14,759 research outputs found
Current reversal and exclusion processes with history-dependent random walks
A class of exclusion processes in which particles perform history-dependent
random walks is introduced, stimulated by dynamic phenomena in some biological
and artificial systems. The particles locally interact with the underlying
substrate by breaking and reforming lattice bonds. We determine the
steady-state current on a ring, and find current-reversal as a function of
particle density. This phenomenon is attributed to the non-local interaction
between the walkers through their trails, which originates from strong
correlations between the dynamics of the particles and the lattice. We
rationalize our findings within an effective description in terms of
quasi-particles which we call front barriers. Our analytical results are
complemented by stochastic simulations.Comment: 5 pages, 6 figure
New spectral classification technique for X-ray sources: quantile analysis
We present a new technique called "quantile analysis" to classify spectral
properties of X-ray sources with limited statistics. The quantile analysis is
superior to the conventional approaches such as X-ray hardness ratio or X-ray
color analysis to study relatively faint sources or to investigate a certain
phase or state of a source in detail, where poor statistics does not allow
spectral fitting using a model. Instead of working with predetermined energy
bands, we determine the energy values that divide the detected photons into
predetermined fractions of the total counts such as median (50%), tercile (33%
& 67%), and quartile (25% & 75%). We use these quantiles as an indicator of the
X-ray hardness or color of the source. We show that the median is an improved
substitute for the conventional X-ray hardness ratio. The median and other
quantiles form a phase space, similar to the conventional X-ray color-color
diagrams. The quantile-based phase space is more evenly sensitive over various
spectral shapes than the conventional color-color diagrams, and it is naturally
arranged to properly represent the statistical similarity of various spectral
shapes. We demonstrate the new technique in the 0.3-8 keV energy range using
Chandra ACIS-S detector response function and a typical aperture photometry
involving background subtraction. The technique can be applied in any energy
band, provided the energy distribution of photons can be obtained.Comment: 11 pages, 9 figures, accepted for publication in Ap
Ferrimagnetic mixed-spin ladders in weak and strong coupling limits
We study two similar spin ladder systems with the ferromagnetic leg coupling.
First model includes two sorts of spins, s= 1/2 and s= 1, and the second model
comprises only s=1/2 legs coupled by a "triangular" rung exchange. The
antiferromagnetic (AF) rung coupling destroys the long-range order and
eventually makes the systems equivalent to the AF s=1/2 Heisenberg chain. We
investigate the situation by different methods in weak and strong rung coupling
limits. Particularly we compare the spin-wave theory and the bosonization
method in the weak coupling regime of the second model. We analyze the spectra
and correlations, and discuss the order parameter of these ladder systems.Comment: 12 pages, 4 figure
On the metal-insulator transition in the two-chain model of correlated fermions
The doping-induced metal-insulator transition in two-chain systems of
correlated fermions is studied using a solvable limit of the t-J model and the
fact that various strong- and weak-coupling limits of the two-chain model are
in the same phase, i.e. have the same low-energy properties. It is shown that
the Luttinger-liquid parameter K_\rho takes the universal value unity as the
insulating state (half-filling) is approached, implying dominant d-type
superconducting fluctuations, independently of the interaction strength. The
crossover to insulating behavior of correlations as the transition is
approached is discussed.Comment: 7 pages, 1 figur
Surveys of Galaxy Clusters with the Sunyaev Zel'dovich Effect
We have created mock Sunyaev-Zel'dovich effect (SZE) surveys of galaxy
clusters using high resolution N-body simulations. To the pure surveys we add
`noise' contributions appropriate to instrument and primary CMB anisotropies.
Applying various cluster finding strategies to these mock surveys we generate
catalogues which can be compared to the known positions and masses of the
clusters in the simulations. We thus show that the completeness and efficiency
that can be achieved depend strongly on the frequency coverage, noise and beam
characteristics of the instruments, as well as on the candidate threshold. We
study the effects of matched filtering techniques on completeness, and bias. We
suggest a gentler filtering method than matched filtering in single frequency
analyses. We summarize the complications that arise when analyzing the SZE
signal at a single frequency, and assess the limitations of such an analysis.
Our results suggest that some sophistication is required when searching for
`clusters' within an SZE map.Comment: 8 pages, 7 figure
The Superconducting Instabilities of the non half-filled Hubbard Model in Two Dimensions
The problem of weakly correlated electrons on a square lattice is formulated
in terms of one-loop renormalization group. Starting from the action for the
entire Brillouin zone (and not with a low-energy effective action) we reduce
successively the cutoff about the Fermi surface and follow the
renormalization of the coupling as a function of three energy-momenta. We
calculate the intrinsic scale where the renormalization group flow
crosses over from the regime () where the electron-electron
(e-e) and electron-hole (e-h) terms are equally important to the regime
() where only the e-e term plays a role. In the low energy
regime only the pairing interaction is marginally relevant, containing
contributions from all renormalization group steps of the regime . After diagonalization of , we identify its most
attractive eigenvalue . At low filling,
corresponds to the representation ( symmetry), while near half
filling the strongest attraction occurs in the representation
( symmetry). In the direction of the van Hove singularities, the
order parameter shows peaks with increasing strength as one approaches half
filling. Using the form of pairing and the structure of the renormalization
group equations in the low energy regime, we give our interpretation of ARPES
experiments trying to determine the symmetry of the order parameter in the
Bi2212 high- compound.Comment: 24 pages (RevTeX) + 11 figures (the tex file appeared incomplete
Green Function Monte Carlo with Stochastic Reconfiguration
A new method for the stabilization of the sign problem in the Green Function
Monte Carlo technique is proposed. The method is devised for real lattice
Hamiltonians and is based on an iterative ''stochastic reconfiguration'' scheme
which introduces some bias but allows a stable simulation with constant sign.
The systematic reduction of this bias is in principle possible. The method is
applied to the frustrated J1-J2 Heisenberg model, and tested against exact
diagonalization data. Evidence of a finite spin gap for J2/J1 >~ 0.4 is found
in the thermodynamic limit.Comment: 13 pages, RevTeX + 3 encapsulated postscript figure
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