Lifshitz Transition in a Single-Atom-Thick GdxYb1-xPb3 Kagome Compound on Si(111).

Lanthanide (Ln) elements Gd and Yb alloyed with a Pb monolayer on the Si(111) substrate form LnPb3 compounds having the same crystal structure. They comprise a single-atom-thick Pb layer arranged in a slightly distorted kagome lattice with Ln atoms filling the hexagonal voids. They have similar electronic band structures except for the Fermi level position, which varies between the divalent Yb- and trivalent Gd-containing compounds by ∼0.47 eV. The ability to create a 2D solid solution with the unified continuous Pb layer and hexagonal voids randomly filled with either Gd or Yb atoms allows precise control of the Fermi level position. Small alteration of the Fermi level triggers drastic changes in the Fermi surface topology due to the Lifshitz transition, hence in the physical properties. In particular, the sheet resistance of the GdxYb1-xPb3/Si(111) system can be controllably varied over an order of magnitude range.

Gold Interlayer Promotes Superconductivity in Single and Double Atomic Pb Layers on Si(100).

Introducing an atomic Au monolayer between a Pb film and a Si(100) substrate allows us to fabricate Pb films with single- and double-atom thicknesses. The Pb films have a 2D square-lattice structure with the 1D atomic chains of Pb adatoms on their top, forming Si(100)1 × 7-(Pb, Au) and Si(100)5 × 1-(Pb, Au) superstructures for single and double atomic Pb layers, respectively. Their common characteristic feature is the occurrence of bundles of quasi-1D metallic bands. Transport measurements showed that samples with a Au interlayer demonstrate enhanced superconductor properties, as compared to Pb layers grown on the bare Si(100) surface. Toward improved superconductor properties, the (Pb, Au)/Si(100) system successively avoids risks associated with possible intermixing between adsorbate layers and substrate, as well as with possible Peierls transition into an insulator state, typical for the 1D systems. This finding opens new ways to control low-dimensional superconductivity at the atomic-scale limit.

Weak Antilocalization at the Atomic-Scale Limit of Metal Film Thickness.

Creation of the 2D metallic layers with the thickness as small as a few atomic layers and investigation of their properties are interesting and challenging tasks of the modern condensed-matter physics. One of the possible ways to grow such layers resides in the synthesis of the so-called metal-induced reconstructions on silicon (i.e., silicon substrates covered with ordered metal films of monolayer or submonolayer thickness). The 2D Au-Tl compound on Si(111) surface having [Formula: see text] periodicity belongs to the family of the reconstructions incorporating heavy-metal atoms with a strong spin-orbit coupling (SOC). In such systems, strong SOC results in the spin-splitting of surface-state bands due to the Rashba effect, the occurrence of which was experimentally proved. Another remarkable consequence of a strong SOC that manifests itself in the transport properties is a weak antilocalization (WAL) effect, which has never been explored in the metal layers of atomic thickness. In the present study, the transport and magnetotransport properties of the 2D Au-Tl compound on Si(111) surface were investigated at low temperatures down to ∼2.0 K. The compound was proved to show behavior of the 2D nearly free electron gas system with metallic conduction, as indicated by Ioffe-Regel criterion. It demonstrates the WAL effect which is interpreted in the framework of Hikami-Larkin-Nagaoka theory, and possible mechanisms of the electron decoherence are discussed. Bearing in mind that besides the (Au, Tl)/Si(111)[Formula: see text] system, there are many other ordered atomic-layer metal films on silicon differing by composition, structure, strength of SOC, and spin texture, which provide a promising area for prospective investigations of the WAL effect at the atomic-scale limit when the film thickness is less than the electron wavelength.