LaAu${}_2$ and CeAu${}_2$ surface intermetallic compounds grown by high-temperature deposition on Au(111)

We report on the crystal structure and electronic bands of LaAu${}_2$ and CeAu${}_2$ surface intermetallic compounds grown by high-temperature deposition on Au(111). By scanning-tunneling microscopy we study the formation of different alloy phases as a function of growth temperature and lanthanide coverage. We determine the specific growth conditions to achieve monolayers and bilayers of LaAu${}_2$ and CeAu${}_2$ with high crystalline quality. Due to lattice mismatch with the underlying Au substrate, both LaAu${}_2$ and CeAu${}_2$ exhibit long-range moiré patterns, which can serve as templates for further nanostructure growth. By angle-resolved photoemission we map the two-dimensional band structure of these surface alloys, discussing the nature of the different spectral features in the light of first-principles calculations.

Figure 1

STM images of the LaAu${}_2$ surface intermetallic compound taken at different length scales. (a) At a large scale, the hexagonal moiré superstructure is characterized by slightly undulated rows of triangles. (b) The wavy appearance of the moiré is linked to a variable misalignment of the atomic lattice with respect to the moiré structure, with a maximum of $4^{\circ }$, as shown in the overlaid drawing. The LaAu${}_2$ unit cell is sketched in the inset. (c) and (d) Atomically resolved STM images of the LaAu${}_2$ surface alloy revealing one protrusion (La) per unit cell (marked black) in (c) and two protrusions (Au atoms) in a honeycomb arrangement (marked red) in (d). Image parameters: (a, b) $-2$ V, 0.1 nA, (c) $-2$ V, 1 nA, and (d) $-2$ V, 0.1 nA.

Figure 2

(a) Large-scale STM view obtained after evaporation of Ce on Au(111) at 750 K. Around 70% of the surface is covered by CeAu${}_2$ moiré phase. (b) Atomically resolved STM images of the CeAu${}_2$ moiré phase. The inset shows the CeAu${}_2$ unit cell. In the image, the large unit cell marks the moiré superstructure while the small hexagonal unit cell refers to the CeAu${}_2$ atomic lattice. (c) Morphology of the Ce-Au alloy at triangular cluster areas away from the CeAu${}_2$ moiré lattice. The three lobes that form a single cluster are arranged forming a triangle. The circles in the bottom part of the image denote the quasihexagonal arrangement, where each circle represents a cluster. The surface unit cell is marked in black. The triangular clusters with a double-chain structure display an overall rectangular surface unit cell that is represented by the small circles in the top part of the image. Image parameters: (a) $-1.5$ V, 0.5 nA, (b) 0.05 V, 2 nA, and (c) 1 V, 1 nA.

Figure 3

STM pictures for Ce evaporated on Au(111) at 650 K. The growth conditions permit the nucleation of a pure CeAu${}_2$ moiré phase, eliminating the triangular clusters of Fig. 2c. However, the lower substrate temperature leads to a rougher morphology with coexisting monolayer and bilayer islands. (a) Topography and line profile. (b) Current image enhancing the moiré corrugation, which is attenuated in 2 ML thick islands with respect to the 1 ML structure.

Figure 4

4$f$ Au core-level spectra for Au(111) compared to one and two monolayers of LaAu${}_2$ and one monolayer of CeAu${}_2$, both grown by high-temperature deposition. Surface core-level shifts are marked as peaks 2 and 4. The 4$f$ component related to the surface alloy appears slightly shifted with respect to bulk Au metal (peaks 1 and 3). The insets show the intensity (peak height) variation of the Shockley surface state of Au(111) and the Au 4$f$ intermetallic compound core-level line (peak 3) across the evaporated wedges of La and Ce. The surface state quenching marks the 1 ML coverage.

Figure 5

Top: Two-dimensional bands measured along the $\overline{\Gamma K}$ symmetry direction of (a) 1 ML CeAu${}_2$, (b) 1 ML LaAu${}_2$, and (c) 2 ML LaAu${}_2$ surface intermetallic compounds. The different bands are labeled as in GdAu${}_2$,[8] due to the clear analogy among the three different lanthanide intermetallic compounds. All the data correspond to the second derivative of the photoemission spectra, and have been measured with $h\nu =44$ eV for CeAu${}_2$, and $h\nu =33$ eV for LaAu${}_2$. The corresponding bands for GdAu${}_2$ have been published elsewhere.[8] Bottom: corresponding theoretical band structures. The $\overline{\Gamma KM\Gamma }$ symmetry directions refer to those of the LaAu${}_2$ 2D crystal. The width of the bands gives the relative weight of orbital projections on the $m=0$ channel for all angular momenta. Relative shifts (in particular for 2 ML LaAu${}_2$) are due to the imposition of commensurability with the substrate and the large resulting strain. The spin-up polarization is shown for the CeAu${}_2$.

Figure 6

Constant energy surfaces at various energies for (a) 1 ML LaAu${}_2$, (b) CeAu${}_2$, and (c) GdAu${}_2$, all measured with $h\nu =44$ eV. Maps correspond to the second derivative of the photoemission intensity. Several hexagonal moiré Brillouin zones are indicated and the labels correspond to the ones used in Fig. 5. Some features appear replicated due to the moiré superlattice. At the Fermi level, the dominant intensity corresponds to the star-like band C${}^{\prime }$, which is brought to the zone center through moiré umklapps. At $-0.65$ eV in (a) and (b) and at $-0.6$ eV in (c), A and C merge into a single moiré-nested band.

Figure 7

Energy distribution curves for (a) 1 ML and (b) 2 ML, as measured at $\overline{\Gamma }$ ($k_{\left|\right|}=0$) for the different REAu${}_2$ surface alloys, including GdAu${}_2$ from Ref. 7. Peak energies are listed in Table .