Direct evidence of interaction-induced Dirac cones in a monolayer silicene/Ag(111) system

Abstract

Silicene, analogous to graphene, is a one-atom-thick 2D crystal of silicon, which is expected to share many of the remarkable properties of graphene. The buckled honeycomb structure of silicene, along with enhanced spin-orbit coupling, endows silicene with considerable advantages over graphene in that the spin-split states in silicene are tunable with external fields. Although the low-energy Dirac cone states lie at the heart of all novel quantum phenomena in a pristine sheet of silicene, a hotly debated question is whether these key states can survive when silicene is grown or supported on a substrate. Here we report our direct observation of Dirac cones in monolayer silicene grown on a Ag(111) substrate. By performing angle-resolved photoemission measurements on silicene(3 × 3)/Ag(111), we reveal the presence of six pairs of Dirac cones located on the edges of the first Brillouin zone of Ag(111), which is in sharp contrast to the expected six Dirac cones centered at the K points of the primary silicene(1 × 1) Brillouin zone. Our analysis shows clearly that the unusual Dirac cone structure we have observed is not tied to pristine silicene alone but originates from the combined effects of silicene(3 × 3) and the Ag(111) substrate. Our study thus identifies the case of a unique type of Dirac cone generated through the interaction of two different constituents. The observation of Dirac cones in silicene/Ag(111) opens a unique materials platform for investigating unusual quantum phenomena and for applications based on 2D silicon systems.S ilicene is theoretically predicted to be stable in the honeycomb lattice and, like its cousin graphene, it harbors the characteristic low-energy Dirac cone states (1-6). Silicene can thus be expected to share many of the remarkable quantum properties of graphene (7-10). Distinct from graphene, however, which is dominated by sp 2 bonding and assumes an essentially flat structure, the crystal structure of silicene is naturally buckled as a result of the mixed sp 2 /sp 3 bonding (2-5). The stronger spinorbit coupling strength in silicene (11, 12) leads to a larger energy gap at the Dirac points, which would make it possible to access the quantum spin Hall effect experimentally at a higher temperature (11, 12). The buckled honeycomb structure drives a number of new phenomena and properties in silicene. In particular, the gap at the Dirac point can be tuned by applying external electric and magnetic fields to realize a variety of different phases and topological phase transitions (13-17). The unique advantages of silicene and its compatibility with the traditional silicon industry make it an attractive materials platform for next-generation nanoelectronics applications (18-23).Single-layer and multilayer silicenes have been grown on various supporting materials, with the Ag(111) surface being the most common substrate (24-30). The buckled structure of silicene naturally leads to the formation of a variety of configurations beyond the primary (1 × 1) structure under different preparatio...

Superstructure-Induced Splitting of Dirac Cones in Silicene

“…The band structure along theK-M-K direction also shows a linear dispersion. These results indicate the existence of a pair of Dirac cones at the BZ boundary of Ag(111), which is consistent with previous ARPES results [24]. It should be noted that our results were obtained with a different photon energy; this is reasonable because these bands originate from the surface layers and have a two-dimensional character.…”

“…As is the case with graphene, theoretical calculations have predicted that free-standing silicene hosts a Dirac cone at each K (K ′ ) point [19,20]. In 3×3-silicene/Ag(111), however, early angle-resolved photoemission spectroscopy (ARPES) measurements and theoretical calculations showed that the Dirac cones of free-standing silicene are destroyed because of the strong band hybridization between silicene and Ag(111) [16,[21][22][23][24][25]. Recently, high-resolution ARPES experiments revealed the existence of six pairs of Dirac cones at the Brillouin zone (BZ) boundary of Ag(111), but these Dirac cones originate from the substrate-overlayer interaction instead of from the free-standing silicene [24].…”

“…In 3×3-silicene/Ag(111), however, early angle-resolved photoemission spectroscopy (ARPES) measurements and theoretical calculations showed that the Dirac cones of free-standing silicene are destroyed because of the strong band hybridization between silicene and Ag(111) [16,[21][22][23][24][25]. Recently, high-resolution ARPES experiments revealed the existence of six pairs of Dirac cones at the Brillouin zone (BZ) boundary of Ag(111), but these Dirac cones originate from the substrate-overlayer interaction instead of from the free-standing silicene [24]. To date, the physics on the origin of the Dirac cone pairs is still unclear despite the recent success of band unfolding analysis based on density functional theory (DFT) [26,27].…”

Large area planar stanene epitaxially grown on Ag(1 1 1)

“…The Dirac band structure of free-standing stanene may be destroyed for the present system of epitaxial stanene on a Ag 2 Sn surface alloy. However, further experimental and theoretical studies, may reveal still unnoticed enticing properties, such as the actual presence of interaction induced Dirac cones at previously insufficiently explored regions of the band structure, as has been revealed recently for epitaxial silicene on Ag (1 1 1) [32,33], which was considered before as loosing its most exciting properties [34]. We also recall that structural flatness of stanene is predicted to drive RT QSH effects [19].…”

Two-Dimensional Massless Dirac Fermions in Antiferromagnetic AFe2As2 Chen1, Wang2, Song3 et al. 2017Phys. Rev. Lett.266220View full textAdd to dashboardCite

“…Therefore, it is a challenge to achieve MDF in magnetic ground states of solid-state electronic systems [11,12]. Moreover, two-dimensional (2D) MDF, which were realized in 2D materials and on the surfaces of threedimensional (3D) topological insulators [1][2][3][13][14][15][16][17], have rarely been observed in the bulk of 3D crystals [11]. The discovery of iron-arsenide superconductors not only brings people a new class of unconventional superconductivity [18], but also sheds light on seeking 2D MDF in magnetic compounds [19,20].…”