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Remi Jullien

Publications and source records attributed to Remi Jullien.

13 recordsLinked to original sources
arXiv2006

Magnetoresistance in a soft billiard: giant peak near the percolation threshold

By numerical simulation, we study the classical magnetoresistance of two-dimensional electrons in the presence of weak short range scattering. A critical magnetic field defines the percolation threshold, above which the longitudinal resistance vanishes. Unexpectedely, just below this threshold we find a shrp narrow peak, where the resistance may increase 15 times compared to its zero-field value. By considering the complex topology of the effective potential landscape for the center of the cyclotron circle, we show that this phenomenon is related to infinite equipotential lines, which exists only in a narrow magnetic field interval below the percolation threshold

cond-mat.mes-hall↗
arXiv2003

Sodium diffusion through amorphous silica surfaces: A molecular dynamics study

We have studied the diffusion inside the silica network of sodium atoms initially located outside the surfaces of an amorphous silica film. We have focused our attention on structural and dynamical quantities, and we have found that the local environment of the sodium atoms is close to the local environment of the sodium atoms inside bulk sodo-silicate glasses obtained by quench. This is in agreement with recent experimental results.

cond-mat.dis-nn↗
arXiv2002

Anomalous Low-Field Classical Magnetoresistance in Two Dimensions

The magnetoresistance of classical two-dimensional electrons scattered by randomly distributed impurities is investigated by numerical simulation. At low magnetic fields, we find for the first time a negative magnetoresistance proportional to |B|. This unexpected behavior is shown to be due to a memory effect specific for backscattering events, which was not considered previously.

cond-mat.mes-hall↗
arXiv2002

Characterization of the channel diffusion in a sodium tetrasilicate glass via molecular-dynamics simulations

We study the structural and dynamical characteristics of the sodium atoms inside and outside the ``diffusion channels'' in glassy Na$_2$O-4SiO$_2$ (NS4) using classical molecular dynamics. We show that on average neither energetic arguments nor local environment considerations can explain the increased density of sodium atoms inside the subspace made of the channels. Nevertheless we show that at low temperature the mean square displacement of the sodium atoms inside this subspace is significantly larger than the one of the atoms outside the channels.

cond-mat.dis-nn↗
arXiv2001

Structural and electronic properties of the sodium tetrasilicate glass Na${}_2$Si${}_4$O${}_9$ from classical and ab initio molecular-dynamics simulations

The structure and the electronic properties of a sodium tetrasilicate (Na${}_2$Si${}_4$O${}_9$) glass were studied by combined Car-Parrinello and classical molecular-dynamics simulations. The glass sample was prepared using a method employed recently in a study of a silica glass [M. Benoit {\it et al}, Euro. Phys. J. B {\bf 13}, 631 (2000)]. First we generated a NS4 glass by classical molecular-dynamics and then we took it as the initial configuration of a first-principles molecular-dynamics simulation. In the {\it ab initio} molecular-dynamics simulation, the electronic structure was computed in the framework of the Kohn-Sham density functional theory within the generalized gradient approximation using a B-LYP functional. The Car-Parrinello dynamics is remarkably stable during the considered trajectory and, as soon as it is swit ched on, some significant structural changes occur. The {\it ab initio} description improves the comparison of the structural characteristics with experimental data, in particular concerning the Si-O and Na-O bond lengths. From an electronic point of view, we find that the introduction of the sodium oxide in the silica network lowers the band gap and leads to a highly non-localized effect on the charges of the network atoms.

cond-mat.dis-nn↗
arXiv2001

Channel diffusion of sodium in a silicate glass

We use classical molecular dynamics simulations to study the dynamics of sodium atoms in amorphous Na$_2$O-4SiO$_2$. We find that the sodium trajectories form a well connected network of pockets and channels. Inside these channels the motion of the atoms is not cooperative but rather given by independent thermally activated hops of individual atoms between the pockets. By determining the probability that an atom returns to a given starting site, we show that such events are not important for the dynamics of this system.

cond-mat.stat-mech↗
arXiv2000

Random walks on fractals and stretched exponential relaxation

Stretched exponential relaxation ($\exp{-(t/τ)}^{β_K}$) is observed in a large variety of systems but has not been explained so far. Studying random walks on percolation clusters in curved spaces whose dimensions range from 2 to 7, we show that the relaxation is accurately a stretched exponential and is directly connected to the fractal nature of these clusters. Thus we find that in each dimension the decay exponent $β_K$ is related to well-known exponents of the percolation theory in the corresponding flat space. We suggest that the stretched exponential behavior observed in many complex systems (polymers, colloids, glasses...) is due to the fractal character of their configuration space.

cond-mat.dis-nn↗
arXiv1999

Computer investigation of the energy landscape of amorphous silica

The multidimensional topography of the collective potential energy function of a so-called strong glass former (silica) is analyzed by means of classical molecular dynamics calculations. Features qualitatively similar to those of fragile glasses are recovered at high temperatures : in particular an intrinsic characteristic temperature $T_c\simeq 3500$K is evidenced above which the system starts to investigate non-harmonic potential energy basins. It is shown that the anharmonicities are essentially characterized by a roughness appearing in the potential energy valleys explored by the system for temperatures above $T_c$.

cond-mat.dis-nn↗
arXiv1999

Molecular dynamics calculation of the thermal conductivity of vitreous silica

We use extensive classical molecular dynamics simulations to calculate the thermal conductivity of a model silica glass. Apart from the potential parameters, this is done with no other adjustable quantity and the standard equations of heat transport are used directly in the simulation box. The calculations have been done between 10 and 1000 Kelvin and the results are in good agreement with the experimental data at temperatures above 20K. The plateau observed around 10K can be accounted for by correcting our results taking into account finite size effects in a phenomenological way.

cond-mat.dis-nn↗
arXiv1998

Numerical investigation of the influence of the history on the local structure of glasses

By means of molecular dynamics simulations and the Voronoi tessellation, we study the influence of the history on the low temperature characteristics of soft sphere and silica glasses. The quench from the liquid is interrupted at an intermediate temperature $T_i$ for a given relaxation time, and then the cooling process is continued down to 0K. The local structure at 0K depends on the temperature $T_i$ and the effect is larger for $T_i$ close to the glass transition temperature $T_g$. This dependence, coherent with recent results, is expected in the strong glass former where the characteristics of a particular state depend on its history. In the soft-sphere case, because of crystallization effects, the dependence of the local structure of the glassy samples on their history can only be detected in the supercooled liquid region.

cond-mat.soft↗
arXiv1998

The glass transition in a model silica glass: evolution of the local structure

We use molecular dynamics simulations and the Voronoi tessellation to study the geometrical modifications as a function of temperature in a model silica glass. The standard deviation of the cell volumes, which is a measure of the local density fluctuations, decreases with decreasing temperature, as if it would like to vanish at zero temperature. This evolution towards an ordered state is frozen out at the glass transition and consequently an amorphous sample is obtained at low temperature. This structural freezing following upon the glass transition is noticeable in all the other geometric characteristics of the Voronoi cells and a possible interpretation in terms of geometrical frustration is proposed.

cond-mat.dis-nn↗
arXiv1998

Crystallization in a model glass: influence of the boundary conditions

Using molecular dynamics calculations and the Voronoi tessellation, we study the evolution of the local structure of a soft-sphere glass versus temperature starting from the liquid phase at different quenching rates. This study is done for different sizes and for two different boundary conditions namely the usual cubic periodic boundary conditions and the isotropic hyperspherical boundary conditions for which the particles evolve on the surface of a hypersphere in four dimensions. Our results show that for small system sizes, crystallization can indeed be induced by the cubic boundary conditions. On the other hand we show that finite size effects are more pronounced on the hypersphere and that crystallization is artificially inhibited even for large system sizes.

cond-mat.dis-nn↗