6,078 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
Radion Cosmology in Theories with Universal Extra Dimensions
We discuss cosmology of models with universal extra dimensions, where the
Standard Model degrees of freedom live in a dimensional brane, with
compact and small extra spatial dimensions. In these models, the simplest way
to obtain the conventional 4-dimensional Planck scale starting with a low
string scale is to have also some larger extra dimensions, where only gravity
propagates. In such theories, dimensional reduction generically leads to at
least two radion fields, one associated with the total volume of the extra
spatial dimensions, and the other with the ratio of the sizes of small and
large extra dimensions. In this paper, we discuss the impact of the radion
fields on cosmology. We emphasize various aspects of radion physics such as
radion coupling to the Standard Model fields, bare and dressed radion masses
during inflation, dynamical stabilization of radions during and after
inflation, radion decay life time and its late dominance in thermal history of
the Universe as well as its quantum fluctuations during inflation. We argue
that models where the radion plays the role of an inflaton or the inflaton is a
brane scalar field, run into problems. We then present a successful inflation
model with bulk scalar fields that seems to have all the desired properties. We
also briefly discuss the possibility of radion as a cold dark matter candidate.Comment: 37 pages + 3 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
Pinned Low Energy Electronic Excitation in Metal Exchanged Vanadium Oxide Nanoscrolls
We measured the optical properties of mixed valent vanadium oxide nanoscrolls
and their metal exchanged derivatives in order to investigate the charge
dynamics in these compounds. In contrast to the prediction of a metallic state
for the metal exchanged derivatives within a rigid band model, we find that the
injected charges in Mn exchanged vanadium oxide nanoscrolls are pinned.
A low-energy electronic excitation associated with the pinned carriers appears
in the far infrared and persists at low temperature, suggesting that the
nanoscrolls are weak metals in their bulk form, dominated by inhomogeneous
charge disproportionation and Madelung energy effects.Comment: 4 figure
Towards Scalable Visual Exploration of Very Large RDF Graphs
In this paper, we outline our work on developing a disk-based infrastructure
for efficient visualization and graph exploration operations over very large
graphs. The proposed platform, called graphVizdb, is based on a novel technique
for indexing and storing the graph. Particularly, the graph layout is indexed
with a spatial data structure, i.e., an R-tree, and stored in a database. In
runtime, user operations are translated into efficient spatial operations
(i.e., window queries) in the backend.Comment: 12th Extended Semantic Web Conference (ESWC 2015
- …