Anisotropic Metal-Insulator Transition in Epitaxial Thin Films

By comparing the properties of In and Pb quantum wells in a scanning tunneling microscopy subsurface imaging experiment, we found the existence of lateral bound states, a 2D Mott-Hubbard correlation gap, induced by transverse confinement. Its formation is attributed to spin or charge overscreening of quasi-2D excitations. The signature of the 2D confinement-deconfinement transition is also experimentally observed, with the correlation gap being pinned in the middle of the conduction band. A self-organized 2D Anderson lattice is suggested as a new ground state.

Figure 1

A typical STM image of In islands on Si(111), obtained at $V=0.5\mathrm{V}$, $I=0.1\mathrm{nA}$. The image size is $1600\times 1600\mathrm{n}{\mathrm{m}}^{\mathrm{2}}$.

Figure 2

(a) The density of states spectra for different In islands. (b) The dependence of ${\Delta }^{-1}$ on the number of atomic layers. (c) Spectral shift introduced by 1 ML thickness change in a wedge-shaped island. (d) Height distribution histogram for uniform thickness In(111) islands.

Figure 3

(a)–(d) The $400\times 400{\AA }^2$ STM images of 7 ML island, obtained at different tip voltages: (a) $V=250\mathrm{mV}$, (b) $V=-250\mathrm{mV}$, (c) $V=-450\mathrm{mV}$, (d) $V=-850\mathrm{mV}$; ($I=0.2\mathrm{nA}$). (e) Reappearance of a resonant image at $-3.4\mathrm{V}$. (f) The cross sections of Figs. 3b, 3c where large and small peaks originate from subsurface corner holes and dimer dislocations. (g) Hole Fermi surfaces of In and Pb (inner). (h) Anisotropic excitations (vertical columns) and Fermi liquid quasiparticles.

Figure 4

Normalized tunnel $I\mathrm{\text{−}}V$ characteristics of In and Pb films, obtained in $\pm 4\mathrm{V}$ bias range. Upper inset: Tunnel $I\mathrm{\text{−}}V$ characteristic of In island in a bias range $\pm 2\mathrm{V}$. Lower inset: The energy window of 0D states is pinned in the middle of the conduction band: (a) half-filled band, (b) less than half-filled band.