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.

Emergence of quasi-1D spin-polarized states in ultrathin Bi films on InAs(111)A for spintronics applications.

The discovery, characterization, and control of heavy-fermion low-dimensional materials are central to nanoscience since quantum phenomena acquire an exotic and highly tunable character. In this work, through a variety of comprehensive experimental and theoretical techniques, it was observed and predicted that the synthesis of ultrathin Bi films on the InAs(111)A surface produces quasi-one-dimensional spin-polarized states, providing a platform for the realization of a unique spin-transport regime in the system. Scanning tunneling microscopy and low-energy electron diffraction measurements revealed that the InAs(111)A substrate facilitates the formation of the Bi-dimer phase of 2√3 × 3 periodicity with an admixture of the Bi-bilayer phase under submonolayer Bi deposition. X-ray photoelectron spectroscopy (XPS) measurements have shown the chemical stability of the Bi-induced phases, while spin and angle resolved photoemission spectroscopy (SARPES) observations combined with state-of-the-art DFT calculations have revealed that the electronic spectrum of the Bi-dimer phase holds a quasi-1D hole-like spin-split state at the Fermi level with advanced spin texture, whereas the Bi-bilayer phase demonstrates metallic states with large Rashba spin-splitting. The band structure of the Bi/InAs(111)A interface is discovered to hold great potential as a high-performance spintronics material fabricated in the ultimate two-dimensional limit.

Interplay between magnetic order and electronic band structure in ultrathin GdGe2 metalloxene films.

Dimensionality can strongly influence the magnetic structure of solid systems. Here, we predict theoretically and confirm experimentally that the antiferromagnetic (AFM) ground state of bulk gadolinium germanide metalloxene, which has a quasi-layered defective GdGe2 structure, is preserved in the ultrathin film limit. Ab initio calculations demonstrate that ultrathin GdGe2 films present in-plane intra-layer ferromagnetic coupling and AFM inter-layer coupling in the ground state. Angle-resolved photoemission spectroscopy finds the AFM-induced band splitting expected for the 2 and 3 GdGe2 trilayer (TL) films, which disappear above the Néel temperature. The comparative analysis of isostructural ultrathin DyGe2 and GdSi2 films confirms the magnetic origin of the observed band splitting. These findings are in contrast with the recent report of ferromagnetism in ultrathin metalloxene films, which we ascribe to the presence of uncompensated magnetic moments.

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.

A 2D heavy fermion CePb3 kagome material on silicon: emergence of unique spin polarized states for spintronics.

We report on the successful synthesis of a 2D atomically thin heavy-fermion CePb3 kagome compound on a Si(111) surface. Growth and morphology were controlled and characterized through scanning tunneling microscopy observations revealing the high crystalline quality of the sample. Angle-resolved photoelectron spectroscopy measurements revealed the giant highly-anisotropic Rashba-like spin splitting of the surface states and semi-metallic character of the spectrum. According to the DFT calculations, the occupied hole and unoccupied electron states with huge spin-orbit splitting and out-of-plane spin polarization reside at the M̄ points near the Fermi level EF, which is ≈100 meV above the experimental one. The out-of-plane FM magnetization was found to be preferred with Ce spin and orbital magnetic momenta values of 0.895µB and -0.840µB, respectively. The spin-split states near EF are primarily formed by Pb pxy orbitals with the admixing of Ce d and f electrons due to the Ce f-d hybridization acquired asymmetry with respect to the sign of k∥. The observed electronic structure of the CePb3/Si(111)√3 × âˆš3 system is rather unique and in the hole-doped state, like in our experiment, can be enabled in the tunable spin current regime, which makes it a prospective 2D material for spintronic applications.

Unveiling the hybridization between the Cr-impurity-mediated flat band and the Rashba-split state of the α-Au/Si(111) surface.

Adsorption of foreign atoms onto 2D materials can either lead to ordinary electron doping or the emergence of new electronic effects including topology, superconductivity, and quantum anomalous Hall and Kondo states. We have investigated the effect of Cr doping on the electronic structure of the α-Au/Si(111)- surface and its adsorbate-modified family. It has been found that below a critical coverage of ∼0.05 monolayer, Cr adatoms penetrate beneath the Au and topmost Si layers and induce the occupied resonance flat band in the electronic spectrum as revealed by angle-resolved photoelectron spectroscopy. Further deposition of Cr leads to the growth of the 3D islands spoiling the surface homogeneity. Using density functional theory calculations, we have disclosed the effects of Cr doping on the electronic band structure and revealed the nature of hybridization between the Cr-induced magnetic-split band and the Au-induced Rashba-split surface state. We believe that the synthesized 2D phases and electronic effects produced by magnetic atom doping in the ultimate two-dimensional limit will stimulate further investigations related to the highly correlated phases and will find practical applications in nanoelectronic devices.

Metal Sheet of Atomic Thickness Embedded in Silicon.

The controlled confinement of the metallic delta-layer to a single atomic plane has so far remained an unsolved problem. In the present study, the delta-type structure with atomic sheet of NiSi2 silicide embedded into a crystalline Si matrix has been fabricated using room-temperature overgrowth of a Si film onto the Tl/NiSi2/Si(111) atomic sandwich in ultrahigh vacuum. Tl atoms segregate at the growing Si film surface, and the 1.5-3.0 nm thick epitaxially crystalline Si layer forms atop the NiSi2 sheet. Confinement of the NiSi2 layer to a single atomic plane has been directly confirmed by transmission electron microscopy. The NiSi2 delta-layer demonstrates a p-type conductivity associated with the electronic transport through the two hole-like and one electron-like interface-state bands. The basic structural and electronic properties of the NiSi2 delta-layer remain after keeping the sample in air for one year.

Solving a Long-Standing Problem Regarding Atomic Structure of Si(100)2×3-Ag.

The atomic structure of the Si(100)2×3-Ag reconstruction has remained unknown for more than 25 years since its first observation with scanning tunneling microscopy, despite a relatively small unit cell and seeming abundance of the available experimental data. We propose a structural model of the Si(100)3×2-Ag reconstruction which comfortably fits all the principal experimental findings, including our own and those reported in the literature. The model incorporates 3 Si atoms and 4 Ag atoms per the 2 × 3 unit cell forming linear atomic chains along the 3aSi-periodic direction. A peculiar feature of the Si(100)2×3-Ag structure is the occurrence of the inner Si dimers in the second atomic layer from the top of the Si(100) substrate. The reconstruction is proved to possess semiconducting properties.

Two-Dimensional In-Sb Compound on Silicon as a Quantum Spin Hall Insulator.

Two-dimensional (2D) topological insulator is a promising quantum phase for achieving dissipationless transport due to the robustness of the gapless edge states resided in the insulating gap providing realization of the quantum spin Hall effect. Searching for two-dimensional realistic materials that are able to provide the quantum spin Hall effect and possessing the feasibility of their experimental preparation is a growing field. Here we report on the two-dimensional (In, Sb)2[Formula: see text]2[Formula: see text] compound synthesized on Si(111) substrate and its comprehensive experimental and theoretical investigations based on an atomic-scale characterization by using scanning tunneling microscopy and angle-resolved photoelectron spectroscopy as well as ab initio density functional theory calculations identifying the synthesized 2D compound as a suitable system for realization of the quantum spin Hall effect without additional functionalization like chemical adsorption, applying strain, or gating.

Synthesis of two-dimensional Tl(x)Bi(1-x) compounds and Archimedean encoding of their atomic structure.

Crystalline atomic layers on solid surfaces are composed of a single building block, unit cell, that is copied and stacked together to form the entire two-dimensional crystal structure. However, it appears that this is not an unique possibility. We report here on synthesis and characterization of the one-atomic-layer-thick Tl(x)Bi(1-x) compounds which display quite a different arrangement. It represents a quasi-periodic tiling structures that are built by a set of tiling elements as building blocks. Though the layer is lacking strict periodicity, it shows up as an ideally-packed tiling of basic elements without any skips or halting. The two-dimensional Tl(x)Bi(1-x) compounds were formed by depositing Bi onto the Tl-covered Si(111) surface where Bi atoms substitute appropriate amount of Tl atoms. Atomic structure of each tiling element as well as arrangement of Tl(x)Bi(1-x) compounds were established in a detail. Electronic properties and spin texture of the selected compounds having periodic structures were characterized. The shown example demonstrates possibility for the formation of the exotic low-dimensional materials via unusual growth mechanisms.