Anisotropic Charge Ordering on the Gallium Surface

Low-temperature scanning tunneling microscopy of atomically flat Ga(001) islands revealed the 2D electronic superlattice composed of stripe domains. Tunnel spectroscopy of these surface electrons indicates the formation of a 2D charge-ordered state of Wigner-crystal type driven by competition of short-range and long-range Coulomb energies. At the boundaries of stripe domains the energy spectra exhibit splitting due to charged excitonic states and shift due to charge doping, altogether indicating the self-assembly of 1D hole stripes. The size distribution of stripe domains is broadened around $4a$.

Figure 1

(a) The elementary unit cell of Ga is composed of dimer molecules kept by covalent bonds (shown as thick rods) and additionally interconnected to each other through 2D network of metallic bonds (shown as thin rods). (b) Top view of Ga unit cell: $a\times a=4.5\times 4.5{\AA }^2$. (c) STM image of flattop Ga(001) islands epitaxially grown on Si(100) surface. The image sizes are $400\times 400{\mathrm{nm}}^2$. The image was obtained at tunneling current $I=0.2\mathrm{nA}$ and tip voltage $V_{\mathrm{tip}}=-500\mathrm{mV}$.

Figure 2

STM image obtained at 77 K on top of Ga(001) island on Si(100). The image sizes are $155\times 120{\AA }^2$. The electronic superlattice, whose periodicity is $9.0\times 9.0{\AA }^2$, is composed of unidirectional stripe domains. Some of the domain boundaries are indicated by dashed lines. The image was obtained at $I=0.2\mathrm{nA}$, $V_{\mathrm{tip}}=-500\mathrm{mV}$.

Figure 3

Tunnel spectra of Ga(001) at 77 K reveal fractionally filled surface Hubbard bands: (a) single-gap spectra in the central portion of domains, (b) double-gap spectra at the domain boundaries. Because of hole doping of domain boundaries, the upper and the lower Hubbard bands (labeled as UHB and LHB) are shifted towards higher energies. Simultaneously, two spectral satellites with relative energy of $-V$ develop below these bands. The inset shows the schematics of energy levels.

Figure 4

(a) STM image obtained at 77 K on top of Ga(001) island on Si(111). The image sizes are $215\times 200{\AA }^2$. The domain boundaries are indicated by dashed lines. The image was obtained at $I=0.2\mathrm{nA}$ and $V_{\mathrm{tip}}=-500\mathrm{mV}$. (b) The Fourier transform of STM image in (a).