Numerical study on new functionality of spin-heat cross effect Name: ○Qinfang Zhang Laboratory at RIKEN: Computational Condensed Matter Physics Lab

Stimulated by miraculous properties of graphene, scientists show great interests in other two dimensional (2D) monolayer materials, such as silicene, h-BN and boron sheet. Among them, silicene, a silicon film of one atomic thickness as the counterpart of graphene, has been theoretically predicted and experimentally synthesized on Ag(111), Ir(111), and ZrB2(0001) substrates recently. Different from the flat honeycomb lattice of graphene, silicene has to be stabilized by a low buckling of about 0.44 Å as predicted by density functional theory (DFT) calculations. Most importantly, silicene resembles the unique linear Dirac cone of graphene; thus massless fermions in silicene possess ultrahigh Fermi velocity of about 106 m/s, comparable to that of graphene. Such unique electronic property of silicene sheet can be either retained or tailored when it is supported on semiconducting BN and SiC substrates. Moreover, integration of silicene into microelectronic devices is very tempting since it may be compatible with the mature silicon-based semiconductor technology. To date, although the free-standing silicene sheets have not been isolated yet, silicene is believed to have a bright future and the relevant studies are in the stage of booming development in vigor. It is thus urgent to theoretically explore the structure, stability and physical properties of free-standing silicene in advance. Among those fundamental issues, defects are crucial for production and future applications of silicene monolayer materials. Previous results showed that most of the outstanding properties of silicene and graphene rely on the defect-free perfect structures. Even though the formation energies of defects in graphene are rather high (e.g., ~7.5 eV for a single vacancy), there is a cornucopia of reports on the defects in graphene from both experimental observations and theoretical calculations. Naturally, one expect certain amount of defects must also exist in silicene, and some distinct structural defects were actually found in the STM images from recent experiments. In the fabrication of two-dimensional films, the most commonly found defects are grain boundaries, which might be avoided by further improving the growth technique. However, for the purpose of characterization and device applications, the as-prepared 2D films are usually exposed under high-energy irradiations of laser, electrons, and ions, which would certainly induce local point defects, such as Stone-Wales rotation, single and double vacancies (abbreviated as SW, SV and DV, respectively, hereafter). It is conceivable that the intrinsic properties of these monolayer sheets might be remarkably altered once certain amount of defects were generated. In the case of graphene, SV defect and C adatom will result in local magnetic moments. Chen et al. have observed magnetism in defective graphene without presence of transition metal elements, regardless of the specific type of defects. Meanwhile, it was found that SW and DV defects would introduce small gaps in the band structures of graphene but retain the nonmagnetic behavior. In addition, the initial structural defects may be transformed into other defects by knock-off atoms, bond rotation, migration, and aggregation, which rely on the formation energies of various defects and the diffusion barriers on the transformation path. Therefore, understanding the formation and migration of defects as well as their influences on the electronic/magnetic properties of these novel 2D materials (such as graphene and silicene) not only is meaningful for fundamental research, but also provides a powerful route to tailor their physical properties and to control their functional applications in future devices. So far, many efforts have been devoted to the defects in graphene. However, to the best of our knowledge, there is no such study for the recently synthesized silicene. In this paper, we systematically explored a variety of representative point defects in silicene sheet, including SW defect, SVs, DVs, and Si adatom. The atomic structures and their scanning tunneling microscope (STM) images were obtained by ab initio calculations to provide visible guidance for experimental observations. Besides, the formation energies and diffusion barriers of these defects as well as their influences on the local electronic/magnetic properties of silicene were discussed in detail. These results present primitive knowledge of defects in silicene and can give valuable information to avoid or take advantage of defects in future applications of silicene-based materials and devices.