Rashba splitting of graphene-covered Au(111) revealed by quasiparticle interference mapping

Abstract

We report on low-temperature scanning tunneling spectroscopy measurements on epitaxial graphene flakes on Au(111). We show that using quasiparticle interference (QPI) mapping, we can discriminate between the electronic systems of graphene and Au(111). Beyond the scattering vectors, which can be ascribed to the elastic scattering within each of the systems, we observe QPI features related to the scattering process between graphene states and the Au(111) surface state. This additional interband scattering process at the graphene/Au(111) interface allows the direct quantitative determination of the Rashba-splitting of the Au(111) surface state, which cannot be evaluated from QPI measurements on pure Au(111). This experiment demonstrates a unique local spectroscopic approach to investigate the Rashba-split bands at weakly interacting epitaxial graphene/substrate interfaces. The spin-orbit (SO) interaction in combination with broken space inversion symmetry at surfaces leads to a spin splitting of surface states in heavy metals as experimentally observed by photoemission for instance at the Au(111) [1][2][3], Bi(111) [4,5], and Sb(111) [6] surfaces. This so called Rashba-Bychkov effect [7] is further enhanced for surface alloys, such as Bi/Ag(111) [8,9], Pb/Ag(111) [10], and Sb/Ag(111) [11], leading to a giant Rashba splitting. Moreover, recent studies point out the possibility of an induced extrinsic Rashba splitting in graphene on Au [12][13][14]. Compared with the intrinsic spin-orbit coupling (SOC) in graphene, which is in the range of 50 μeV [15][16][17], the Rashba-type splitting induced by the presence of Au has recently been reported to reach 100 meV [14].Although the Rashba-split bands are readily visible in angle-resolved photoemission experiments [1-3], their observation at the local scale by means of scanning tunneling microscopy (STM) employing quasiparticle interference (QPI) mappings is challenging. Within such an experiment, one probes the local density of states (LDOS) oscillations generated by QPIs arising from elastic scattering between different momentum eigenstates. These standing wave patterns give rise to characteristic features in Fourier-transformed local density of states (FT-LDOS) maps, which can be understood using joint density of states (JDOS) considerations [18][19][20], i.e., the simple evaluation of principle scattering vectors connecting states on the constant-energy contour (CEC) of the system. However, in the case of spin-split bands only scattering between states with the same spin polarization occurs, making it impossible to reconstruct the full spin-split band structure by analysis of QPI [21][22][23][24]. In particular cases, the presence of both spin-split and spin-degenerate surface state bands allows interband transitions, which yield the information about the Rashba splitting [25]. In Au(111), however, only spinconserved backscattering within the surface state is observed, which masks the actual spin splitting, hence necessitating a different approach for the local observat...

Valley-dependent spin-orbit torques in two-dimensional hexagonal crystals

“…The underlying nonmagnetic substrate can be either a heavy metal (such as Pt, W, etc.) or a topological insulator and provides Rashba spin-orbit coupling [29,30,41,42]. Topological insulators present the advantage of having both strong interfacial spin orbit coupling and vanishing bulk conductance [41], thereby promoting carrier transport at their surface.…”

“…In parallel to the development of SOT in ferromagnetic structures, the study of spin-orbit coupled transport has also been extended to low-dimensional hexagonal crystals such as graphene. Experimentally, a spin-splitting induced by Rashba spin-orbit coupling has been observed in graphene grown on heavy metals or surface alloys [28][29][30]. Furthermore, a ferromagnetic insulator EuO was successfully deposited on graphene and spin-polarized states were detected [31][32][33].…”

Local electronic properties of the graphene-protected giant Rashba-split BiAg2 surface

“…Here, the energetically unfavorable model of diluted Au atoms underneath graphene on Ni(111) was proposed [23]. However, recent scanning tunneling microscopy/spectroscopy (STM/STS) experiments have not shown any hints of such * elena.voloshina@hu-berlin.de † yuriy.dedkov@uni-konstanz.de ‡ mikhail.fonin@uni-konstanz.de a splitting [27], leaving the question of induced spin-orbit interaction in graphene still open. In order to resolve this controversy and to evaluate the role of the substrate, further experiments on graphene, which is adsorbed on materials demonstrating a strong spin-orbit interaction, are required.…”

Enhancement of spin polarization by chaos in graphene quantum dot systems

“…The range of E is between zero and a fraction of t, the nearest-neighbor hopping energy of graphene, and the maximum value of ∆ R is set to be 0.07t ≈ 200meV, which is the currently experimentally achievable value [24][25][26] . Effect of chaos and Rashba interaction on conductance and spin-polarization fluctuations.…”

“…There are two types of spin-orbit coupling: intrinsic and external. In graphene, the intrinsic spin-orbit coupling is usually quite weak, but significant interaction (e.g., characterized by energy splitting on the order of 200meV) can be realized [24][25][26] through the Rashba effect by depositing graphene on the surface of Ni(111) or Ir(111). Rashba spin-orbit interaction preserves the time-reversal symmetry but breaks the inversion symmetry in the direction perpendicular to the two-dimensional material plane, and has wide applications in spin transport devices [27][28][29][30][31][32][33] .…”