Spin-orbit coupling induced gap in graphene on Pt(111) with intercalated Pb monolayer

Graphene is one of the most promising materials for nanoelectronics owing to its unique Dirac cone-like dispersion of the electronic state and high mobility of the charge carriers. However, to facilitate the implementation of the graphene-based devices, an essential change of its electronic structure, a creation of the band gap should controllably be done. Brought about by two fundamentally different mechanisms, a sublattice symmetry breaking or an induced strong spin-orbit interaction, the band gap appearance can drive graphene into a narrow-gap semiconductor or a 2D topological insulator phase, respectively, with both cases being technologically relevant. The later case, characterized by a spin-orbit gap between the valence and conduction bands, can give rise to the spin-polarized topologically protected edge states. Here, we study the effect of the spin-orbit interaction enhancement in graphene placed in contact with a lead monolayer. By means of angle-resolved photoemission spectroscopy, we show that intercalation of the Pb interlayer between the graphene sheet and the Pt(111) surface leads to formation of a gap of 200 meV at the Dirac point of graphene. Spin-resolved measurements confirm the splitting to be of a spin-orbit nature, and the measured near-gap spin structure resembles that of the quantum spin Hall state in graphene, proposed by Kane and Mele [ Phys. Rev. Lett. 2005, 95, 226801 ]. With a bandstructure tuned in this way, graphene acquires a functionality going beyond its intrinsic properties and becomes more attractive for possible spintronic applications.The work was partially supported by grants from Saint Petersburg State University for scientific investigations (Nos. 11.38.271.2014 and 15.61.202.2015) and DFG - SPbU grant No. 11.65.42.2017. We acknowledge financial support from the University of Basque Country UPV/EHU (Grant No. GIC07-IT-756-13), the Departamento de Educacion del Gobierno Vasco and the Spanish Ministerio de Ciencia e Innovacion (Grant No. FIS2010-19609-C02-01), the Spanish Ministry of Economy and Competitiveness MINECO (Grant No. FIS2013-48286-C2-1-P), and the Tomsk State University Academic D. I. Mendeleev Fund Program in 2015 (Research Grant No. 8.1.05.2015).Peer Reviewe

Reply to "comment on 'spin-orbit coupling induced gap in graphene on Pt(111) with intercalated Pb monolayer'"

In a recent article, we studied the electronic and spin structure of graphene on Pt(111) with intercalated Pb monolayer. By means of ARPES, we have shown that the Dirac cone of graphene is characterized by the gap between the π and π* states in this system. The spin texture of the π and π* states and its correspondence with the Kane and Mele model for graphene with spin−orbit gap have been unveiled by spinresolved ARPES. In the Comment on our paper, Dedkov and Voloshina claim that (1) the notation of the superstructure used in our work is incorrect, and (2) spin-resolved data treatment is inappropriate. In this Reply, we show that the superstructure notation is indeed correct. Moreover, the statistical analysis of the reported spin-resolved ARPES data and new experimental data with better statistics support the main conclusion of our article.The work was partially supported by grant of Saint Petersburg State University for scientific investigations (No. 15.61.202.2015) and DFG - SPbU grant No. 11.65.42.2017. We acknowledge support by the University of the Basque Country (Grant Nos. GIC07IT36607 and IT-756-13), the Spanish Ministry of Science and Innovation (Grant Nos. FIS2013-48286-C02-02-P, FIS2013-48286-C02-01-P, and FIS2016-75862-P), and Tomsk State University competitiveness improvement programme (Project No. 8.1.01.2017). The part of photoemission measurements was supported by Russian Science Foundation (Project No. 17-12-01047). The authors acknowledge support from the Russian−German laboratory at BESSY II and the “German−Russian Interdisciplinary Science Center”(G-RISC) program.Peer Reviewe

Direct Spectroscopic Evidence of Magnetic Proximity Effect in MoS2 Monolayer on Graphene/Co

A magnetic field modifies optical properties and provides valley splitting in a molybdenum disulfide (MoS2) monolayer. Here we demonstrate a scalable approach to the epitaxial synthesis of MoS2 monolayer on a magnetic graphene/Co system. Using spin- and angle-resolved photoemission spectroscopy we observe a magnetic proximity effect that causes a 20 meV spin-splitting at the (Gamma) over bar point and canting of spins at the (K) over bar point in the valence band toward the in-plane direction of cobalt magnetization. Our density functional theory calculations reveal that the in-plane spin component at (K) over bar is localized on Co atoms in the valence band, while in the conduction band it is localized on the MoS2 layer. The calculations also predict a 16 meV spin-splitting at the (Gamma) over bar point and 8 meV (K) over bar-(K) over bar' valley asymmetry for an out-of-plane magnetization. These findings suggest control over optical transitions in MoS2 via Co magnetization. Our estimations show that the magnetic proximity effect is equivalent to the action of the magnetic field as large as 100 T

Narrow photoluminescence and Raman peaks of epitaxial MoS2 on graphene/Ir(111)

We report on the observation of photoluminescence (PL) with a narrow 18 meV peak width from molecular beam epitaxy grown MoS2 on graphene/lr(1 1 1). This observation is explained in terms of a weak graphene-MoS2 interaction that prevents PL quenching expected for a metallic substrate. The weak interaction of MoS2 with the graphene is highlighted by angle-resolved photoemission spectroscopy and temperature dependent Raman spectroscopy. These methods reveal that there is no hybridization between electronic states of graphene and MoS2 as well as a different thermal expansion of both materials. Molecular beam epitaxy grown MoS2 on graphene is therefore an important platform for optoelectronics which allows for large area growth with controlled properties

Magneto-spin−orbit graphene: Interplay between exchange and spin−orbit couplings

A rich class of spintronics-relevant phenomena require implementation of robust magnetism and/or strong spin-orbit coupling (SOC) to graphene, but both properties are completely alien to it. Here, we for the first time experimentally demonstrate that a quasi-freestanding character, strong exchange splitting and giant SOC are perfectly achievable in graphene at once. Using angle- and spin-resolved photoemission spectroscopy, we show that the Dirac state in the Au-intercalated graphene on Co(0001) experiences giant splitting (up to 0.2 eV) while being by no means distorted due to interaction with the substrate. Our calculations, based on the density functional theory, reveal the splitting to stem from the combined action of the Co thin film in-plane exchange field and Au-induced Rashba SOC. Scanning tunneling microscopy data suggest that the peculiar reconstruction of the Au/Co(0001) interface is responsible for the exchange field transfer to graphene. The realization of this "magneto-spin-orbit" version of graphene opens new frontiers for both applied and fundamental studies using its unusual electronic bandstructure.The authors acknowledge support by the Saint Petersburg State University (Grant 15.61.202.2015), German-Russian Interdisciplinary Science Center (G-RISC) funded by the German Federal Foreign Office via the German Academic Exchange Service (DAAD) and Russian-German laboratory at BESSY II (Helmholtz-Zentrum Berlin). The funding by the University of the Basque Country (Grants GIC07IT36607 and IT-756-13), the Spanish Ministry of Science and Innovation (Grants FIS2013-48286-C02-02-P, FIS2013-48286-C02-01-P, and FIS2016-75862-P) and Tomsk State University competitiveness improvement programme (Project No. 8.1.01.2017) is also gratefully acknowledged. I.P.R. acknowledges support by the Ministry of Education and Science of the Russian Federation within the framework of the governmental program “Megagrants” (state task no. 3.8895.2017/P220).Peer reviewe

Probe-dependent Dirac-point gap in the gadolinium-doped thallium-based topological insulator TlBi0.9Gd0.1Se2

A tunable gap in the topological surface state is of great interest for novel spintronic devices and applications in quantum computing. Here, we study the surface electronic structure and magnetic properties of the Gd-doped topological insulator TlBi0.9Gd0.1Se2. Utilizing superconducting quantum interference device magnetometry, we show paramagnetic behavior down to 2 K. Combining spin- and angle-resolved photoemission spectroscopy with different polarizations of light, we demonstrate that the topological surface state is characterized by the Dirac cone with a helical spin structure and confirm its localization within the bulk band gap. By using different light sources in photoemission spectroscopy, various Dirac-point gap values were observed: 50 meV for hν=18eV and 20 meV for hν=6.3eV. Here, we discuss the gap observation by the angle-resolved photoemission spectroscopy method as a consequence of the scattering processes. Simulating the corresponding spectral function, we demonstrate that the asymmetric energy-distribution curve of the surface state leads to an overestimation of the corresponding gap value. We speculate that 20 meV in our case is a trustworthy value and attribute this gap to be originated by scattering both on magnetic and charge impurities provided by Gd atoms and surface defects. Given the complexity and importance of scattering processes in the topological surface state together with our observations of distinctive photoemission asymmetry, we believe our results are important for research of the massive Dirac fermions in novel quantum materials.This work was supported by St. Petersburg State University Project (ID No. 51126254), by the Russian Science Foundation (Grant No. 18-12-00062), by the Ministry of Science and Higher Education of the Russian Federation (Grant No 2020-1902-01-058), and by the Science Development Foundation under the President of the Republic of Azerbaijan (Grant No. EIF-BGM-4-RFTF-1/2017-1/04/1-M-02). The studies were also carried out at the resource centers of St. Petersburg State University “Physical Methods for Surface Investigation” and “Diagnosis of Functional Materials for Medicine, Pharmacology, and Nanoelectronics.” In addition, the work was supported by the German-Russian Interdisciplinary Science Center (G-RISC) funded by the German Federal Foreign Office via the German Academic Exchange Service (DAAD) and Russian-German Laboratory at BESSY II (Helmholtz Zentrum, Berlin). We thank the Hiroshima Synchrotron Radiation Center (Proposal No. 18BG026), Helmholtz-Zentrum Berlin für Materialien und Energie for the allocation of synchrotron radiation beam times, and the N-BARD, Hiroshima University for supplying liquid helium. A.K. was financially supported by KAKENHI (Grants No. 17H06138, No. 17H06152, and No. 18H03683).Peer reviewe

Large-Scale Sublattice Asymmetry in Pure and Boron-Doped Graphene

The implementation of future graphene-based electronics is essentially restricted by the absence of a band gap in the electronic structure of graphene. Options of how to create a band gap in a reproducible and processing compatible manner are very limited at the moment. A promising approach for the graphene band gap engineering is to introduce a large-scale sublattice asymmetry. Using photoelectron diffraction and spectroscopy we have demonstrated a selective incorporation of boron impurities into only one of the two graphene sublattices. We have shown that in the well-oriented graphene on the Co(0001) surface the carbon atoms occupy two nonequivalent positions with respect to the Co lattice, namely top and hollow sites. Boron impurities embedded into the graphene lattice preferably occupy the hollow sites due to a site-specific interaction with the Co pattern. Our theoretical calculations predict that such boron-doped graphene possesses a band gap that can be precisely controlled by the dopant concentration. B-graphene with doping asymmetry is, thus, a novel material, which is worth considering as a good candidate for electronic applications

Origin of Giant Rashba Effect in Graphene on Pt/SiC

Intercalation of noble metals can produce giant Rashba-type spin–orbit splittings in graphene. The spin–orbit splitting of more than 100 meV has yet to be achieved in graphene on metal or semiconductor substrates. Here, we report the p-type graphene obtained by Pt intercalation of zero-layer graphene on SiC substrate. The spin splitting of ∼200 meV was observed at a wide range of binding energies. Comparing the results of theoretical studies of different models with the experimental ones measured by spin-ARPES, XPS and STM methods, we concluded that inducing giant spin–orbit splitting requires not only a relatively close distance between graphene and Pt layer but also the presence of graphene corrugation caused by a non-flat Pt layer. This makes it possible to find a compromise between strong hybridization and increased spin–orbit interaction. In our case, the Pt submonolayer possesses nanometer-scale lateral ordering under graphene