19,044 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
The partition semantics of questions, syntactically
Groenendijk and Stokhof (1984, 1996; Groenendijk 1999) provide a logically
attractive theory of the semantics of natural language questions, commonly
referred to as the partition theory. Two central notions in this theory are
entailment between questions and answerhood. For example, the question "Who is
going to the party?" entails the question "Is John going to the party?", and
"John is going to the party" counts as an answer to both. Groenendijk and
Stokhof define these two notions in terms of partitions of a set of possible
worlds.
We provide a syntactic characterization of entailment between questions and
answerhood . We show that answers are, in some sense, exactly those formulas
that are built up from instances of the question. This result lets us compare
the partition theory with other approaches to interrogation -- both linguistic
analyses, such as Hamblin's and Karttunen's semantics, and computational
systems, such as Prolog. Our comparison separates a notion of answerhood into
three aspects: equivalence (when two questions or answers are interchangeable),
atomic answers (what instances of a question count as answers), and compound
answers (how answers compose).Comment: 14 page
Dominant Superconducting Fluctuations in the One-Dimensional Extended Holstein-Extended Hubbard model
The search for realistic one-dimensional (1D) models that exhibit dominant
superconducting (SC) fluctuations effects has a long history. In these 1D
systems, the effects of commensurate band fillings--strongest at
half-filling--and electronic repulsions typically lead to a finite charge gap
and the favoring of insulating density wave ordering over superconductivity.
Accordingly, recent proposals suggesting a gapless metallic state in the
Holstein-Hubbard (HH) model, possibly superconducting, have generated
considerable interest and controversy, with the most recent work demonstrating
that the putative dominant superconducting state likely does not exist. In this
paper we study a model with non-local electron-phonon interactions, in addition
to electron-electron interactions, this model unambiguously possesses dominant
superconducting fluctuations at half filling in a large region of parameter
space. Using both the numerical multi-scale functional renormalization group
for the full model and an analytic conventional renormalization group for a
bosonized version of the model, we demonstrate the existence of dominant
superconducting (SC) fluctuations. These dominant SC fluctuations arise because
the spin-charge coupling at high energy is weakened by the non-local
electron-phonon interaction and the charge gap is destroyed by the resultant
suppression of the Umklapp process. The existence of the dominant SC pairing
instability in this half-filled 1D system suggests that non-local
boson-mediated interactions may be important in the superconductivity observed
in the organic superconductors.Comment: 8 pages, 4 figure
Improved Spin Dynamics Simulations of Magnetic Excitations
Using Suzuki-Trotter decompositions of exponential operators we describe new
algorithms for the numerical integration of the equations of motion for
classical spin systems. These techniques conserve spin length exactly and, in
special cases, also conserve the energy and maintain time reversibility. We
investigate integration schemes of up to eighth order and show that these new
algorithms can be used with much larger time steps than a well established
predictor-corrector method. These methods may lead to a substantial speedup of
spin dynamics simulations, however, the choice of which order method to use is
not always straightforward.Comment: J. Mod. Phys. C (in press
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