Stochastic Resonance and Dynamic First-Order Pseudo-Phase Transitions in the Irreversible Growth of Thin Films under Spatially Periodic Magnetic Fields

We study the irreversible growth of magnetic thin films under the influence of spatially periodic fields by means of extensive Monte Carlo simulations. We find first-order pseudo-phase transitions that separate a dynamically disordered phase from a dynamically ordered phase. By analogy with time-dependent oscillating fields applied to Ising-type models, we qualitatively associate this dynamic transition with the localization/delocalization transition of "spatial hysteresis" loops. Depending on the relative width of the magnetic film, $L$, compared to the wavelength of the external field, $\lambda$, different transition regimes are observed. For small systems ($L<\lambda$), the transition is associated with the Standard Stochastic Resonance regime, while, for large systems ($L>\lambda$), the transition is driven by Anomalous Stochastic Resonance. The origin of the latter is identified as due to the emergence of an additional relevant lengthscale, namely the roughness of the spin domain switching interface. The distinction between different stochastic resonance regimes is discussed at length, both qualitatively by means of snapshot configurations, as well as quantitatively via residence-length and order-parameter probability distributions.Comment: 21 pages, 8 figures. To appear in Phys. Rev.

Glassy behavior of the site frustrated percolation model

The dynamical properties of the site frustrated percolation model are investigated and compared with those of glass forming liquids. When the density of the particles on the lattice becomes high enough, the dynamics of the model becomes very slow, due to geometrical constraints, and rearrangement on large scales is needed to allow relaxation. The autocorrelation functions, the specific volume for different cooling rates, and the mean square displacement are evaluated, and are found to exhibit glassy behavior.Comment: 8 pages, RevTeX, 11 fig

Performance Counter Measurements of Data Structures: Implementations for Multi-Objective Optimisation

Solving multi-objective optimisation problems using evolutionary computation methods involve the implementation of algorithms and data structures for the storage of tempo- rary solutions. Computational efficiency of these systems becomes important as problems increase in complexity and the number of solutions maintained becomes large. Many data structures and algorithms have been proposed looking to decrease computa- tional times. The effectiveness of a data structure/algorithm can be characterised using wall-clock time. This is a widely used parameter in the literature, however it is strongly dependent on the underlying computer architecture and hence not a reliable measure of absolute performance. A commonly used approach to avoid architectural dependencies is to compare the performance of the data structure being evaluated to the equivalent implementation using a linked list. Modern processors offer built-in hardware performance counters, giving access to a wide set of parameters that can be used to explore performance. In this dissertation we study the efficiency of a non-dominated quad-tree data structure in combination with different evolutionary algorithms using hardware performance counters. We also compare the re- sults for the quad-tree data structure to a linked list as it is the standard practice, however we find non-scalable hardware dependencies might appear

Quantum Simulations of Relativistic Quantum Physics in Circuit QED

We present a scheme for simulating relativistic quantum physics in circuit quantum electrodynamics. By using three classical microwave drives, we show that a superconducting qubit strongly-coupled to a resonator field mode can be used to simulate the dynamics of the Dirac equation and Klein paradox in all regimes. Using the same setup we also propose the implementation of the Foldy-Wouthuysen canonical transformation, after which the time derivative of the position operator becomes a constant of the motion.Comment: 13 pages, 3 figure

Quantum Simulation of Dissipative Processes without Reservoir Engineering

We present a quantum algorithm to simulate general finite dimensional Lindblad master equations without the requirement of engineering the system-environment interactions. The proposed method is able to simulate both Markovian and non-Markovian quantum dynamics. It consists in the quantum computation of the dissipative corrections to the unitary evolution of the system of interest, via the reconstruction of the response functions associated with the Lindblad operators. Our approach is equally applicable to dynamics generated by effectively non-Hermitian Hamiltonians. We confirm the quality of our method providing specific error bounds that quantify itss accuracy.Comment: 7 pages + Supplemental Material (6 pages

Dynamic heterogeneities in attractive colloids

We study the formation of a colloidal gel by means of Molecular Dynamics simulations of a model for colloidal suspensions. A slowing down with gel-like features is observed at low temperatures and low volume fractions, due to the formation of persistent structures. We show that at low volume fraction the dynamic susceptibility, which describes dynamic heterogeneities, exhibits a large plateau, dominated by clusters of long living bonds. At higher volume fraction, where the effect of the crowding of the particles starts to be present, it crosses over towards a regime characterized by a peak. We introduce a suitable mean cluster size of clusters of monomers connected by "persistent" bonds which well describes the dynamic susceptibility.Comment: 4 pages, 4 figure