Glassy Aging with Modified Kohlrausch-Williams-Watts Form

In this report we address the question whether aging in the non equilibrium glassy state is controlled by the equilibrium alpha-relaxation process which occur at temperatures above Tg. Recently Lunkenheimer et. al. [Phys. Rev. Lett. 95, 055702 (2005)] proposed a model for the glassy aging data of dielectric relaxation using a modified Kohlrausch-Williams-Watts (KWW) form. The aging time dependence of the relaxation time is defined by these authors through a functional relation involving the corresponding frequency but the stretching exponent is same as the alpha-relaxation stretching exponent. We present here an alternative functional form directly involving the relaxation time itself. The proposed model fits the data of Lunkenheimer et. al. perfectly with a stretching exponent different from the alpha-relaxation stretching exponent.Comment: 1 TeX file, 10 eps figure

Plasticity-Induced Magnetization in Amorphous Magnetic Solids

Amorphous magnetic solids, like metallic glasses, exhibit a novel effect: the growth of magnetic order as a function of mechanical strain under athermal conditions in the presence of a magnetic field. The magnetic moment increases in steps whenever there is a plastic event. Thus plasticity induces the magnetic ordering, acting as the effective noise driving the system towards equilibrium. We present results of atomistic simulations of this effect in a model of a magnetic amorphous solid subjected to pure shear and a magnetic field. To elucidate the dependence on external strain and magnetic field we offer a mean-field theory that provides an adequate qualitative understanding of the observed phenomenon

Phase Separation in Binary Fluid Mixture with Quenched Disorder

Quenched or frozen-in, structural disorder is ubiquitous in real experimental systems. Much of the progress is achieved in understanding the phase separation of such systems using diffusion-driven coarsening in Ising model with quenched disorder. But there is a paucity of research in the phase-separation kinetics in fluids with quenched disorder. In this letter, we present results from a detailed Molecular dynamics (MD) simulation the effects of randomly placed localized impurities on the phase separating kinetics of binary fluid mixture. Our system resembles the fluid imbibed into a porous medium. We observe a dramatic slowing down in the pattern formation with increasing pin particle concentration. The domain growth follows the power-law with a disorder-dependent exponent. Due to the energetically favorable positions, the domain boundary roughens which modifies the correlation function and structure factor to a non-Porod behavior. The correlation function and structure factor provide clear evidence that the superuniversality does not hold in our system

Kinetics Of Vapor-Liquid Phase Separation Under Gravity

We study the kinetics of vapor-liquid phase separation of a hydrodynamics preserving three-dimensional one component Lennard Jones system in the presence of external gravitational field using extensive molecular dynamic simulation. A bicontinuous domain structure is formed when the homogeneous system near the critical density is quenched inside the coexistence region. In the absence of gravity, the domain morphology is statistically self-similar and the length scale grows linearly with time. However, the presence of gravity destroys the isotropy of the system and affects the scaling laws. We observe an accelerated domain growth in the direction of the field at late time which resembles sedimentation process. Consequently, a new length scale emerges which strongly depends on the field strength. Similar behavior is observed in the direction perpendicular to the applied field, with a different growth rate. Finally, the validity of Porod's law and Superuniversality in such anisotropic systems is verified in terms of two-point equal time order parameter correlation function and static structure factor

Atomistic Simulations of Magnetic Amorphous Solids: Magnetostriction, Barkhausen noise and novel singularities

We present results of atomistic simulations of a new model of a magnetic amorphous solid subjected to external mechanical strains and magnetic fields. The model employed offers new perspectives on important effects like Barkhausen noise and magnetostriction. It is shown that the plastic response in such systems exhibit singularities characterized by unexpected exponents requiring careful theoretical reasoning. The spatial structure of the plastic events requires a new coarse grained elasto-magnetic theory which is provided here

Shear Transformation Zones: State Determined or Protocol Dependent?

The concept of a Shear Transformation Zone (STZ) refers to a region in an amorphous solid that undergoes a plastic event when the material is put under an external mechanical load. An important question that had accompanied the development of the theory of plasticity in amorphous solids for many years now is whether an STZ is a {\em region} existing in the material (which can be predicted by analyzing the unloaded material), or is it an {\em event} that depends on the loading protocol (i.e., the event cannot be predicted without following the protocol itself). In this Letter we present strong evidence that the latter is the case. Infinitesimal changes of protocol result in macroscopically big jumps in the positions of plastic events, meaning that these can never be predicted from considering the unloaded material.Comment: 4 pages, 5 figure