35,111 research outputs found
Simulations of a classical spin system with competing superexchange and double-exchange interactions
Monte-Carlo simulations and ground-state calculations have been used to map
out the phase diagram of a system of classical spins, on a simple cubic
lattice, where nearest-neighbor pairs of spins are coupled via competing
antiferromagnetic superexchange and ferromagnetic double-exchange interactions.
For a certain range of parameters, this model is relevant for some magnetic
materials, such as doped manganites, which exhibit the remarkable colossal
magnetoresistance effect. The phase diagram includes two regions in which the
two sublattice magnetizations differ in magnitude. Spin-dynamics simulations
have been used to compute the time- and space-displaced spin-spin correlation
functions, and their Fourier transforms, which yield the dynamic structure
factor for this system. Effects of the double-exchange
interaction on the dispersion curves are shown.Comment: Latex, 3 pages, 3 figure
Global dispersive solutions for the Gross-Pitaevskii equation in two and three dimensions
We study asymptotic behaviour at time infinity of solutions close to the
non-zero constant equilibrium for the Gross-Pitaevskii equation in two and
three spatial dimensions. We construct a class of global solutions with
prescribed dispersive asymptotic behavior, which is given in terms of the
linearized evolution
Constraints on the long-period moment-dip tradeoff for the Tohoku earthquake
Since the work of Kanamori and Given (1981), it has been recognized that shallow, pure dip-slip earthquakes excite long-period surface waves such that it is difficult to independently constrain the moment (M_0) and the dip (ÎŽ) of the source mechanism, with only the product M_0 sin(2ÎŽ) being well constrained. Because of this, it is often assumed that the primary discrepancies between the moments of shallow, thrust earthquakes are due to this moment-dip tradeoff. In this work, we quantify how severe this moment-dip tradeoff is depending on the depth of the earthquake, the station distribution, the closeness of the mechanism to pure dip-slip, and the quality of the data. We find that both long-period Rayleigh and Love wave modes have moment-dip resolving power even for shallow events, especially when stations are close to certain azimuths with respect to mechanism strike and when source depth is well determined. We apply these results to USGS W phase inversions of the recent M9.0 Tohoku, Japan earthquake and estimate the likely uncertainties in dip and moment associated with the moment- dip tradeoff. After discussing some of the important sources of moment and dip error, we suggest two methods for potentially improving this uncertainty
Phonon-mediated tuning of instabilities in the Hubbard model at half-filling
We obtain the phase diagram of the half-filled two-dimensional Hubbard model
on a square lattice in the presence of Einstein phonons. We find that the
interplay between the instantaneous electron-electron repulsion and
electron-phonon interaction leads to new phases. In particular, a
d-wave superconducting phase emerges when both anisotropic phonons
and repulsive Hubbard interaction are present. For large electron-phonon
couplings, charge-density-wave and s-wave superconducting regions also appear
in the phase diagram, and the widths of these regions are strongly dependent on
the phonon frequency, indicating that retardation effects play an important
role. Since at half-filling the Fermi surface is nested, spin-density-wave is
recovered when the repulsive interaction dominates. We employ a functional
multiscale renormalization-group method that includes both electron-electron
and electron-phonon interactions, and take retardation effects fully into
account.Comment: 8 pages, 5 figure
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