We investigate the electronic properties of graphene nanoflakes on Ag(111) and Au(111) surfaces by means of scanning tunneling microscopy and spectroscopy as well as density functional theory calculations. Quasiparticle interference mapping allows for the clear distinction of substrate-derived contributions in scattering and those originating from graphene nanoflakes. Our analysis shows that the parabolic dispersion of Au(111) and Ag(111) surface states remains unchanged with the band minimum shifted to higher energies for the regions of the metal surface covered by graphene, reflecting a rather weak interaction between graphene and the metal surface. The analysis of graphene-related scattering on single nanoflakes yields a linear dispersion relation E(k), with a slight p-doping for graphene/Au(111) and a larger n-doping for graphene/Ag(111). The obtained experimental data (doping level, band dispersions around EF, and Fermi velocity) are very well reproduced within DFT-D2/D3 approaches, which provide a detailed insight into the site-specific interaction between graphene and the underlying substrate.

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...

Chirality-induced spin polarization at chiral electrodes probed by magnetic resonance.

By using Fourier-transform scanning tunneling spectroscopy we measure the interference patterns produced by the impurity scattering of confined Dirac quasiparticles in epitaxial graphene nanoflakes. Upon comparison of the experimental results with tight-binding calculations of realistic model flakes, we show that the characteristic features observed in the Fourier-transformed local density of states are related to scattering between different transverse modes (subbands) of a graphene nanoflake and allow direct insight into the gapped electronic spectrum of graphene. We also observe a strong reduction of quasiparticle lifetime which is attributed to the interaction with the underlying substrate. In addition, we show that the distribution of the on-site energies at flower defects leads to an effectively broken pseudospin selection rule, where intravalley backscattering is allowed. [17,18]. With regard to the electronic transport through graphene nanoribbons, the effect of impurity scattering and edge disorder becomes an important issue. A powerful tool to examine the quasiparticle interference (QPI) effects in graphene due to scattering at defects and edges is scanning tunneling microscopy and spectroscopy [19][20][21][22]. The observed QPI is directly related to modulations in the local density of states (LDOS) [23,24] and provides access to the present scattering vectors and thus to the electronic structure of graphene [25][26][27][28]. Recently, the influence of local scattering centers on the local density of states in graphene nanoribbons has been studied theoretically [29]. The interplay between single impurity scattering and size quantization was shown to generate characteristic spectral features in the Fourier transform (FT) LDOS that can be related to the transverse modes of the nanoribbon.Here we present a comprehensive study of size quantization in epitaxial graphene nanoflakes (GNFs) on Ag(111) upon analysis of QPI by STM and tight-binding simulations of realistic model flakes. We indeed find the characteristic features in the FT-LDOS related to scattering between different transverse modes of a GNF as predicted by theory. Detailed analysis of the scattering features allows one to gain a profound insight into the behavior of charge carriers in graphene flakes, including discrete electronic spectrum and quasiparticle lifetimes, as well as effects of pseudospin.Graphene nanoflakes were initially grown on Ir(111) and decoupled by noble metal intercalation as described elsewhere [13,30]. STM and STS measurements were carried out in an Omicron cryogenic STM setup in ultrahigh vacuum at T = 5−10 K. Differential conductance (dI/dV ) maps were * mikhail.fonin@uni-konstanz.de obtained using a standard lock-in technique with modulation voltages V mod = 3 mV(rms) and at frequencies f mod = 600−800 Hz. Tight-binding calculations were performed using an atomistic recursive Green's function formalism including the effects of trigonal warping in order to account for the relatively large doping level of graphene on A...

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