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L. Padilla

Publications and source records attributed to L. Padilla.

8 recordsLinked to original sources

Local Einstein relation for fractals

We study single random walks and the electrical resistance for fractals obtained as the limit of a sequence of periodic structures. In the long-scale regime, power laws describe both the mean-square displacement of a random walk as a function of time and the electrical resistance as a function of length. We show that the corresponding power-law exponents satisfy the Einstein relation. For shorter scales, where these exponents depend on length, we find how the Einstein relation can be generalized to hold locally. All these findings were analytically derived and confirmed by numerical simulations.

cond-mat.stat-mech

Universal behavior for single-file diffusion on a disordered fractal

We study single-file diffusion on a one-dimensional lattice with a random fractal distribution of hopping rates. For finite lattices, this problem shows three clearly different regimes, namely, nearly independent particles, highly interacting particles, and saturation. The mean-square displacement of a tagged particle as a function of time follows a power law in each regime. The first crossover time $t_s$, between the first and the second regime, depends on the particle density. The other crossover time $t_l$, between the second and the third regime, depends on the lattice length. We find analytic expressions for these dependencies and show how the general behavior can be characterized by an universal form. We also show that the mean-square displacement of the center of mass presents two regimes; anomalous diffusion for times shorter than $t_l$, and normal diffusion for times longer than $t_l$. We study single-file diffusion on a one-dimensional lattice with a random fractal distribution of hopping rates. For finite lattices, this problem shows three clearly different regimes, namely, nearly independent particles, highly interacting particles, and saturation. The mean-square displacement of a tagged particle as a function of time follows a power law in each regime. The first crossover time $t_s$, between the first and the second regime, depends on the particle density. The other crossover time $t_l$, between the second and the third regime, depends on the lattice length. We find analytic expressions for these dependencies and show how the general behavior can be characterized by an universal form. We also show that the mean-square displacement of the center of mass presents two regimes; anomalous diffusion for times shorter than $t_l$, and normal diffusion for times longer than $t_l$.

cond-mat.stat-mech

Intermediate Range Structure in Ion-Conducting Tellurite Glasses

We present ac conductivity spectra of tellurite glasses at several temperatures. For the first time, we report oscillatory modulations at frequencies around MHz. This effect is more pronounced the lower the temperature, and washes out when approaching the glass transition temperature $T_g$. We show, by using a minimal model, how this modulation may be attributed to the fractal structure of the glass at intermediate mesoscopic length scales.

cond-mat.dis-nn

Anisotropic anomalous diffusion modulated by log-periodic oscillations

We introduce finite ramified self-affine substrates in two dimensions with a set of appropriate hopping rates between nearest-neighbor sites, where the diffusion of a single random walk presents an anomalous {\it anisotropic} behavior modulated by log-periodic oscillations. The anisotropy is revealed by two different random walk exponents, $ν_x$ and $ν_y$, in the {\it x} and {\it y} direction, respectively. The values of these exponents, as well as the period of the oscillation, are analytically obtained and confirmed by Monte Carlo simulations.

cond-mat.stat-mech

Anomalous diffusion with log-periodic modulation in a selected time interval

On certain self-similar substrates the time behavior of a random walk is modulated by logarithmic periodic oscillations on all time scales. We show that if disorder is introduced in a way that self-similarity holds only in average, the modulating oscillations are washed out but subdiffusion remains as in the perfect self-similar case. Also, if disorder distribution is appropriately chosen the oscillations are localized in a selected time interval. Both the overall random walk exponent and the period of the oscillations are analytically obtained and confirmed by Monte Carlo simulations.

cond-mat.stat-mech

Log-periodic oscillations for diffusion on self-similar finitely ramified structures

Under certain circumstances, the time behavior of a random walk is modulated by logarithmic periodic oscillations. The goal of this paper is to present a simple and pedagogical explanation of the origin of this modulation for diffusion on a substrate with two properties: self-similarity and finite ramification order. On these media, the time dependence of the mean-square displacement shows log-periodic modulations around a leading power law, which can be understood on the base of a hierarchical set of diffusion constants. Both the random walk exponent and the period of oscillations are analytically obtained for a pair of examples, one fractal, the other non-fractal, and confirmed by Monte Carlo simulations.

cond-mat.stat-mech

Log-periodic modulation in one-dimensional random walks

We have studied the diffusion of a single particle on a one-dimensional lattice. It is shown that, for a self-similar distribution of hopping rates, the time dependence of the mean-square displacement follows an anomalous power law modulated by logarithmic periodic oscillations. The origin of this modulation is traced to the dependence on the length of the diffusion coefficient. Both the random walk exponent and the period of the modulation are analytically calculated and confirmed by Monte Carlo simulations.

cond-mat.stat-mech

Search for gamma-ray bursts above 20 TeV with the HEGRA AIROBICC Cherenkov array

A search for gamma-ray bursts (GRBs) above 20 TeV within the field of view (1 sr) of the HEGRA AIROBICC Cherenkov array (29N, 18W, 2200 m a.s.l.) has been performed using data taken between March 1992 and March 1993. The search is based on an all-sky survey using four time scales, 10 seconds, 1 minute, 4 minutes and 1 hour. No evidence for TeV-emission has been found for the data sample. Flux upper limits are given. A special analysis has been performed for GRBs detected by BATSE and WATCH. Two partially and two fully contained GRBs in our field of view (FOV) were studied. For GRB 920925c which was fully contained in our FOV, the most significant excess has a probability of 7.7 10**-8 (corresponding to 5.4 sigmas) of being caused by a background fluctuation. Correcting this probability with the appropriate trial factor, yields a 99.7% confidence level for this excess to be related to the GRB (corresponding to 2.7 sigmas). This result is discussed within the framework of the WATCH detection.

astro-ph