Finite temperature QMC study of the one-dimensional polarized Fermi gas

Quantum Monte Carlo (QMC) techniques are used to provide an approximation-free investigation of the phases of the one-dimensional attractive Hubbard Hamiltonian in the presence of population imbalance. The temperature at which the "Fulde-Ferrell-Larkin-Ovchinnikov" (FFLO) phase is destroyed by thermal fluctuations is determined as a function of the polarization. It is shown that the presence of a confining potential does not dramatically alter the FFLO regime, and that recent experiments on trapped atomic gases likely lie just within the stable temperature range.Comment: 10 pages, 13 figures We added a discussion of the behaviour of the FFLO peak as a function of the attractive interaction strengt

Hydrogenography, a thin film optical combinatorial study of hydrogen storage materials

Griessen, R.P. [Promotor]Dam, B. [Copromotor

Attractive Hubbard Model on a Honeycomb Lattice

We study the attractive fermionic Hubbard model on a honeycomb lattice using determinantal quantum Monte Carlo simulations. By increasing the interaction strength U (relative to the hopping parameter t) at half-filling and zero temperature, the system undergoes a quantum phase transition at 5.0 < U_c/t < 5.1 from a semi-metal to a phase displaying simultaneously superfluid behavior and density order. Doping away from half-filling, and increasing the interaction strength at finite but low temperature T, the system always appears to be a superfluid exhibiting a crossover between a BCS and a molecular regime. These different regimes are analyzed by studying the spectral function. The formation of pairs and the emergence of phase coherence throughout the sample are studied as U is increased and T is lowered

Solving the Direction Field for Discrete Agent Motion

Models for pedestrian dynamics are often based on microscopic approaches allowing for individual agent navigation. To reach a given destination, the agent has to consider environmental obstacles. We propose a direction field calculated on a regular grid with a Moore neighborhood, where obstacles are represented by occupied cells. Our developed algorithm exactly reproduces the shortest path with regard to the Euclidean metric.Comment: 8 pages, 4 figure

Extreme events driven glassy behaviour in granular media

Motivated by recent experiments on the approach to jamming of a weakly forced granular medium using an immersed torsion oscillator [Nature 413 (2001) 407], we propose a simple model which relates the microscopic dynamics to macroscopic rearrangements and accounts for the following experimental facts: (1) the control parameter is the spatial amplitude of the perturbation and not its reduced peak acceleration; (2) a Vogel-Fulcher-Tammann-like form for the relaxation time. The model draws a parallel between macroscopic rearrangements in the system and extreme events whose probability of occurrence (and thus the typical relaxation time) is estimated using extreme-value statistics. The range of validity of this description in terms of the control parameter is discussed as well as the existence of other regimes.Comment: 7 pages, to appear in Europhys. Let

Statistics of S-matrix poles in Few-Channel Chaotic Scattering: Crossover from Isolated to Overlapping Resonances

We derive the explicit expression for the distribution of resonance widths in a chaotic quantum system coupled to continua via M equivalent open channels. It describes a crossover from the $\chi^2$ distribution (regime of isolated resonances) to a broad power-like distribution typical for the regime of overlapping resonances. The first moment is found to reproduce exactly the Moldauer-Simonius relation between the mean resonance width and the transmission coefficient. This fact may serve as another manifestation of equivalence between the spectral and the ensemble averaging.Comment: 4 two-column pages, RevTex. text is slightly modified; some misprints are correcte

Structural and optical properties of Mg_xAl_1-xH_y gradient thin films: a combinatorial approach

The structural, optical and dc electrical properties of MgxAl1-x (0.2≤x≤0.9) gradient thin films covered with Pd/Mg are investigated before and after exposure to hydrogen. We use hydrogenography, a novel high-throughput optical technique, to map simultaneously all the hydride forming compositions and the kinetics thereof in the gradient thin film. Metallic Mg in the MgxAl1-x layer undergoes a metal-to-semiconductor transition and MgH₂ is formed for all Mg fractions x investigated. The presence of an amorphous Mg-Al phase in the thin film phase diagram enhances strongly the kinetics of hydrogenation. In the Al-rich part of the film, a complex H-induced segregation of MgH₂ and Al occurs. This uncommon large-scale segregation is evidenced by metal and hydrogen profiling using Rutherford backscattering spectrometry and resonant nuclear analysis based on the reaction ¹H(¹⁵N,αγ)¹²C. Besides MgH₂, an additional semiconducting phase is found by electrical conductivity measurements around an atomic [Al]/[Mg] ratio of 2 (x=0.33). This suggests that the film is partially transformed into Mg(AlH₄)₂ at around this composition

A link between short-range and long-range properties of random sphere packings

We present a high precision particle-by-particle 3D reconstruction of granular systems composed of monodispersed spheres (sphere packings); the experimental approach is based on magnetic resonance imaging techniques. Our measurements revealed a strong correlation between the volume defined by the distance to the first nearest neighbor and the long-range average density. The main contribution to the amplitude decay of the correlation function can be described as exponential rather than power law up to a range equal to 7 sphere diameters. No evidence of geometrical structural changes as a function of the density was observed and neither regular crystallites nor any other statistically significant structures could be ascribed to a specific local arrangement. We concluded that granular compaction is the result of a process through which the system changes the average size of local structures without changing their local geometrical characteristics. These conclusions are supported by two-body correlation functions and Voronoi polyhedra space decomposition. The results provide a different perspective on the mechanisms underlying compaction with respect to previous works, and allow to discriminate between the different existing theoretical approache