11,198 research outputs found
Superconductivity and incommensurate spin fluctuations in a generalized t-J model for the cuprates
We consider the slave-fermion Schwinger-boson decomposition of an effective
model obtained through a systematic low-energy reduction of the three-band
Hubbard Hamiltonian. The model includes a three-site term t'' similar to that
obtained in the large-U limit of the Hubbard model but of opposite sign for
realistic or large O-O hopping. For parameters close to the most realistic ones
for the cuprates, the mean-field solution exhibits d+s superconductivity
(predominantly d_{x^2-y^2}) with a dependence on doping x very similar to the
experimentally observed. We also obtained incommensurate peaks at wave vectors
near in the spin structure factor, which also agree with
experiment.Comment: 9 pages, latex, 2 figures, to appear in Europhys. Let
Parallel structurally-symmetric sparse matrix-vector products on multi-core processors
We consider the problem of developing an efficient multi-threaded
implementation of the matrix-vector multiplication algorithm for sparse
matrices with structural symmetry. Matrices are stored using the compressed
sparse row-column format (CSRC), designed for profiting from the symmetric
non-zero pattern observed in global finite element matrices. Unlike classical
compressed storage formats, performing the sparse matrix-vector product using
the CSRC requires thread-safe access to the destination vector. To avoid race
conditions, we have implemented two partitioning strategies. In the first one,
each thread allocates an array for storing its contributions, which are later
combined in an accumulation step. We analyze how to perform this accumulation
in four different ways. The second strategy employs a coloring algorithm for
grouping rows that can be concurrently processed by threads. Our results
indicate that, although incurring an increase in the working set size, the
former approach leads to the best performance improvements for most matrices.Comment: 17 pages, 17 figures, reviewed related work section, fixed typo
Tipping time of a quantum rod
The behaviour of a quantum rod, pivoted at its lower end on an impenetrable
floor and restricted to moving in the vertical plane under the gravitational
potential is studied analytically under the approximation that the rod is
initially localised to a small-enough neighbourhood around the point of
classical unstable equilibrium. It is shown that the rod evolves out of this
neighbourhood. The time required for this to happen, i.e., the tipping time is
calculated using the semi-classical path integral. It is shown that equilibrium
is recovered in the classical limit, and that our calculations are consistent
with the uncertainty principle.Comment: 10 pages, 1 figure, To appear in Euro. J. Phy
Hamiltonian Reduction and the Construction of q-Deformed Extensions of the Virasoro Algebra
In this paper we employ the construction of Dirac bracket for the remaining
current of deformed Kac-Moody algebra when constraints similar to
those connecting the -WZW model and the Liouville theory are imposed and
show that it satisfy the q-Virasoro algebra proposed by Frenkel and
Reshetikhin. The crucial assumption considered in our calculation is the
existence of a classical Poisson bracket algebra induced, in a consistent
manner by the correspondence principle, mapping the quantum generators into
commuting objects of classical nature preserving their algebra.Comment: 6 pages, late
Statistics of finite-time Lyapunov exponents in the Ulam map
The statistical properties of finite-time Lyapunov exponents at the Ulam
point of the logistic map are investigated. The exact analytical expression for
the autocorrelation function of one-step Lyapunov exponents is obtained,
allowing the calculation of the variance of exponents computed over time
intervals of length . The variance anomalously decays as . The
probability density of finite-time exponents noticeably deviates from the
Gaussian shape, decaying with exponential tails and presenting spikes
that narrow and accumulate close to the mean value with increasing . The
asymptotic expression for this probability distribution function is derived. It
provides an adequate smooth approximation to describe numerical histograms
built for not too small , where the finiteness of bin size trimmes the sharp
peaks.Comment: 6 pages, 4 figures, to appear in Phys. Rev.
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