Tunable Spin-Dependent Properties of Zigzag Silicene Nanoribbons Nam B. Le, 1,2 Tran Doan Huan, 3 and Lilia M. Woods 1,* 1 Department of Physics, University of South Florida, 4202 East Fowler Avenue, Tampa, Florida 33620, USA 2 Institute of Engineering Physics, Hanoi University of Science and Technology, 1 Dai Co Viet, Hai Ba Trung, Hanoi 10000, Vietnam 3 Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, Unit 3136, Storrs, Connecticut 06269-3136, USA (Received 30 January 2014; revised manuscript received 10 April 2014; published 11 June 2014) Silicene zigzag nanoribbons are studied using ab initio simulation methods. We find novel structure- property relations influenced by several factors, such as the magnitude of the width, spin polarization, spin- orbit coupling, and extended topological defects. It is obtained that while defect-free silicene nanoribbons experience antiferromagnetic-ferromagnetic transition as a function of the width, all defective nanoribbons are ferromagnets. At the same time, the spin-orbit coupling role is significant as it leads to spin-dependent energy gaps in the electronic structure. The origin of edged spin polarization is also studied in terms of the balance between the exchange correlation and kinetic energy contributions. The uncovered unique spin- dependent properties may be useful for the application of silicene nanoribbons in spintronic applications. DOI: 10.1103/PhysRevApplied.1.054002 I. INTRODUCTION Two-dimensional graphene, a monolayer of honeycomb lattice of C atoms [1,2], and its quasi-one-dimensional derivatives, such as nanoribbons [3–5], have been studied extensively due to their extraordinary properties. Even though graphene and its nanoribbons may have desired characteristics for high speed electronics applications [6], their integration into Si-based electronic devices has been challenging. There has been considerable interest in seek- ing analogous materials, which possess the extraordinary properties of graphene, but they are more compatible with existing electronic devices. Silicene, composed of Si atoms in a 2D hexagonal lattice, is a great candidate, which may be better suited for practical electronic applications [7]. Other Si nano- structures have also generated much scientific interest [8–21]. Theoretical studies and density functional theory (DFT) calculations have shown that this Si-based graphene analog and silicene nanoribbons (SiNRs) possess not only graphenelike properties, but also other unique character- istics [8–14]. Recent experimental advances towards syn- thesis [15–21] have opened up new perspectives for applications. Although energy bands in silicene and gra- phene have similar dispersions at characteristic points of the Brillouin zone, the massive nature of the Dirac carriers has made silicene suitable for realizing quantum Hall spin effects and topological insulator features [13,14,22]. Although silicene has not been synthesized in a free- standing form yet, it has been successfully grown on Ag substrates by depositing Si atoms onto Ag(110) [15–17] and Ag(111) [18–20] surfaces. It can also be formed through surface segregation on ZrB 2 thin films grown on Si wafers [21]. The honeycomb structure of silicene is identified using scanning tunneling microscopy [15,16,18], in combination with low-energy electron diffraction [19] or angular-resolved photoemission spectroscopy [20]. While C atoms are in a 2D configuration, characterized by an sp 2 orbital hybridization, Si atoms prefer a low-buckled honeycomb structure [8,9,12,19]. As a result, some sp 3 hybridization can be found in silicene. DFT calculations reveal that the π and π Ã states around the Fermi level of silicene are graphenelike; namely, their energy bands are linear at the Dirac points of the hexagonal Brillouin zone [12]. Silicene, however, has a large spin-orbit coupling (SOC) [14], strengthened by the buckled lattice. The large SOC can be used to control the massive silicene Dirac electron, which can lead to prominent topological phase transitions [23]. Despite the predictions for many exotic features in silicene systems, their structure-property relationships are not fully known and understood yet. The quasi-1D nature, edges, and strong SOC are of particular interest. In this paper, we present computational studies using DFT meth- ods for zigzag SiNRs exploring how their characteristics are influenced by the size of the width, magnetic orienta- tion, and the presence of extended topological defects. It turns out that the SOC plays a critical role for the electronic structure and that there is a width-dependent antiferromag- netic-ferromagnetic transition. In addition, extended topo- logical defects, containing pentagonal and octagonal rings embedded in the hexagonal buckled structure, result in novel spin-polarized properties. * lmwoods@usf.edu PHYSICAL REVIEW APPLIED 1, 054002 (2014) 2331-7019=14=1(5)=054002(6) 054002-1 © 2014 American Physical Society