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Bismuth-based binary alloy β-Bi2Pd has recently been suggested as a connate topological superconductor with immense promise for Majorana zero modes. However, the epitaxial thin films down to the ultimate level of single unit-cell have been challenging. Here, we employed the state-of-the-art molecular beam epitaxy to prepare single unit-cell superconducting β-Bi2Pd films with lateral dimensions of hundreds of nanometers. Utilizing in situ scanning tunneling microscopy/spectroscopy, we uncover evidence of anisotropic superconductivity whose spectra are well fit by anisotropic s-wave gap functions and demonstrate that the epitaxial growth of β-Bi2Pd films relies substantially on the substrate temperature, flux ratio, growth rate, and coverage. The high-quality epitaxial films provide a promising platform to investigate the topological superconductivity at the two-dimensional limit. [ABSTRACT FROM AUTHOR]

We demonstrate fluorine intercalation into Bi2Sr2CaCu2O8+x (BSCCO) flakes, as thick as 100 nm, by back-gating a single crystalline LaF3 substrate. Element-sensitive analysis not only confirms the presence of fluorine in BSCCO after gating but also reveals lateral diffusion of fluorine in BSCCO. Transport measurements further demonstrate the hole-doping effect of fluoride ions in BSCCO through effective modulation of the superconducting transitions. Our work introduces a distinct type of ions (F−) that can be intercalated into materials via solid state back-gating. It broadens the toolbox of ion back-gating and is useful for addressing exotic phenomena in the heavily hole doped regime. [ABSTRACT FROM AUTHOR]

It is widely believed that the undoped Mott insulator is crucial to understand the origin of high-TC superconductivity and the complex phase diagram in cuprates. Tetragonal CuO, the simplest structure among cuprates, is regarded as an archetypal model to explore the ground state physics of Mott insulator. Here, we report on the epitaxial growth and surface characterization of tetragonal CuO thin film by combining ozone-assisted molecular beam epitaxy and multi-analytical techniques. In situ scanning tunneling microscopy measurement reveals two sets of Cu2+ square lattices with a reversed contrast of their apparent heights between neighboring domains. A spectroscopy study demonstrates an upper bound of the charge transfer gap as 3.68 eV, and quantitatively agrees with the previous photoemission and x-ray photoelectron results. Our work will deepen the understanding of high-energy electronic structures in parent cuprates. [ABSTRACT FROM AUTHOR]

Low-dimensional materials exhibit exotic properties and have attracted widespread attention. However, many low-dimensional materials are highly sensitive to air, making it challenging to investigate their intrinsic properties with ex situ measurements. To overcome such challenges, here, we developed a system combined with sample growth, electrode deposition, and in situ electrical transport measurement under ultra-high vacuum condition. The in situ deposition of electrodes enables desired ohmic electrical contacts between the probes and samples, which allows continuous temperature dependent resistance (R–T) measurements. Combined with a scanning tunneling microscope, surface morphology, electronic structure, and electrical transport properties of the same sample can be systematically investigated. We demonstrate the performance of this in situ electrical transport measurement system with three-unit-cell thick FeSe films grown on Nb-doped SrTiO3(001) substrates, where a low-noise R–T curve with a zero-resistance superconducting transition temperature of ∼30 K is observed. [ABSTRACT FROM AUTHOR]

We investigate the atomic structure and electronic properties of monolayers of copper phthalocyanines (CuPc) deposited on epitaxial graphene substrate. We focus in particular on hexadecafluorophthalocyanine (F16CuPc), using both theoretical and experimental (scanning tunneling microscopy - STM) studies. For the individual CuPc and F16CuPc molecules, we calculated the electronic and optical properties using density functional theory (DFT) and time-dependent DFT and found a red-shift in the absorption peaks of F16CuPc relative to those of CuPc. In F16CuPc, the electronic wavefunctions are more polarized toward the electronegative fluorine atoms and away from the Cu atom at the center of the molecule. When adsorbed on graphene, the molecules lie flat and form closely packed patterns: F16CuPc forms a hexagonal pattern with two well-ordered alternating α and β stripes while CuPc arranges into a square lattice. The competition between molecule-substrate and intermolecular van der Waals interactions plays a crucial role in establishing the molecular patterns leading to tunable electron transfer from graphene to the molecules. This transfer is controlled by the layer thickness of, or the applied voltage on, epitaxial graphene resulting in selective F16CuPc adsorption, as observed in STM experiments. In addition, phthalocyanine adsorption modifies the electronic structure of the underlying graphene substrate introducing intensity smoothing in the range of 2-3 eV below the Dirac point (ED) and a small peak in the density of states at ∼0.4 eV above ED. [ABSTRACT FROM AUTHOR]

The self-assembled structure of submonolayer manganese phthalocyanine (MnPc) on Pb(111) surface is investigated by using low-temperature scanning tunneling microscopy (STM). A 'holelike' superlattice, which is superimposed on the self-assembled nearly quadratic network, is observed. High resolution STM images reveal that there are two distinct azimuthal orientations of MnPc molecules. It is found that by taking the two different orientations the self-assembly can further be optimized energetically by maximizing intermolecular orbital overlapping. It is this intralayer energy minimization process that leads to the characteristic holelike superlattice. [ABSTRACT FROM AUTHOR]

The transport property of a superconducting Pb nanobridge, which is carved by focus ion beam technique from an atomically flat single-crystal Pb thin film grown on Si(111) substrate, is investigated. Below the superconducting transition temperature TC, the nanobridge exhibits a series of sharp voltage steps as a function of current. The multiple voltage steps are interpreted as a consequence of spatially localized phase slip centers or hot-spot formation in the bridge. Just below the critical current, the voltages versus current curve shows a power-law behavior in the low temperature region, but Ohmic near the TC. The thermally activated phase slip, quantum phase slip, and imhomogeneity in a one-dimensional superconducting system may contribute to the observed results. [ABSTRACT FROM AUTHOR]

Molecular beam epitaxy (MBE) is a powerful technique to grow high quality thin films with atomic precision, and multiprobe scanning tunneling potentiometry (STP) is an ideal tool to probe electronic transportation in nanometer scale. We combine the two advanced techniques together and successfully construct a unique system of MBE, STP, and four-probe in situ transport measurement. Excellent functions of this system have been demonstrated by experiments on several materials under ultrahigh vacuum conditions. The system provides an ideal platform for in situ study of electronic transport properties of various thin films, such as two-dimensional superconductors and topological insulators. [ABSTRACT FROM AUTHOR]

The inherent susceptibility of low-dimensional materials to thermal fluctuations has long been expected to pose a major challenge to achieve intrinsic long-range ferromagnetic order in two-dimensional materials. The recent explosion of interest in atomically thin materials and their assembly into van der Waals heterostructures has renewed interest in two-dimensional ferromagnetism, which is interesting from a fundamental scientific point of view and also offers a missing ingredient necessary for the realization of spintronic functionality in van der Waals heterostructures. Recently, several atomically thin materials have been shown to be robust ferromagnets. Such ferromagnetism is thought to be enabled by magnetocrystalline anisotropy which suppresses thermal fluctuations. In this article, we review recent progress in two-dimensional ferromagnetism in detail and predict new possible two-dimensional ferromagnetic materials. We also discuss the prospects for applications of atomically thin ferromagnets in novel dissipationless electronics, spintronics, and other conventional magnetic technologies. Particularly, atomically thin ferromagnets are promising to realize time reversal symmetry breaking in two-dimensional topological systems, providing a platform for electronic devices based on the quantum anomalous Hall effect showing dissipationless transport. Our proposed directions will assist the scientific community to explore novel two-dimensional ferromagnetic families which can spawn new technologies and further improve the fundamental understanding of this fascinating area. [ABSTRACT FROM AUTHOR]

Ultrathin Pb films with a thickness of three monolayers (ML) were prepared on α-[formula]Pb/Si(111) (Pb-SIC) substrate by molecular beam epitaxy. Despite significant defect scattering, low temperature scanning tunneling microscopy reveals a high superconducting transition temperature Tc of 6.9 K, compared with the bulk Tc (7.2 K). By applying external magnetic field, magnetic vortices were directly imaged, which demonstrates the robustness of superconductivity. By comparing to nearly free-standing Pb films on graphitized SiC (0001) substrate, we suggest that the higher Tc of 3 ML Pb films on Pb-SIC originates from the combined effects of quantum confinement and substrate-enhanced electron-phonon coupling. [ABSTRACT FROM AUTHOR]

We report on molecular beam epitaxy growth of bilayer Bi(111) films on topological insulator Bi2Te3. In situ scanning tunneling microscopy/spectroscopy shows that Bi growth mode changes from quasi bilayer-by-bilayer to step-flow with increasing substrate temperature. Bilayer Bi(111) exhibits an electron donor behavior, causing an 80 meV downshift of the Dirac point of Bi2Te3. Local work function difference between the bilayer films and Bi2Te3 films is measured to be 390 meV. Based on the observations, we propose a schematic energy-band diagram which reveals band bending effect at the Bi/Bi2Te3 interface. Our work paves a way to explore the exotic topological properties of bilayer islands and thin films of Bi. [ABSTRACT FROM AUTHOR]

We have prepared Ca-intercalated multilayer epitaxial graphene films on silicon carbide and observed superconductivity in them with both magnetic and transport measurements. Superconducting transition has been detected at temperature up to 7 K in Ca-intercalated epitaxial graphene with the thickness down to 10 layers grown on both Si-face and C-face of silicon carbide. The result demonstrates intercalated epitaxial graphene as a good platform to study graphene-based superconductivity. [ABSTRACT FROM AUTHOR]

We investigate the far-field scattering property of a single Ag nanoring. Under oblique excitation, two-focus scatterings with distinct intensities were observed. We show that the two-focus scatterings result from the interference of far-field scattering light from the ring circumference, and the local field enhancement effect of surface plasmons plays the key role in the focus intensity. By finite difference time domain and numerical integer methods, we calculated far-field scattering and surface plasmons' distributions, and the results are in good agreement with the experiment. [ABSTRACT FROM AUTHOR]

Bi2Se3 is a typical three-dimensional topological insulator but always exhibits metallic behavior due to heavy n-type doping. With in situ angle-resolved photoemission spectroscopy, we have systematically studied the doping effects of Sb and Pb on the electronic structure of Bi2Se3 films prepared by molecular beam epitaxy. The surface chemical potential of Bi2Se3 can be tuned by 110 and 145 meV by doping Sb and Pb atoms, respectively. By codoping Pb and Sb, the Fermi level can be shifted from above to below the Dirac point. The underlying mechanism in different doping effects of Sb and Pb is discussed. [ABSTRACT FROM AUTHOR]

Atomically flat thin films of topological insulator Bi2Se3 have been grown on double-layer graphene formed on 6H-SiC(0001) substrate by molecular beam epitaxy. By a combined study of reflection high energy electron diffraction and scanning tunneling microscopy, we identified the Se-rich condition and temperature criterion for layer-by-layer growth of epitaxial Bi2Se3 films. The as-grown films without doping exhibit a low defect density of 1.0±0.2×1011/cm2, and become a bulk insulator at a thickness of ten quintuple layers, as revealed by in situ angle resolved photoemission spectroscopy measurement. [ABSTRACT FROM AUTHOR]

We have calculated the energy barriers for an adatom climbing up onto a Pb mesa top either over a facet-facet edge or through a facet-step joint, using a modified embedded atom method. We found that the second process is not only thermodynamically more favorable than the first one but also much faster with a diminishing barrier. Our results provide a plausible explanation for the experimentally observed intriguing growth behavior of a Pb mesa. The underlying mechanisms can be generally applicable to other systems. [ABSTRACT FROM AUTHOR]

Using a low temperature method, the authors have prepared atomically flat Pb ultrathin films on Si(111)-7×7 surface. Room temperature annealing of the films results in a percolation morphology with fractal vacancy islands where the Si substrate is exposed. The fractal film with a nominal thickness of 23 ML exhibits enhanced onset superconducting transition temperature of 7.0 K and negative magnetoresistance with wide magnetoresistance terrace under perpendicular magnetic field when the film is in superconducting state. They attribute the phenomena to the coexistence of two superconducting phases in this fractal film. [ABSTRACT FROM AUTHOR]

Varied temperature photoemission study is performed to investigate the quantum size effects on the thermal property of atomically flat Pb films grown on Si(111). The binding energies of the quantum well states for the films with thicknesses from 10 to 24 ML exhibit a linear increase with increasing temperature from 75 to 270 K. Under free electron approximation, thermal expansion coefficients of the thin films are determined, which manifest a large enhancement and oscillation behavior. The large enhancement is interpreted by a model based on the quantum confinement along the film normal direction. The oscillation is shown to be closely related to the structural instability and is a result of the formation of the quantized electronic states in thin films. [ABSTRACT FROM AUTHOR]

The thickness dependence of the local work function (LWF) and its relationship with the quantum well states (QWSs) are studied. The measured LWF shows an oscillatory behavior between adjacent layers with a period of 2 ML and, in addition, an envelope beating pattern with a period of 9 ML. Scanning tunneling spectroscopy investigations reveal that the oscillatory LWF correlates perfectly with the formation of the QWSs: the higher the occupied QWS is, the smaller the LWF is. Through the role of the LWF, this study establishes the importance of quantum size effects in thin films for surface reactions and catalysis. [ABSTRACT FROM AUTHOR]

A two-dimensional artificial crystal, which is made up of artificial atoms—identical Al clusters with nanometer size and spacing, was fabricated by taking advantage of surface-mediated clustering on a growth template. In situ scanning tunneling microscopy analysis and first-principles total energy calculations were used to determine the atomic structure of the Al nanoclusters. The Al clusters exhibit more remarkable thermal stability than the In clusters we reported previously. Based on our systematic observations and calculations, the formation mechanism and the high stability of these magic clusters are discussed. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Atomic structure of FeSi[sub 2] or CoSi[sub 2] grown on the Si(100) 2xn surface has been investigated by scanning tunneling microscopy. After annealing the Fe or Co covered Si(100) 2xn substrate at ∼800 °C, an ordered adatom vacancy array appears on the nominal 1x1 surface of the formed FeSi[sub 2] or CoSi[sub 2] islands, which has not been observed for silicide on the Si(100)-2xl. Upon further annealing to ∼ll00 °C, the vacancies coalesce into striped domains along one of the <011> directions. These nanostructured features are a result of the Ni impurities, and can be a promising template for fabricating nanodot arrays. [ABSTRACT FROM AUTHOR]

Growth of In on the Si(001)-2×n nanostructured surface is investigated by an in situ scanning tunneling microscope (STM). The deposited In atoms predominantly occupy the normal 2×1 dimer-row structure, and develop into a uniform array of In nanowires at a coverage of ∼0.2 ML. High-resolution STM images show that the In atoms form a stable local 2×2 reconstruction that removes surface Si dangling bonds states and saturates all In valency. Since the dimensions of the Si(001)-2×n vacancy line structure depend on impurity concentrations, this study demonstrates that the 2×n surface can be used for spontaneous fabrication of various metal nanowire arrays. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report comprehensive x-ray diffraction studies of the crystal structure and epitaxy of thin films of the topological insulator Bi2Te3 grown on Si (1 1 1). The films are single crystals of high crystalline quality, which strongly depend on that of their substrates, with in-plane epitaxial relationships of Bi2Te3 [2 1 -3 0] | Si [1 -1 0] and Bi2Te3 [0 1 -1 0] | Si [1 1 -2] along which the lattices of 1 × 3 Bi2Te3 and 2 × 2 Si supercells are well matched. As the samples age, we observe loss of crystalline Bi2Te3 film thickness accompanied with roughening of the crystalline interfaces, formation of new crystalline phases as well as compositional and structural modification of the Si substrate, consistent with the diffusion of Te into the Si substrate. [ABSTRACT FROM AUTHOR]

Using first-principles calculations based on density-functional theory, we systematically study the adsorption of O(2) molecules on ultrathin Pb(111) films ranging from 3 to 11 monolayers (MLs). It is found that no matter how thick the film is, the O(2) molecule prefers to adsorb at the threefold hcp hollow site where it lies parallel to the surface. The adsorption mechanism is discussed from the hybridization of p orbitals of O(2) and Pb. The adsorption energy of O(2) on the Pb(111) film, about several hundred meV, shows a 2 ML oscillation with the thickness. This study well confirms the modulation of the surface reactivity of Pb films induced by the quantum well states, which is compatible with the previous experimental observation.