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 $\Lambda$ about the Fermi surface and follow the renormalization of the coupling $U$ as a function of three energy-momenta. We calculate the intrinsic scale $T_{co}$ where the renormalization group flow crosses over from the regime ($\Lambda > T_{co}$) where the electron-electron (e-e) and electron-hole (e-h) terms are equally important to the regime ($\Lambda < T_{co}$) where only the e-e term plays a role. In the low energy regime only the pairing interaction $V$ is marginally relevant, containing contributions from all renormalization group steps of the regime $\Lambda > T_{co}$. After diagonalization of $V_{\Lambda =T_{co}}$, we identify its most attractive eigenvalue $\lambda _{\min}$. At low filling, $\lambda _{\min}$ corresponds to the $B_2$ representation ($d_{xy}$ symmetry), while near half filling the strongest attraction occurs in the $B_1$ representation ($d_{x^2-y^2}$ 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-$T_{c}$ 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