Experimental evidence of monolayer ${\mathrm{AlB}}_2$ with symmetry-protected Dirac cones

Monolayer ${\mathrm{AlB}}_2$ is composed of two atomic layers: honeycomb borophene and triangular aluminum. In contrast with the bulk phase, monolayer ${\mathrm{AlB}}_2$ is predicted to be a superconductor with a high critical temperature. Here, we demonstrate that monolayer ${\mathrm{AlB}}_2$ can be synthesized on Al(111) via molecular beam epitaxy. Our theoretical calculations revealed that the monolayer ${\mathrm{AlB}}_2$ hosts several Dirac cones along the $\mathrm{\Gamma }\text{−}M$ and $\mathrm{\Gamma }\text{−}K$ directions; these Dirac cones are protected by crystal symmetries and are thus resistant to external perturbations. The extraordinary electronic structure of the monolayer ${\mathrm{AlB}}_2$ was confirmed via angle-resolved photoemission spectroscopy measurements. These results are likely to stimulate further research interest to explore the exotic properties arising from the interplay of Dirac fermions and superconductivity in two-dimensional materials.

Figure 1

(a) An STM image of boron on Al(111) showing the triangular corrugations. (b) Magnified STM image showing the honeycomb lattice of boron. (c) and (d) LEED patterns of Al(111) and B/Al(111), respectively. (e) and (f) Top and side views of the structure model of B/Al(111). The white rhombus indicates a unit cell of honeycomb borophene or monolayer ${\mathrm{AlB}}_2$. The boron and topmost Al atoms are indicated by yellow and blue balls, respectively. The underlying Al atoms are indicated by gray balls.

Figure 2

(a) Calculated phonon spectrum of the honeycomb borophene. (b), (c) Calculated phonon spectrum of the monolayer ${\mathrm{AlB}}_2$ with lattice constants of 2.98 and 2.86 Å, respectively. The optimized lattice constant of the freestanding ${\mathrm{AlB}}_2$ monolayer is 2.98 Å. (d) Calculated band structure of the monolayer ${\mathrm{AlB}}_2$. The three characteristic bands are indicated by $\alpha$, $\beta$, and $\gamma$. Red arrows indicate the Dirac cones protected by mirror symmetry along the high-symmetry lines. The “$+$” and “$-$” signs (in blue) along $\mathrm{\Gamma }\text{−}M$ and $\mathrm{\Gamma }\text{−}K$ are the mirror eigenvalues of ${\mathrm{M}}_{\mathrm{\Gamma }M}$ and ${\mathrm{M}}_{\mathrm{\Gamma }K}$, respectively. The red circle indicates the Dirac cone derived from the $p_z$ orbitals of boron.

Figure 3

(a) ARPES second derivative image of pristine Al(111) measured with 25-eV photons. (b) and (c) ARPES second derivative images of ${\mathrm{AlB}}_2$/Al(111) along the $\mathrm{\Gamma }\text{−}K$ and $\mathrm{\Gamma }\text{−}M$ directions, respectively. $\alpha$, $\beta$, and $\gamma$ indicate the three characteristic bands of freestanding ${\mathrm{AlB}}_2$. The Dirac points are indicated by black arrows. The incident photon energy is 35 eV. (d) ARPES second derivative image of ${\mathrm{AlB}}_2$/Al(111) along the $\mathrm{\Gamma }\text{−}K$ direction measured with 40-eV photons. Red arrows indicate the surface states of Al(111); black arrows indicate the Dirac points of the monolayer ${\mathrm{AlB}}_2$. The black dashed lines are guides for the eye.

Figure 4

Calculated band structures of ${\mathrm{AlB}}_2$/Al(111) with different additional separations between ${\mathrm{AlB}}_2$ and Al(111): 0, 0.8, 1.4, and 1.8 Å. The calculation results were projected on the surface ${\mathrm{AlB}}_2$ layer. Red ellipses indicate the $\gamma$ and ${\gamma }^{\prime }$ bands. The thicknesses of the lines correspond to the spectral weight of the bands.