Topological insulator gap in graphene contacted with Pb

Graphene is the most promising two dimensional material for nanoelectronic applications featuring the relativistic-like electronic spectrum. Contact of graphene with various materials and its functionalization allows to manipulate the electronic structure, e.g. to change the conductivity type and band gap creation. The latter is of great interest due to the requirements for graphene transistor realization. Furthermore, graphene contact with heavy/magnetic metals results in the lifting of the spin degeneracy of the Dirac cone, opening the spintronics field for its applications. However, up to now the proposed modifications do not allow to introduce graphene to existing electronic devices.
Another way to vary the Dirac cone structure had been proposed by D. Haldane and developed by C. Kane and E. Mele. It was found that the relativistic effect – spin-orbit interaction – could create a band gap at the Dirac point of graphene; this gap is topologically non-equivalent to band gap in “classical” insulating materials. Such a state is called quantum spin Hall phase and represents a 2D topological insulators class. In order to increase the “intrinsic” spin-orbit coupling in graphene the contact with heavy Pb atoms was recently proposed. This work was devoted to study the electronic structure of graphene/Pb system and to explore the spin-orbit gap formation in this system.
Electronic structure measurements were carried out at the BaDElPh beamline at Elettra by means of Angle-Resolved Photoemission Spectroscopy (ARPES). Graphene was grown on Pt(111) substrate and monolayer of Pb atoms had been intercalated underneath graphene. Fig. 1a presents the scheme of the synthesized system where lead atoms form the rectangular structure that whose ordering was confirmed by low energy electron diffraction. ARPES image of the system taken at the photon energy of 34 eV is shown in Fig. 1b where the graphene Dirac cone is clearly visible. Owing to the charge transfer from Pb to C atoms the upper part of the Dirac cone becomes occupied allowing the study of the dispersion relations near the Dirac point. One can see that the two parts of the cone do not intersect, forming a gap with the width of ~200 meV. Moreover the analysis of the spin structure of the Dirac cone by means of spin-resolved ARPES experiments performed at BESSY II allowed us to separate the “intrinsic” and so-called “Rashba” spin-orbit interactions demostrating the spin-orbit character of the formed band gap.
These results provide a new way for band gap creation in graphene, which can be turned to topological insulator and open the field of topological quantum computations for its applications.

Figure 1a) Sketch of the studied system, the Pb atoms presented by yellow spheres; b) ARPES image of graphene/Pb/Pt(111) in the region of K point, taken as a sum of two spectra with p-and s-polarization of light; c) schematic spin structure of the graphene states in the case of large “intrinsic” spin-orbit interaction d) ARPES mapping of the system in two orthogonal k-directions near the K point of graphene.

 

This research was conducted by the following research team:

Ilya I. Klimovskikh1, Mikhail M. Otrokov2,3, Vladimir Yu. Voroshnin1, Daria Sostina1, Luca Petaccia4, Giovanni Di Santo4, Sangeeta Thakur4, Evgueni V. Chulkov1,2,3,5, and Alexander M. Shikin1


1 Saint Petersburg State University, Saint Petersburg, Russia
2 Tomsk State University, Tomsk, Russian Federation
3 Donostia International Physics Center (DIPC), San Sebastiań/Donostia, Basque Country, Spain
4 Elettra - Sincrotrone Trieste SCpA, Trieste, Italy
5 Departamento de Fıśica de Materiales UPV/EHU, Centro de Fıśica de Materiales CFM-MPCand Centro Mixto CSIC-UPV/EHU, SanSebastiań/Donostia, Basque Country, Spain


Contact persons:

Ilya I. Klimovskikh, email: klimovskih_ilya@mail.ru
Luca Petaccia, email: luca.petaccia@elettra.eu
Alexander M. Shikin, email: ashikin@inbox.ru

 

Reference

Ilya I. Klimovskikh,Mikhail M. Otrokov,Vladimir Yu. Voroshnin,Daria Sostina,Luca Petaccia, Giovanni Di Santo,Sangeeta Thakur,Evgueni V. Chulkov,and Alexander M. Shikin, "Spin−Orbit Coupling Induced Gap in Graphene on Pt(111) with Intercalated Pb Monolayer", ACS Nano 11, 368 (2017); DOI: 10.1021/acsnano.6b05982 & ibid. ACS Nano 11, 10630 (2017);  DOI: 10.1021/acsnano.7b06779

 
Last Updated on Wednesday, 20 December 2017 14:13