报告题目：What Drives the Formation of Twin Domain Boundary in   Two-Dimensional Materials

报告人：Shengbai Zhang教授

报告人单位: Rensselaer Polytechnic Institute

报告时间： 2018年6月04日（周一）下午13:30

报告地点：嘉定园区学术活动中心307房间

报告摘要：Recent high-resolution STM experiment observes twin domain boundaries (TDBs) on the surface of layer-structured single-crystal WTe2. None of the TDBs are thermally-generated, which is in contrast to the current belief on the TBD formation in two-dimensional (2D) systems. Instead, the formation of the TDBs are always associated with surface rippling, indicative of the existence of a strain field in the samples. To resolve the controversy, we preform density functional theory calculations combined with elasticity theory analysis, which shows that the nucleation of TDBs in WTe2 takes place by individual atoms. For such a nucleation to occur, a large critical shear strain (6.4%) is theoretically predicated, which is in good agreement with the experiment (7%).

We further note that, while twinninng mechanism in a three-dimensional (3D) metal such as Al, Ni, or Cu is widely known, twinning mechanism in 2D and layered materials can be fundamentally different. In a 3D metal, the neighboring atomic planes can slip over each other to nucleate a partial dislocation, whereas in 2D such a mechanism does not apply. Instead, twinning in 2D must take a different pathway within the monolayer that has no 3D analogue due to its intrinsically higher activation energy barrier.

报告题目： Recent development in phase change memory materials using time-dependent density functional theory

报告人：Shengbai Zhang教授

报告人单位: Rensselaer Polytechnic Institute

报告时间： 2018年6月04日（周一）下午13:30

报告地点：嘉定园区学术活动中心307房间

报告摘要： Phase change memory (PCM) materials exhibit fascinating physics as SET (i.e., its amorphiszation) happens in less than a 100 fs, while RESET (i.e., its recrystallization) happens in a matter of only a few ns, which defies exclusively all the phase-change phenomena known for solid state. They are the backbone of DVD, as well as that of electronic memory devices such as IBM’s storage class memory and the recent Micron’s X-Point technology. Most recently, PCM also gained momentum for developing non-von Neumann architecture, beyond CMOS, and in-memory computing. Our journey on the quest of the PCM materials started with the understanding of phase transitions using static  and quasi-static  first-principles calculations. Nonetheless, a non-thermal nature of the ultrafast PCM material amorphization under a high electronic excitation was unveiled for the first time , which has since gained considerable momentum. Using the recently developed time-dependent density functional theory (TDDFT)-molecular dynamics (MD), we not only confirmed the previous predictions, but also uncovered the dependence on the excitation energies – a high enough excitation energy inevitably leads to a significant carrier multiplication effect. As such, a phase transition takes place well before the lattice can be heated up. A similar phenomenon found in standard semiconductors, coined with the name plasma quenching, was also explained by our theory . Most recently, we found that, in ferroelectric materials, an equally ultrafast (< a few 100’s fs) phases transition can take place between crystalline phases. While this proposition is in startle contrast to our na？ve intuition, it is in full agreement with experiments .