1,849 research outputs found
Unconventional metallic conduction in two-dimensional Hubbard-Wigner lattices
The interplay between long-range and local Coulomb repulsion in strongly
interacting electron systems is explored through a two-dimensional
Hubbard-Wigner model. An unconventional metallic state is found in which
collective low-energy excitations characteristic of the Wigner crystal induce a
flow of electrical current despite the absence of one-electron spectral weight
at the Fermi surface. Photoemission experiments on certain quarter-filled
layered molecular crystals should observe a gap in the excitation spectrum
whereas optical spectroscopy should find a finite Drude weight indicating
metallic behavior.Comment: 10 pages, accepted for publication in PR
Protosymbols that integrate recognition and response
We explore two controversial hypotheses through robotic implementation: (1) Processes involved in recognition and response are tightly coupled both in their operation and epigenesis; and (2) processes involved in symbol emergence should respect the integrity of recognition and response while exploiting the periodicity of biological motion. To that end, this paper proposes a method of recognizing and generating motion patterns based on nonlinear principal component neural networks that are constrained to model both periodic and transitional movements. The method is evaluated by an examination of its ability to segment and generalize different kinds of soccer playing activity during a RoboCup match
Superconducting Pairing Symmetries in Anisotropic Triangular Quantum Antiferromagnets
Motivated by the recent discovery of a low temperature spin liquid phase in
layered organic compound -(ET)Cu(CN) which becomes a
superconductor under pressure, we examine the phase transition of Mott
insulating and superconducting (SC) states in a Hubbard-Heisenberg model on an
anisotropic triangular lattice. We use a renormalized mean field theory to
study the Gutzwiller projected BCS wavefucntions. The half filled electron
system is a Mott insulator at large on-site repulsion , and is a
superconductor at a moderate . The symmetry of the SC state depends on the
anisotropy, and is gapful with symmetry near the
isotropic limit and is gapless with symmetry at small anisotropy
ratio.Comment: 6 pages, 5 figure
NMR evidence for very slow carrier density fluctuations in the organic metal (TMTSF)ClO
We have investigated the origin of the large increase in spin-echo decay
rates for the Se nuclear spins at temperatures near to in the
organic superconductor (TMTSF)ClO. The measured angular dependence of
demonstrates that the source of the spin-echo decays lies with
carrier density fluctuations rather than fluctuations in TMTSF molecular
orientation. The very long time scales are directly associated with the
dynamics of the anion ordering occurring at , and the inhomogeneously
broadened spectra at lower temperatures result from finite domain sizes. Our
results are similar to observations of line-broadening effects associated with
charge-ordering transitions in quasi-two dimensional organic conductors.Comment: 5 pages, 4 figure
Electronic and magnetic properties of the ionic Hubbard model on the striped triangular lattice at 3/4 filling
We report a detailed study of a model Hamiltonian which exhibits a rich
interplay of geometrical spin frustration, strong electronic correlations, and
charge ordering. The character of the insulating phase depends on the magnitude
of Delta/|t| and on the sign of t. We find a Mott insulator for Delta >> U >>
|t|; a charge transfer insulator for U >> \Delta >> |t|; and a correlated
covalent insulator for U >> \Delta ~ |t|. The charge transfer insulating state
is investigated using a strong coupling expansion. The frustration of the
triangular lattice can lead to antiferromagnetism or ferromagnetism depending
on the sign of the hopping matrix element, t. We identify the "ring" exchange
process around a triangular plaquette which determines the sign of the magnetic
interactions. Exact diagonalization calculations are performed on the model for
a wide range of parameters and compared to the strong coupling expansion. The
regime U >> \Delta ~ |t| and t<0 is relevant to Na05CoO2. The calculated
optical conductivity and the spectral density are discussed in the light of
recent experiments on Na05CoO2.Comment: 15 pages, 15 figure
Evidence for the formation of a Mott state in potassium-intercalated pentacene
We investigate electronic transport through pentacene thin-films intercalated
with potassium. From temperature-dependent conductivity measurements we find
that potassium-intercalated pentacene shows metallic behavior in a broad range
of potassium concentrations. Surprisingly, the conductivity exhibits a
re-entrance into an insulating state when the potassium concentration is
increased past one atom per molecule. We analyze our observations theoretically
by means of electronic structure calculations, and we conclude that the
phenomenon originates from a Mott metal-insulator transition, driven by
electron-electron interactions.Comment: 8 pages, 6 figure
Non-local correlations in metals close to a charge order insulator transition
The charge ordering transition induced by the nearest-neighbor Coulomb
repulsion, V, in the 1/4-filled extended Hubbard model is investigated using
Cellular Dynamical Mean-Field Theory. We find a transition to a strongly
renormalized charge ordered Fermi liquid at VCO and a metal-to-insulator
transition at VMI>VCO. Short range antiferromagnetism occurs concomitanly with
the CO transition. Approaching the charge ordered insulator the Fermi surface
deforms and the scattering rate of electrons develops momentum dependence on
the Fermi surface.Comment: 4 pages, 6 Figures, accepted for publication in Physical Review
Letter
Phase diagram of the one-dimensional Hubbard model with next-nearest-neighbor hopping
We study the one-dimensional Hubbard model with nearest-neighbor and
next-nearest-neighbor hopping integrals by using the density-matrix
renormalization group (DMRG) method and Hartree-Fock approximation. Based on
the calculated results for the spin gap, total-spin quantum number, and
Tomonaga-Luttinger-liquid parameter, we determine the ground-state phase
diagram of the model in the entire filling and wide parameter region. We show
that, in contrast to the weak-coupling regime where a spin-gapped liquid phase
is predicted in the region with four Fermi points, the spin gap vanishes in a
substantial region in the strong-coupling regime. It is remarkable that a large
variety of phases, such as the paramagnetic metallic phase, spin-gapped liquid
phase, singlet and triplet superconducting phases, and fully polarized
ferromagnetic phase, appear in such a simple model in the strong-coupling
regime.Comment: 11 pages, 8 figure
Gapped optical excitations from gapless phases: imperfect nesting in unconventional density waves
We consider the effect of imperfect nesting in quasi-one-dimensional
unconventional density waves in the case, when the imperfect nesting and the
gap depends on the same wavevector component.
The phase diagram is very similar to that in a conventional density wave. The
density of states is highly asymmetric with respect to the Fermi energy.
The optical conductivity at T=0 remains unchanged for small deviations from
perfect nesting. For higher imperfect nesting parameter, an optical gap opens,
and considerable amount of spectral weight is transferred to higher
frequencies. This makes the optical response of our system very similar to that
of a conventional density wave. Qualitatively similar results are expected in
d-density waves.Comment: 8 pages, 7 figure
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