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A. Villares Ferrer

Publications and source records attributed to A. Villares Ferrer.

7 recordsLinked to original sources
arXiv2007

Dynamical localization of a particle coupled to a two-level systems thermal reservoir

Using the functional-integral method, we investigate the effect of a two-level systems thermal reservoir on the single particle dynamics. We find that at low temperatures, within the sub-ohmic regime, the particle becomes {}``dynamically'' localized at long times due to an effective potential generated by the particle-reservoir interaction. This behavior is different from the one obtained for the usual bath of harmonic oscillators and is fundamentally related with the non-Markovian character of the dissipative process.

cond-mat.mes-hall↗
arXiv2006

Optical conductivity of charge carriers interacting with a two-level systems reservoir

Using the functional-integral method we investigate the effective dynamics of a charged particle coupled to a set of two-level systems as a function of temperature and external electric field. The optical conductivity and the direct current (dc) resistivity induced by the reservoir are computed. Three different regimes are found depending on the two-level system spectral function, which may lead to a non-Drude optical conductivity in a certain range of parameters. Our results contrast to the behavior found when considering the usual bath of harmonic oscillators which we are able to recover in the limit of very low temperatures.

cond-mat.other↗
arXiv2004

Domain-wall profile in the presence of anisotropic exchange interactions: Effective on-site anisotropy

Starting from a D-dimensional XXZ ferromagnetic Heisenberg model in an hypercubic lattice, it is demonstrated that the anisotropy in the exchange coupling constant leads to a D-dependent effective on-site anisotropy interaction often ignored for D>1. As a result the effective width of the wall depends on the dimensionality of the system. It is shown that the effective one-dimensional Hamiltonian is not the one-dimensional XXZ version as assumed in previous theoretical work. We derive a new expression for the wall profile that generalizes the standard Landau-Lifshitz form. Our results are found to be in very good agreement with earlier numerical work using the Monte Carlo method. Preceding theories concerning the domain wall contribution to magnetoresistance have considered the role of D only through the modification of the density of states in the electronic band structure. This Brief Report reveals that the wall profile itself contains an additional D dependence for the case of anisotropic exchange interactions.

cond-mat.mtrl-sci↗
arXiv2000

Soliton dynamics in 1D quantum antiferromagnets

The problem of dissipative motion of solitons in the case of tetramethyl ammonium manganese chloride (TMMC) is studied as a function of the external magnetic field and the temperature. Two specific situations are analyzed separately; the first, above the transition temperature $T_N$, in which the classical motion of the spin degree of freedom is described by a sine-Gordon (SG) equation of motion and, the second, below $T_N$, in which the system is described by a double sine-Gordon (2SG) equation of motion. The existence of a dissipative regime for the soliton motion and its influence on the dynamical structure factor -which might be experimentally detected- are reported.

cond-mat↗
arXiv1999

Skyrmion dynamics in quantum Hall ferromagnets

Exploring a classical solution of the non-linear sigma model for a quantum Hall ferromagnet, a skyrmion-magnon effective hamiltonian is obtained via the collective coordinates method. Using the Feynman-Vernon functional integral formalism for this model we find the temperature dependent transport coefficients which characterize a single skyrmion dynamics.

cond-mat.mes-hall↗
arXiv1999

Mobility of Bloch Walls via the Collective Coordinate Method

We have studied the problem of the dissipative motion of Bloch walls considering a totally anisotropic one dimensional spin chain in the presence of a magnetic field. Using the so-called "collective coordinate method" we construct an effective Hamiltonian for the Bloch wall coupled to the magnetic excitations of the system. It allows us to analyze the Brownian motion of the wall in terms of the reflection coefficient of the effective potential felt by the excitations due to the existence of the wall. We find that for finite values of the external field the wall mobility is also finite. The spectrum of the potential at large fields is investigated and the dependence of the damping constant on temperature is evaluated. As a result we find the temperature and magnetic field dependence of the wall mobility.

cond-mat.stat-mech↗