arXiv subjects

Joan J. Cerdà

Publications and source records attributed to Joan J. Cerdà.

6 recordsLinked to original sources

arXiv2024

Dynamic response of a ferromagnetic nanofilament under rotating fields: effects of flexibility, thermal fluctuations and hydrodynamics

Using nonequilibrium computer simulations, we study the response of ferromagnetic nanofilaments, consisting of stabilized onedimensional chains of ferromagnetic nanoparticles, under external rotating magnetic fields. In difference with their analogous microscale and stiff counterparts, which have been actively studied in recent years, nonequilibrium properties of rather flexible nanoparticle filaments remain mostly unexplored. By progressively increasing the modeling details, we are able to evidence the qualitative impact of main interactions that can not be neglected at the nanoscale, showing that filament flexibility, thermal fluctuations and hydrodynamic interactions contribute independently to broaden the range of synchronous frequency response in this system. Furthermore, we also show the existence of a limited set of characteristic dynamic filament configurations and discuss in detail the asynchronous response, which at finite temperature becomes probabilistic.

cond-mat.soft↗

arXiv2020

Light Cones in Classical Dipole-Dipole Interacting Systems

The speed at which the magnetic interaction propagates along a chain of classic dipoles is discussed here. While in the quantum information counterpart for long-range interacting spins, where the speed of propagation of the information plays a paramount role, it is not strictly clear whether a light cone exists or not, here we provide numerical evidence that interacting dipoles do posses a linear light cone shortly after a perturbation takes place. Specifically, a power-law expansion occurs which is followed by a linear propagation of the associated interaction. As opposed to the quantum case, and in analogy with the so-called speed of gravity problem, we find that the speed of propagation of information can be arbitrarily large in the classic context. In order to agree with special relativity, we propose the derivation of a frame-independent Landau-Lifshitz equation.

quant-ph↗

arXiv2020

Adsorption transition of a grafted ferromagnetic filament controlled by external magnetic fields

Extensive Langevin dynamics simulations are used to characterize the adsorption transition of a flexible magnetic filament grafted onto an attractive planar surface. Our results identify different structural transitions at different ratios of the thermal energy to the surface attraction strength: filament straightening, adsorption and the magnetic flux closure. The adsorption temperature of a magnetic filament is found to be higher in comparison to an equivalent nonmagnetic chain. The adsorption has been also investigated under the application of a static homogeneous external magnetic field. We found that the strength and the orientation of the field can be used to control the adsorption process, providing a precise switching mechanism. Interestingly, we have observed that the characteristic field strength and tilt angle at the adsorption point are related by a simple power law.

cond-mat.soft↗

arXiv2017

Spectral Ergodicity in Deep Learning Architectures via Surrogate Random Matrices

In this work a novel method to quantify spectral ergodicity for random matrices is presented. The new methodology combines approaches rooted in the metrics of Thirumalai-Mountain (TM) and Kullbach-Leibler (KL) divergence. The method is applied to a general study of deep and recurrent neural networks via the analysis of random matrix ensembles mimicking typical weight matrices of those systems. In particular, we examine circular random matrix ensembles: circular unitary ensemble (CUE), circular orthogonal ensemble (COE), and circular symplectic ensemble (CSE). Eigenvalue spectra and spectral ergodicity are computed for those ensembles as a function of network size. It is observed that as the matrix size increases the level of spectral ergodicity of the ensemble rises, i.e., the eigenvalue spectra obtained for a single realisation at random from the ensemble is closer to the spectra obtained averaging over the whole ensemble. Based on previous results we conjecture that success of deep learning architectures is strongly bound to the concept of spectral ergodicity. The method to compute spectral ergodicity proposed in this work could be used to optimise the size and architecture of deep as well as recurrent neural networks.

stat.ML↗

arXiv2014

Phase diagram for a single flexible Stockmayer polymer at zero field

The equilibrium conformations of a flexible permanent magnetic chain that consists of a sequence of linked magnetic colloidal nanoparticles with short-ranged Lennard-Jones attractive interactions (Stockmayer polymer) are thoroughly analysed via Langevin dynamics simulations. A tentative phase diagram is presented for a chain of length $N=100$. The phase diagram exhibits several unusual conformational phases when compared with the non-magnetic chains. These phases are characterised by a large degree of conformational anisotropy, and consist of closed chains, helicoidal-like states, partially collapsed states, and very compact disordered states. The phase diagram contains several interesting features like the existence of at least two 'triple points'.

cond-mat.soft↗

arXiv2014

Effects of the dipolar interaction on the equilibrium morphologies of a single supramolecular magnetic filament in bulk

We study the equilibrium morphologies of a single supramolecular magnetic filament in a three-dimensional system as a function of the effective strength of the magnetic dipolar interactions. The study is performed by means of Langevin dynamics simulations with a bead-spring chain model of freely rotating dipoles. We demonstrate the existence of three structural regimes as the value of the dipolar coupling parameter is increased: a coil compaction regime, a coil expansion regime and a closed chain regime in which the structures tend progressively to an ideal ring configuration. We discuss the governing effects of each regime, the structural transition between open and closed morphologies, and the reasons why we see no multiloop configurations that have been observed in two-dimensional systems under similar conditions.

cond-mat.mes-hall↗