Sebastiano Caravati

Fast Crystallization of the Phase Change Compound GeTe by Large-Scale Molecular Dynamics Simulations

Phase change materials are of great interest as active layers in rewritable optical disks and novel electronic nonvolatile memories. These applications rest on a fast and reversible transformation between the amorphous and crystalline phases upon heating, taking place on the nanosecond time scale. In this work, we investigate the microscopic origin of the fast crystallization process by means of large-scale molecular dynamics simulations of the phase change compound GeTe. To this end, we use an interatomic potential generated from a Neural Network fitting of a large database of ab initio energies. We demonstrate that in the temperature range of the programming protocols of the electronic memories (500-700 K), nucleation of the crystal in the supercooled liquid is not rate-limiting. In this temperature range, the growth of supercritical nuclei is very fast because of a large atomic mobility, which is, in turn, the consequence of the high fragility of the supercooled liquid and the associated breakdown of the Stokes-Einstein relation between viscosity and diffusivity.

Neural network interatomic potential for the phase change material GeTe

GeTe is a prototypical phase change material of high interest for applications in optical and electronic nonvolatile memories. We present an interatomic potential for the bulk phases of GeTe, which is created using a neural network (NN) representation of the potential-energy surface obtained from reference calculations based on density functional theory. It is demonstrated that the NN potential provides a close to ab initio quality description of a number of properties of liquid, crystalline, and amorphous GeTe. The availability of a reliable classical potential allows addressing a number of issues of interest for the technological applications of phase change materials, which are presently beyond the capability of first-principles molecular dynamics simulations.

Density functional simulations of Sb-rich GeSbTe phase change alloys

We generated models of the amorphous phase of Sb-rich GeSbTe phase change alloys by quenching from the melt within density functional molecular dynamics. We considered the two compositions Ge(1)Sb(1)Te(1) and Ge(2)Sb(4)Te(5). Comparison with previous results on the most studied Ge(2)Sb(2)Te(5) allowed us to draw some conclusions on the dependence of the structural properties of the amorphous phase on the alloy composition. Vibrational and electronic properties were also scrutinized. Phonons at high frequencies above 200 cm(-1) are localized in tetrahedra around Ge atoms in Sb-rich compounds as well as in Ge(2)Sb(2)Te(5). All compounds are semiconducting in the amorphous phase, with a band gap in the range 0.7-1.0 eV.

Vibrational properties of crystalline Sb(2)Te(3) from first principles.

Phonon dispersion relations and infrared and Raman spectra of crystalline Sb(2)Te(3) were computed within density functional perturbation theory. Overall good agreement with experiments is obtained, which allows us to assign the Raman and IR peaks to specific phonons.

On the Role of Nonspherical Cavities in Short Length-Scale Density Fluctuations in Water

Density fluctuations in liquid water are at the heart of numerous phenomena associated with hydrophobic effects such as protein folding and the interaction between biomolecules. One of the most fundamental processes in this regard is the solvation of hydrophobic solutes in water. The vast majority of theoretical and numerical studies examine density fluctuations at the short length scale focusing exclusively on spherical cavities. In this work, we use both first-principles and classical molecular dynamics simulations to demonstrate that density fluctuations in liquid water can deviate significantly from the canonical spherical shapes. We show that regions of empty space are frequently characterized by exotic, highly asymmetric shapes that can be quite delocalized over the hydrogen bond network. Interestingly, density fluctuations of these shapes are characterized by Gaussian statistics with larger fluctuations. An important consequence of this is that the work required to create non spherical cavities can be substantially smaller than that of spheres. This feature is also qualitatively captured by the Lum-Chandler-Weeks theory. The scaling behavior of the free energy as a function of the volume at short length scales is qualitatively different for the nonspherical entities. We also demonstrate that nonspherical density fluctuations are important for accommodating the hydrophobic amino acid alanine and are thus likely to have significant implications when it comes to solvating highly asymmetrical species such as alkanes, polymers, or biomolecules.

Functional Properties of Phase Change Materials from Atomistic Simulations

Chalcogenide alloys are materials of interest for optical recording and electronic nonvolatile memories. These applications rest on an ensemble of functional properties: a fast and reversible transformation between the amorphous and the crystalline phase upon heating and a strong optical and electronic contrast between the two phases that allow discriminating the two states of the memory. We discuss the insights gained from atomistic simulations based on Density Functional Theory on the functional properties of the prototypical phase change compounds Ge\(_2\)Sb\(_2\)Te\(_5\) and GeTe. We review the results on the structural and bonding properties of the crystalline and amorphous phases, the origin of the optical and electronic contrast between the two phases and the source of the fast crystallization of the supercooled liquid. The results on the crystallization kinetics obtained from large scale simulations with interatomic potentials based on Neural Network methods are also discussed.