systems. The period-four charge-order
pattern ...1100... here is accompanied by two distinct bond distortion
patterns, giving rise to bond-charge-density waves (BCDW) of types 1 and 2.
Using quantum Monte Carlo methods, we determine the phase diagram within the
extended Hubbard Hamiltonian that gives both types 1 and 2 BCDW in the
thermodynamic limit. We further investigate the effect of electron-electron and
electron-phonon interactions on the amount of charge disproportionation. Our
results show that between these two bond patterns, one (BCDW2) in general
coexists with a large magnitude charge order, which is highly sensitive to
electron-phonon interactions, while the other (BCDW1) is characterized by weak
charge order. We discuss the relevance of our work to experiments on several
1/4-filled conductors, focusing in particular on the materials (EDO-TTF)_2X and
(DMEDO-TTF)_2X with large amplitude charge-order.Comment: 7 pages, 8 figure
Dynamics of the giant planets of the solar system in the gaseous proto-planetary disk and relationship to the current orbital architecture
We study the orbital evolution of the 4 giant planets of our solar system in
a gas disk. Our investigation extends the previous works by Masset and
Snellgrove (2001) and Morbidelli and Crida (2007, MC07), which focussed on the
dynamics of the Jupiter-Saturn system. The only systems that we found to reach
a steady state are those in which the planets are locked in a quadruple mean
motion resonance (i.e. each planet is in resonance with its neighbor). In total
we found 6 such configurations. For the gas disk parameters found in MC07,
these configurations are characterized by a negligible migration rate. After
the disappearance of the gas, and in absence of planetesimals, only two of
these six configurations (the least compact ones) are stable for a time of
hundreds of millions of years or more. The others become unstable on a
timescale of a few My. Our preliminary simulations show that, when a
planetesimal disk is added beyond the orbit of the outermost planet, the
planets can evolve from the most stable of these configurations to their
current orbits in a fashion qualitatively similar to that described in Tsiganis
et al. (2005).Comment: The Astronomical Journal (17/07/2007) in pres
The Link between General Relativity and Shape Dynamics
We show that one can construct two equivalent gauge theories from a linking
theory and give a general construction principle for linking theories which we
use to construct a linking theory that proves the equivalence of General
Relativity and Shape Dynamics, a theory with fixed foliation but spatial
conformal invariance. This streamlines the rather complicated construction of
this equivalence performed previously. We use this streamlined argument to
extend the result to General Relativity with asymptotically flat boundary
conditions. The improved understanding of linking theories naturally leads to
the Lagrangian formulation of Shape Dynamics, which allows us to partially
relate the degrees of freedom.Comment: 19 pages, LaTeX, no figure
Phase Diagram of the Attractive Hubbard Model with Inhomogeneous Interactions
The phase diagram of the attractive Hubbard model with spatially
inhomogeneous interactions is obtained using a single site dynamical mean field
theory like approach. The model is characterized by three parameters: the
interaction strength, the active fraction (fraction of sites with the
attractive interaction), and electron filling. The calculations indicate that
in a parameter regime with intermediate values of interaction strength
(compared to the bare bandwidth of the electrons), and intermediate values of
the active fraction, "non-BCS" superconductivity is obtained. The results of
this work are likely to be relevant to many systems with spatially
inhomogeneous superconductivity such as strongly correlated oxides, systems
with negative U centers, and, in future, cold atom optical lattices.Comment: 9 pages, 7 figures, to appear in Physical Review
On a partially reduced phase space quantisation of general relativity conformally coupled to a scalar field
The purpose of this paper is twofold: On the one hand, after a thorough
review of the matter free case, we supplement the derivations in our companion
paper on 'loop quantum gravity without the Hamiltonian constraint' with
calculational details and extend the results to standard model matter, a
cosmological constant, and non-compact spatial slices. On the other hand, we
provide a discussion on the role of observables, focussed on the situation of a
symmetry exchange, which is key to our derivation. Furthermore, we comment on
the relation of our model to reduced phase space quantisations based on
deparametrisation.Comment: 51 pages, 5 figures. v2: Gauge condition used shown to coincide with
CMC gauge. Minor clarifications and correction
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