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The application of monatomic Sb material in the phase-change memory has been greatly compromised due to easy crystallization at room temperature. In this work, we alloy 10 at. % Cr into Sb, so that the crystallization temperature of the amorphous Cr10Sb90 thin film has been raised to above 130 °C and the crystallinity can be controlled by different annealing temperatures. We find that Cr10Sb90 thin films possess relatively large electrical and optical contrasts between the amorphous (a-) and crystalline (c-) states, e.g., the resistance of the a-film decreases by three orders of magnitude after crystallization and the real part of the dielectric function of glass is much larger than that of the crystal in the wavelength range of 300 to 1650 nm. The first-principles simulations reveal that Cr doping leads to a more disordered a-state and the Cr–Sb bonds appear to be stronger than Sb–Sb bonds, which explains the enhanced stability of a-Cr10Sb90. Our findings demonstrate that alloying with Cr is an effective way to improve the stability of phase-change materials in the memory applications without damaging the desired properties of these materials.

Phase-change material Sc0.2Sb2Te3 (SST) can remarkably boost the writing speed of memory devices due to the extremely fast crystallization. It was demonstrated that the fast crystallization is because Sc stabilizes the 4-fold rings which act as precursors of nuclei in the amorphous phase. In this work, by using first-principles molecular dynamics simulations, we studied the local structures in the liquid and amorphous SST at various temperatures. The results reveal that Sc-centered configurations are almost in the form of stable octahedral clusters, which enhances the local order of the amorphous phase. Different from Sb- and Te-centered clusters which have lower coordination numbers, Sc-centered clusters are mainly in high-coordinated octahedral structures. These Sc-centered octahedral clusters present a high stability in supercooled liquid and amorphous states, remarkably reducing the incubation time of nucleation and speeding up the crystallization. Our study reveals the role of Sc atoms in the liquid and amorphous structure, paving the way for the application of Sc-based phase-change memory.

The binding sites and diffusion pathways of Si adatoms on a c(4×2) reconstructed Si(001) surface are investigated by a tight-binding method with an environment-dependent silicon potential in conjunction with ab initio calculations using the Car–Parrinello method. A new diffusion pathway along the trough edge driven by dimer flipping is found with a barrier of 0.74 eV, comparable to that of 0.68 eV along the top of the dimer rows.

Structural and dynamical heterogeneities in metallic glasses, while intensely studied, remain an enigma. For instance, whether and how the dynamical and structural heterogeneities are correlated is still an outstanding question. Meanwhile, the nature of the impact of medium-range order (MRO) on the dynamical heterogeneity remains elusive. In this paper, we analyzed the structural and dynamical heterogeneities in both as-quenched and relaxed Cu 64.5Zr35.5 metallic glasses based on the atomistic trajectories collected from molecular dynamics simulations. We found that the majority of the mobile atoms are not involved in icosahedral clusters or Bergman superclusters, indicating that dynamical heterogeneities are strongly correlated with structural heterogeneities. The Bergman-type MRO has an even stronger correlation with the dynamical heterogeneity than the icosahedral short range order. Moreover, we found that the localized soft vibration modes below 1.0 THz are mostly concentrated on the mobile atoms. These results suggest that the vibrational properties can be conveniently utilized to predict the atomic mobility in metallic glasses, which can bridge the studies of dynamical heterogeneity by experiments and simulations.

Creating metallic glasses by cooling liquid melts in molecular dynamics simulations faces a well-known challenge that the cooling rate is too fast compared with experiments. Taking the prototypical Cu64.5Zr35.5 glasses as an example, we propose an efficient cooling strategy in which most of the computer time is spent on a prolonged isothermal process slightly below the glass-transition temperature, Tg. The glassy sample prepared in this way demonstrates significant energetic stability, slow dynamics, and well-developed short-range icosahedral order. By conventional uniform cooling, similar properties can only be obtained using a cooling rate more than 15 times slower.

The formation and development processes of dislocation in graphene are investigated by performing tight-binding molecular dynamics (TBMD) simulation and ab initio total energy calculation. It is found that the coalescence of pentagon-heptagon (5-7) pairs with vacancy defects induces the formation of dislocation due to the separation of two 5-7 pairs. In TBMD simulations, adatoms are ejected and evaporated from graphene surface so that the dislocation is developed. It is observed that diffusing carbon atoms nearby dangling bonds help non-hexagonal rings change into stable hexagonal rings. These results might give some ideas for the control of structural properties by inducing defect structures.

We show that pentagon–heptagon (5–7) pair defects in carbon nanotube play an important role as the center of vacancy reconstruction using tight-binding molecular dynamics simulations and ab initio total energy calculations. Single vacancy defect diffuses toward and coalesces with 5–7 pair defects and the coalescence structure is reconstructed into a new and more stable 5–7 pair defect plus an adatom by an exchange mechanism. In the case of four single vacancy defects, the vacancy defects coalesce with 5–7 pair defects and form defect structures with nonhexagonal rings. Finally, these defective structures reconstruct into two new 5–7 pair defects.

In diamond and zinc blende crystals, the competition between glide and shuffle-set slips has been intensively studied. In particular, the undissociated screw dislocation in Si seen at low temperature about five years ago was generally believed to be shuffle set. In this letter, the authors have performed tight-binding and density functional theory calculations that show that a glide-set C core has lower energy than the shuffle-set A core after period-doubling reconstruction. Since the C core can cross slip between two glide-set planes, it satisfies all the experimental observations to date, and may play important roles in dislocation cross slip and ductile-to-brittle transition in these materials.