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Majorana zero mode, the bound state of Majorana fermion in a condensed matter system, plays a critical role in fault-tolerant topological quantum computation. It can be realized in the vortex of a two-dimensional topological superconductor as a zero-energy excitation. A topological superconductor can be constructed by coupling superconductivity to the topological surface states by the proximity effect. In this review article, we discuss the fabrication of such artificially engineered topological superconductors by molecular beam epitaxy. The topological phase and superconductivity are exhibited in Bi2Se3/NbSe2 and Bi2Te3/NbSe2 heterostructures simultaneously. Several characteristic features of Majorana zero mode have been revealed in the vortex by a low-temperature scanning tunneling microscope and corroborated by theoretical results. The discovery of Majorana zero mode may pave the way for further applications in topological quantum computing. [ABSTRACT FROM AUTHOR]

Magnetic metals deposited on graphene hold the key to applications in spintronics. Here, we present the results of Fe islands grown on graphene/SiC(0001) by molecular beam epitaxy, which are investigated by scanning tunneling microscopy. The two types of islands distinguished by flat or round tops are revealed, indicating bimodal growth of Fe. The atomic structures on the top surfaces of flat islands are also clearly resolved. Our results may improve the understanding of the mechanisms of metals deposited on graphene and pave the way for future spintronic applications of Fe/graphene systems. [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]

The Mn-Bi-Te class of compounds are recently discovered topological insulators with broken time-reversal-symmetry, which host unique quantum anomalous Hall and axion insulator states. Their key characteristics are believed to be sufficiently understood by models in a single-particle picture. Here, we apply scanning tunneling microscopy to study the electronic properties of MnBi2Te4 and MnBi4Te7. Unexpectedly, our quasiparticle interference (QPI) results demonstrate that rotational symmetry of the crystal breaks, i.e. a nematic-like pattern arises, in certain energy range but persists in others. Moreover, our data in the presence of an external magnetic field rule out the possibility of the material magnetism as an origin of the C2 symmetric QPI pattern. This study reveals that the interaction in the Mn-Bi-Te class of topological materials may play an essential role in their electronic states, and thus opens a new path for investigating the interplay between wavefunction topology and symmetry breaking phases. [ABSTRACT FROM AUTHOR]

Abstract: Surface superstructures are studied with scanning tunneling microscopy during the growth of In on In/Si(111). On the inhomogeneous substrate of Si(111) 4×1/-In coexisting surface, the deposition of 1.5 monolayer (ML) In at about 0°C leads to reconstruction on surface and the formation of one-dimensional nanowires on 4×1 surface (1.0ML=7.8×1014 atoms/cm2). Subsequent deposition of In at −100°C gives rise to the appearance of a hexagonal superstructure with periodicity on reconstructed surface, while self-alignment of In dots along one-dimensional nanowires is observed on the initial 4×1 surface. On Si(111) -In surface, the growth of 2.5ML In at about −100°C yields 6×6 superstructure. The strain in the epitaxial In thin films provides a driving force for the formation of self-organized surface structures. [Copyright &y& Elsevier]

Domain boundaries of superlattices formed on the highly oriented pyrolytic graphite surface are studied by scanning tunneling microscopy. Twist and glide boundaries are identified with help of theoretical simulations. The apparent vertical corrugation difference of the superlattices with the same lattice periodicity is explained in terms of overlayer attenuation. Our study reveals that the intra-layer strain has important effect on the electronic structure of the graphite surface. [Copyright &y& Elsevier]

To uncover the critical effect of disorder on superconductivity, the ideal method is to visualize the microscopic crystalline deficiencies in real space while measuring the macroscopic superconducting properties. By using a self-developed multifunctional scanning tunneling microscope, we investigated the correlation between controllably introduced disorder and superconductivity in the Si(111)- 7 × 3 -In surface reconstruction. It is revealed that not only the density but also the spatial distribution of surface vacancies makes a significant influence on the diamagnetic response of the superconducting surface reconstruction. The higher density of vacancies uniformly dispersed on terraces results in a lower critical temperature and critical magnetic field, while the presence of grooves formed by aggregation of vacancies at step edges reduces the critical supercurrent and thus weakens the diamagnetic shielding effect remarkably. [ABSTRACT FROM AUTHOR]

We investigate the surface structure and electronic properties of β-Sn islands deposited on a graphitized 6H-SiC (0001) substrate via low temperature scanning tunneling microscopy and spectroscopy. Owing to the confinement of the island geometry, quantum well states (QWSs) are formed, manifesting as equidistant peaks in the tunneling spectra. Furthermore, a distinct strip feature appears on the surfaces of odd-layer Sn islands, ranging from 15–19 layers, which is not present on the surfaces of even-layer Sn islands. The spatial distribution of strips can be modified by applying a bias pulse, using an STM tip. Furthermore, the strip-like structure shows significant impacts on the QWS. An energy splitting of the lowest unoccupied QWSs is observed in strip regions; this may be ascribed to caused the phase shift of the wave functions of the QWSs on the top surface, due to surface distortions created by the aforementioned strips. [ABSTRACT FROM AUTHOR]

[Display omitted] • Monolayer C 60 film is grown on a SrTiO 3 (0 0 1) surface for the first time. • Monolayer C 60 exhibit a short-range ordered molecular arrangement, while multi-layer C 60 show a quasi-(2 × 2) superstructure with a chiral pinwheel motif. • Monolayer C 60 on STO shows the largest HOMO-LUMO gap comparing to those on any other substrates. Ultrathin fullerene (C 60) films on various metal and semiconductor substrates have been intensively studied but not yet on a perovskite oxide substrate. Using scanning tunneling microscopy and spectroscopy, we investigated the molecular arrangement and electronic structure of C 60 ultrathin films grown on a SrTiO 3 (0 0 1) surface. Subject to strong coupling and weak screening from the substrate, the monolayer C 60 exhibit short-range ordered molecular arrangement with disordered molecular orientation and a substantially large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap (∼4.1 eV). In contrast, C 60 in the 2nd and 3rd layers are self-assembled into hexagon lattices with a quasi-(2 × 2) superstructure resulted from the arrangement of molecular orientations showing a chiral pinwheel structure. The HOMO-LUMO gap of the multi-layer C 60 further expands to ∼ 4.6 eV, very close to that of gas-phase C 60 , indicating a negligible inter-layer screening effect. [ABSTRACT FROM AUTHOR]

Following the first experimental realization of graphene, other ultrathin materials with unprecedented electronic properties have been explored, with particular attention given to the heavy group-IV elements Si, Ge and Sn. Two-dimensional buckled Si-based silicene has been recently realized by molecular beam epitaxy growth, whereas Ge-based germanene was obtained by molecular beam epitaxy and mechanical exfoliation. However, the synthesis of Sn-based stanene has proved challenging so far. Here, we report the successful fabrication of 2D stanene by molecular beam epitaxy, confirmed by atomic and electronic characterization using scanning tunnelling microscopy and angle-resolved photoemission spectroscopy, in combination with first-principles calculations. The synthesis of stanene and its derivatives will stimulate further experimental investigation of their theoretically predicted properties, such as a 2D topological insulating behaviour with a very large bandgap, and the capability to support enhanced thermoelectric performance, topological superconductivity and the near-room-temperature quantum anomalous Hall effect. [ABSTRACT FROM AUTHOR]

A self-assembled iron(II) phthalocyanine single layer adsorbed on the topological insulator Bi2Te3 was investigated by spin-polarized scanning tunneling microscopy and density functional theory calculations. Although the molecule-substrate interaction is dominated by a relatively weak van der Waals force, the local density of states of Fe was found to strongly depend on the adsorption sites, resulting in a supermolecular lattice. Spin-polarized measurements show that the magnetic moment of iron(II) phthalocyanine persists with an in-plane magnetic easy axis, which was fürther confirmed by density functional calculations. [ABSTRACT FROM AUTHOR]

We report high transition temperature superconductivity in one unit-cell (UC) thick FeSe films grown on a Seetched SrTiO3(001) substrate by molecular beam epitaxy (MBE). A superconducting gap as large as 20 meV and the magnetic field induced vortex state revealed by in situ scanning tunneling microscopy (STM) suggest that the superconductivity of the 1 UC FeSe films could occur around 77 K. The control transport measurement shows that the onset superconductivity temperature is well above 50 K. Our work not only demonstrates a powerful way for finding new superconductors and for raising TC, but also provides a well-defined platform for systematic studies of the mechanism of unconventional superconductivity by using different superconducting materials and substrates. [ABSTRACT FROM AUTHOR]

The crystal structure, electronic, and magnetic properties of Gadolinium (Gd) substituted Bi2Se3-represented by Bi1.98Gd0.02Se3-were investigated systematically by scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and superconducting quantum interference device. Gd dopants with valence of 3+ were mainly found to substitute Bi atoms. Each Gd3+ ion has a magnetic moment as large as ∼6.9μB in the bulk paramagnetic state. [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]

Silicene, a monolayer of silicon atoms arranged in honeycomb lattices, can be synthesized on the Ag(111) surface, where it forms several superstructures with different buckling patterns and periodicity. Using scanning tunneling microscopy (STM), we obtained high-resolution images of silicene grown on Ag(111) and revealed its five phases, i.e., 4 × 4 − α, 4 × 4 − β, − α, − β and − γ, some observed for the first time. For each of the phases, we have determined its atomic structure by comparing the atomic-resolution STM images with theoretical simulation results previously reported. We thus eliminate the contradictions of previous studies on the structural models of various silicene phases supported by the Ag(111) surface. [ABSTRACT FROM AUTHOR]