Macroscopic and robust supercurrents are observed by direct electron transport measurements on a silicon surface reconstruction with In adatoms (Si(111)-( √ 7 × √ 3)-In). The superconducting transition manifests itself as an emergence of the zero resistance state below 2.8 K. I − V characteristics exhibit sharp and hysteretic switching between superconducting and normal states with well-defined critical and retrapping currents. The two-dimensional (2D) critical current density J2D,c is estimated to be as high as 1.8 A/m at 1.8 K. The temperature dependence of J2D,c indicates that the surface atomic steps play the role of strongly coupled Josephson junctions.

Recent progress in two-dimensional superconductors with atomic-scale thicknesses is reviewed mainly from the experimental point of view. The superconducting systems treated here involve a variety of materials and forms: elemental-metal ultrathin films and atomic layers on semiconductor surfaces; interfaces and superlattices of heterostructures made of cuprates, perovskite oxides, and rareearth metal heavy-fermion compounds; interfaces of electric-double-layer transistors; graphene and atomic sheets of transition-metal dichalcogenide; iron selenide and organic conductors on oxide and metal surfaces, respectively. Unique phenomena arising from the ultimate two-dimensionality of the system and the physics behind them are discussed.Contents in the atomic-scale limit has been a technical challenge for a long time. This is because, in this limit, the whole system consists entirely of surfaces and interfaces, which are vulnerable to structural and chemical disorder in general. For this purpose, therefore, it is demanded that samples with highly ordered and controlled surfaces and interfaces should be fabricated and their superconducting properties probed through advanced techniques.During this decade, the studies on ultrathin 2D superconductors have seen remarkable progress beyond the traditional experimental framework in various fields, and the existence of 2D superconductors with truly atomic-scale thicknesses has also been established. This was mostly driven by rapid advancements on nanotechnology in recent years, including molecular beam epitaxy (MBE), pulsed laser deposition (PLD), in-situ ultrahigh vacuum (UHV)-low temperature (LT) measurement, scanning tunneling microscopy/spectroscopy (STM/STS), etc. These technologies now allow various types of superconducting materials to be fabricated with the atomic-scale precision and to be characterized in unprecedented details, which will be the subject of the present review article. For example, ultrathin elemental metal films grown on silicon surfaces in a layer-by-layer fashion were found to exhibit robust superconductivity down the monolayer (ML) thickness regime [17,18]. This system also exhibits Tc oscillation as a function of film thickness due to the electron quantum confinement effects [19]. La2-xSrxCuO4, a representative cuprate high-Tc superconductor, also showed superconductivity at one-unit-cell (1-UC) thickness and was electrically tuned to an insulating state, revealing a S-I transition driven by quantum phase fluctuations [20]. Furthermore, the interface between LaAlO3 and SrTiO3, both of them being perovskite oxide insulators, was found to exhibit 2D superconductivity that can coexist or compete with ferromagnetism [21]. Recent technological breakthrough of field-effect transistors (FET) using an electric-double-layer (EDL) gate have enabled carrier doping with an unprecedentedly high level (n2D ~10 14 cm -2 ) at the subsurface region [22]. This led to the successful realization of field-induced 2D superconductivity in various insulating materials includi...

We have studied the superconducting Si(111)-( √ 7 × √ 3)-In surface using a 3 He-based lowtemperature scanning tunneling microscope (STM). Zero-bias conductance (ZBC) images taken over a large surface area reveal that vortices are trapped at atomic steps after magnetic fields are applied. The crossover behavior from Pearl to Josephson vortices is clearly identified from their elongated shapes along the steps and significant recovery of superconductivity within the cores. Our numerical calculations combined with experiments clarify that these characteristic features are determined by the relative strength of the interterrace Josephson coupling at the atomic step.PACS numbers: 74.25. Ha,74.55.+v,74.50.+r The recent discovery of superconductivity in silicon surface reconstructions with metal adatoms was an unexpected surprise, because they are regarded as one of the thinnest two-dimensional (2D) materials ever possible [1][2][3][4][5]. This class of surface 2D materials has now become relevant for extensive superconductor researches in progress [6][7][8][9]. Notably, these new studies have been advanced by surface analytical techniques such as scanning tunneling microscopy (STM) [1,5,7,8] and ultrahigh vacuum (UHV)-compatible transport measurement [2-4, 10, 11].One ubiquitous feature of these surface systems is the presence of atomic steps. Atomic steps are considered to strongly affect electron transport phenomena, because they potentially decouple neighboring surface terraces [12][13][14][15]. This could prevent superconducting currents from running over a long distance. The presence of supercurrents through atomic steps has indeed been demonstrated by direct electron transport measurements [2][3][4], and recent experiments indicated that atomic steps work as Josephson junctions [2,5]. Nevertheless, direct evidence of Josephson coupling has not been obtained yet, and possible local variation of its strength has remained an open issue. This problem is also closely related to Josephson junctions formed at the grain boundaries in thin films of high-T c cuprates, which are of technological importance [16,17].In this Letter, we report on compelling evidence of the Josephson coupling at atomic steps on the surface superconductor Si (111)Zero-bias conductance (ZBC) images taken with a low-temperature (LT) STM reveal that vortices are present at atomic steps after magnetic fields are applied. The crossover behavior from Pearl to Josephson vortices is evident from their characteristic elongated shapes and significant recovery of superconductivity within their cores. This identification is strongly supported by our numerical calculations, which clarify their dependence on the interterrace Josephson coupling at the atomic step.The experiment was performed using a UHV-LT-STM constructed at the Institute of Solid State Physics, University of Tokyo. The STM head was accommodated within a 3 He-based cryostat combined with a solenoid superconducting magnet, where magnetic field was applied in the normal direction to the sampl...

Electron conduction through quasi-one-dimensional (1D) indium atomic wires on silicon (the Si(111)-4×1-In reconstruction) is clarified with the help of local structural analysis using scanning tunneling microscopy. The reconstruction has a conductance per square as high as 100 µS, with global conduction despite numerous surface steps. A complete growth of indium wires up to both the surface steps and the lithographically printed electrodes is essential for the macroscopic transport. The system exhibits a metal-insulator transition at 130 K, consistent with a recent ultraviolet photoemission study [

A new interlayer dielectric for multilevel interconnection using fluorine doped silicon oxide (SiOF) has been developed. The film is deposited by simple technique, which is CzFo addition to conventional TEOS based PECVD. Si-F bond formation in the film is recognized by chemical bonding structural study using FT-IR and XPS. Low dielectric constant and excellent gap filling property are obtained. Therefore a high capable interlayer dielectric formation for advanced VLSI devices can be realiznd with this simple technique.