Figure 1

(a) Constant current $90\times 43{\mathrm{nm}}^2$ STM image of epitaxial graphene on SiC(0001), with two adjacent monolayer (ML) and bilayer (BL) graphene terraces. The spatial derivative of the image is shown, to enhance the corrugation due to the interface states which is higher on ML terraces [10]. Sample bias: $+980\mathrm{mV}$, tunneling current: 0.15 nA. (b) Schematic Fermi surface for electron-doped ML and BL graphene. (c) Illustration of an intervalley scattering process. (d) 3D rendered $50\times 50{\mathrm{nm}}^2$ constant current image of two adjacent ML and BL terraces, taken at low sample bias ($+1\mathrm{mV}$). A long wavelength scattering pattern is found on the BL terrace (left), and not on the ML terrace (right). Tunneling current: 0.2 nA. (e) Schematic of forbidden intravalley backscattering for ML graphene. Small arrows sketch the pseudospin direction. (f) Schematic of intravalley backscattering for BL graphene.Reuse & Permissions

Figure 2

(a), (b) Low-bias STM images of 50 nm wide monolayer (a) and bilayer (b) terraces. Sample bias and tunneling current are, respectively, $+2\mathrm{mV}$ and 0.4 nA for (a), $+4\mathrm{mV}$ and 0.13 nA for (b). (c), (d) Two-dimensional fast Fourier transform (FFT) maps of the STM images (a) and (b). (e) Central region of (c), showing no intravalley backscattering related ring (the arrow points out the position where such a ring should appear). (f) One of the outer pockets of (c). (g) Central region of (d), showing a clear ringlike feature of radius $2q_F$ related to intravalley backscattering. (h) One of the outer pockets of (d). Outer pockets shown in (f) and (h) are centered at the $K$ (or $K^{\prime }$) point and result from intervalley scattering.Reuse & Permissions

Figure 3

(a)–(c) Theoretical calculation of the Fourier Transform (FT) of the LDOS at $E_F$ for different graphene layers with a single delta-function impurity. (a) 3D rendered FT map for graphene monolayer with a Dirac energy $E_D=E_F-0.45\mathrm{eV}$. The calculation predicts no central ring of radius $2q_F$, showing the inefficiency of intravalley backscattering. (b) Same representation as (a) for a bilayer, showing a clear central $2q_F$ ring. $E_D=E_F-0.3\mathrm{eV}$. An asymmetry of $\sim 0.1\mathrm{eV}$ between the two layers has been included, to account for the different electron density reported by ARPES (12,13). (c) Same representation as (a) for an asymmetric monolayer, with $E_D=E_F-0.45\mathrm{eV}$. A site energy difference of 0.25 eV is introduced between the carbon atoms of each unit cell, and the impurity is placed on a carbon atom with high site energy. The symmetry breaking between the two carbon sublattices restores the efficiency of intravalley backscattering as shown by the reappearance of a central ring in the FT map.Reuse & Permissions