Local electronic properties of the graphene-protected giant Rashba-split ${\mathrm{BiAg}}_2$ surface

We report the preparation of an interface between graphene and a strong Rashba-split ${\mathrm{BiAg}}_2$ surface alloy and an investigation of its structure as well as the electronic properties by means of scanning tunneling microscopy/spectroscopy and density functional theory calculations. Upon evaluation of the quasiparticle interference patterns, an unperturbed linear dispersion for the $\pi$ band of $n$-doped graphene is observed. Our results also reveal the intact nature of the giant Rashba-split surface states of the ${\mathrm{BiAg}}_2$ alloy, which demonstrate only a moderate downward energy shift due to the presence of graphene. This effect is explained in the framework of density functional theory by an inward relaxation of the Bi atoms at the interface and subsequent delocalization of the wave function of the surface states. Our findings demonstrate a realistic pathway to prepare a graphene-protected giant Rashba-split ${\mathrm{BiAg}}_2$ for possible spintronic applications.

Figure 1

(a) GNF/Ir(111), (b) GNF/Ag(111)/ Ir(111), and (c) ${\mathrm{GNF}\text{/}\mathrm{BiAg}}_2$/Ir(111). The bottom row shows the corresponding STM and LEED images acquired after every preparation step. Imaging parameters: (a) $U_T=50$ mV, $I_T=500$ pA; (b) $U_T=-150$ mV, $I_T=800$ pA; (c) $U_T=50$ mV, $I_T=700$ pA. LEED images were obtained at 75 eV.

Figure 2

(a) Topographic, $z(x,y)$, and (b) differential conductance, $dI/dV(x,y)$, maps acquired on ${\mathrm{GNF}\text{/}\mathrm{BiAg}}_2$. Imaging parameters: $U_T=+125$ mV, $I_T=900$ pA, $f_{\text{mod}}=687$ Hz, $U_{\text{mod}}=10$ mV. (c) FFT image obtained from (b). The dashed hexagon marks the reciprocal lattice of graphene with vector ${\vec{b}}_{\text{gr}}$. Vector ${\vec{b}}_{\text{Bi}}$ marks the reciprocal lattice of ${\mathrm{BiAg}}_2$. Rectangles are used for areas around the $\mathrm{\Gamma }$ point and the $K$ point of the graphene BZ, zooms of which are shown in (d) and (e), respectively.

Figure 3

(a) STM topography for the simultaneous imaging of areas of ${\mathrm{BiAg}}_2$ and gr/${\mathrm{BiAg}}_2$. Imaging parameters: $75\times 30{\mathrm{nm}}^2,U_T=20$ mV, $I_T=2$ nA. Scattering vector dispersions $k\left(E\right)$ obtained at different $U_T$ corresponding to $D_1,D_2,D_3$ for (b) clean ${\mathrm{BiAg}}_2$ and (c) gr/${\mathrm{BiAg}}_2$. (d) Scattering vector dispersion corresponding to the intervalley scattering in graphene for gr/${\mathrm{BiAg}}_2$.

Figure 4

(a) Top and (b) side views of the gr/${\mathrm{BiAg}}_2$ interface. In (b), the charge difference, $\mathrm{\Delta }{\rho }_{\text{gr}/\text{sub}}\left(r\right)={\rho }_{\text{gr}/\text{sub}}\left(r\right)-[{\rho }_{\text{gr}}\left(r\right)+{\rho }_{\text{sub}}\left(r\right)]$, is color coded, from red $\left(+2e/{\mathrm{nm}}^3\right)$ to blue $\left(-2e/{\mathrm{nm}}^3\right)$.

Figure 5

Calculated energy band dispersions for (a) clean ${\mathrm{BiAg}}_2$ (system A), (b) gr/${\mathrm{BiAg}}_2$ fixed (system B), and (c), (d) gr/${\mathrm{BiAg}}_2$ relaxed (system C).