物理科学与技术学院物理讲坛（2019年第16-18讲）

报告题目：Ab-initio Simulation of Heat Transport in Liquids and Glasses.

报告人：Stefano Baroni，教授

单 位：Scuola Internazionale Superiore di Studi Avanzati

报告时间：2019年7月3日 下午13:00-14:00

报告地点：赛思学术会堂
报告摘要：I will present a few novel theoretical ideas recently developed in my group, that allow one to simulate heat transport in different classes of materials, hitherto considered out of the scope of ab-initio methods based on electronic-structure theory, i.e. liquids and disordered solids. The main new concept on which all of our developments stand is that of gauge invariance of transport coefficients: in a nutshell, transport coefficients are largely independent of the detailed microscopic representation of the conserved density from which they are derived through the Green-Kubo relation, provided this density integrates to the correct conserved value (the energy in our case) and its correlations are short ranged. Building on this concept, we have developed a full quantum-mechanical expression for the energy current, based on density-functional theory, that allow us to compute the thermal conductivity from ab-initio molecular dynamics and the Green-Kubo theory of linear response. This theory has been then combined with standard lattice dynamics to obtain a compact expression for the heat conductivity, valid in the weakly anharmonic regime, which naturally bridges the Boltzmann-Peierls expression in crystalline solids with a generalisation of the Allen-Feldman model in glasses, thus encompassing the case of weakly disorder solids, for which no solid theory existed so far. A number of numerical benchmarks are presented, along with realistic applications to water and silica and silicon glasses..

报告题目：Controlling the Structural, Electronic and Topological Properties of Substrate Supported Two-dimensional Systems through Interfacial Interaction

报告人：周苗，教授

单 位：北京航空航天大学

报告时间：2019年7月3日 下午14:15-15:15

报告地点：赛思学术会堂
报告摘要：When Nobel laureate Herbert Kroemer coined the famous phrase that “the interface is the device”, he referred to the astonishing success of devices based on thin semiconductor films (especially Si) for photonic and electronic applications that started more than 40 years ago. During the last decade, tremendous efforts have been made to explore the physical properties of two-dimensional (2D) structures, such as graphene, MoS2 and many others. In practice, these structures are always supported on certain substrate, whether during synthesis or device setup, and the substrate tends to affect the intrinsic properties of 2D materials. However, such interfacial interaction can be hardly explored in experiments. In this talk, we will present our progress on the understanding and design of novel physical phenomena and electronic devices via theoretical approaches. Especially, by combining tight-binding modeling, density functional theory and non-equilibrium Green’s function, we designed various topological states on Si(111) substrate (Fig. 1), in which atomic orbital filtering at the interface play an essential role in determining the physical phenomena [1]. We studied the control of the structural and electronic properties of 2D MoS2 supported on metal substrate for fields of electrochemical H2 production and field effect transistors, where in interfacial interaction between MoS2 and metal surface can be effectively tuned [2]. At last, we would also like to show how to manipulate much weaker interfacial interactions for practical interaction, particularly in 2D van der Waals heterostructures. We highlight these theoretical findings in guiding related experiments, and hope to stimulate considerable interest among communities from physics, chemistry and materials science [3]

[1] Proc. Natl. Acad. Sci. U. S. A 111, 14378 (2014); Phys. Rev. Lett. 113, 236802 (2014); Nano Lett. 16, 404 (2016); Science 357, 287 (2017).

[2] Nanotechnology 29, 03LT01 (2017); Phys. Chem. Chem. Phys. 20, 26083 (2018); J. Mater. Chem. C 6, 12245 (2018).

[3] npj Computational Materials 5, 20 (2019); Electron. Struct. 1, 015010 (2019).

报告题目：Graphene on Nickel surfaces: theory and experiments

报告人：Maria Peressi，教授

单 位：University of Trieste

报告时间：2019年7月3日 下午 15:30-16:30

报告地点：赛思学术会堂
报告摘要：The morphologic, electronic and chemical properties of epitaxial graphene layers are strongly determined by the substrate orientation and flatness. By combining extensive numerical simulations (DFT, MD, KMC) with a multi-technique experimental approach (STM, STS, XPS), we performed atomic-scale investigation of graphene grown both on single-crystal and polycrystalline Ni surfaces, unraveling also transient phenomena taking place during the growth process. The structural and bonding configuration of graphene overlayers varies from flat and uniform on Ni(111) [1] to corrugated and with variable strength in moiré patterns on Ni(001) (Figure 1a-f) [2]. Continuous regular 2D carbon layers can also form on stepped polycrystalline Ni surfaces, involving a rearrangement of the substrate and a peculiar steps dynamics [3]. We observed a temperature-driven change of the edge structure from substrate to hydrogen passivation in graphene flakes growing on Ni(111) [4]. On the same substrate, we followed the action of single nickel adatoms diffusing on the surface. These atoms drive the growth process by temporarily attaching at kink sites along the graphene flake edges and facilitating the incorporation of new C atoms in the carbon network (Figure 1g) [5].

This work is partially supported by the Italian Ministry of Foreign Affairs and International Cooperation and the University of Trieste (FRA2018).

[1] F. Bianchini et al, J. Phys. Chem. Lett. 5 (2014) 467-473; V. Carnevali et al, Nanoscale (2019), in press, DOI: 10.1039/C9NR01072F

[2] Z. Zou et al, Carbon 130 (2018) 441-447; A. Sala et al, in preparation

[3] Z. Zou et al, submitted

[4] L.L. Patera et al, Nano Lett. 15 (2015) 56-62

[5] L.L. Patera et al, Science 359 (2018) 1243-1246