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Search for superconductors with a T c above the liquid nitrogen temperature (77 K) led to the discovery of a high-T c cuprate with a T c above 130 K over two decades ago 16 . Even though the value of T c is only 26 K in the first Fe-based superconductor, LaFeAsO (ref. 17) with the earlier work on superconducting oxides interfaces [3][4][5][6] , demonstrates that interface between two different materials provide not only a rich system for studying two-dimensional (2D) superconductivity, but also a potential pathway to high-T c superconductivity [7][8][9][10][11][12] .Indeed, recent angle-resolved photoemission spectroscopy (ARPES) experiments on the FeSe/STO system revealed different electronic structure from those of bulk FeSe and possible occurrence of superconductivity around 65 K (refs. 13,14). An ex situ transport measurements performed on FeSe/STO protected by multiple layers of FeTe and amorphous Si overlay revealed a zero-resistance T c of 23.5 K and an onset T c > 40 K (ref. 15). Evidently the addition of protection layers suppresses superconductivity in single-layer FeSe. In this work, we report electrical transport measurements on single-layer films of FeSe grown on Nb-doped SrTiO 3 substrate using an in situ 4-point probe (4PP) technique. We found that superconductivity could be obtained even at a temperature as high as 109 K.Single-layer films of FeSe were grown on Nb-doped SrTiO 3 (001) surface by the same method as reported previously 1 , employing extra Se flux in an MBE system equipped with STM/STS and 4PP capabilities. The growth process was monitored by reflection high-energy electron diffraction (RHEED) (Fig. 1a), which allows the precise control of film growth needed to achieve one unit-cell thickness as exactly as possible. The crystal nature of the films was confirmed by STM imaging at both large and atomic scales, as shown in Figs grown on a conducting substrate, it does mean zero resistance of the film as the film shorts the conducting substrate. However, when the 4PP detects a finite voltage, it may not necessarily mean that the sample is not superconducting. Indeed, the 4PP technique is a powerful tool for investigating superconductivity in films that cannot be taken out of a UHV system or an interface that is not accessible by surface probes.Two typical I-V curves collected at 3 K in C1423 and C1234 are shown in Figs. 2b and 2c, respectively. The data demonstrate explicitly that the film is superconducting, with the critical current (I c ) defined by the current value for which the superconducting top layer can no longer short the conducting substrate with a finite resistance. Even though interpretation of the finite voltage in the superconducting I-V curves are complicated, the essentially zero voltage seen at low currents cannot be resulted from artificial effects of the contact, since all four tips of the 4PP have Ohmic contacts with the sample individually (Supplementary Information). It is also interesting to note that the extracted I c have similar values for both measurement con...

The two-dimensional (2D) superconducting state is a fragile state of matter susceptible to quantum phase fluctuations. Although superconductivity has been observed in ultrathin metal films down to a few layers 1-10 , it is still not known whether a single layer of ordered metal atoms, which represents the ultimate 2D limit of a crystalline film, could be superconducting. Here we report scanning tunnelling microscopy measurements on single atomic layers of Pb and In grown epitaxially on Si(111) substrate, and demonstrate unambiguously that superconductivity does exist at such a 2D extreme. The film shows a superconducting transition temperature of 1.83 K for an atom areal density n = 10.44 Pb atoms nm −2 , 1.52 K for n = 9.40 Pb atoms nm −2 and 3.18 K for n = 9.40 In atoms nm −2 , respectively. We confirm the occurrence of superconductivity by the presence of superconducting vortices under magnetic field. In situ angle-resolved photoemission spectroscopy measurements reveal that the observed superconductivity is due to the interplay between the Pb-Pb (In-In) metallic and the Pb-Si (In-Si) covalent bondings.The one-atomic-layer films of Pb and In studied here were grown with atomic precision on bulk-terminated Si(111) substrate using molecular beam epitaxy. The one-atomic-layer films of Pb have two different structural phases depending on the coverage (for sample preparation, see the Methods section). Figure 1a,d shows the schematic structure and scanning tunnelling microscopy (STM) topograph of the so-called striped incommensurate (SIC) phase, which has a Pb coverage of 4/3 monolayers (ML;. Here 1 ML is defined as the surface atomic density of the Si(111) with areal density n = 7.84 atoms nm −2 . In a unit cell of the SIC-Pb phase, there are four Pb atoms per three surface Si atoms. Three of the four Pb atoms each form a covalent bond with an underlying Si atom, leaving one Pb atom without bonding to the Si substrate. Besides the covalent bonds with the Si substrate, the metal atoms also form metallic bonds within the metal overlayer. As all Pb atoms are located exactly in the same atomic-layer sheet (see the large-scale STM image and cross-section height profiles in Supplementary Fig. S1), the resulting areal density of Pb atoms is 10.44 nm −2 . Compared with the bulk Pb(111) plane, the lattice of the SIC phase is compressed by 5%.Ultralow-temperature (down to 0.40 K) scanning tunnelling spectroscopy (STS) on the SIC phase reveals a clear signature of superconductivity. Figure 2a shows the tunnelling spectra taken on the SIC phase using a superconducting Nb tip. At 0.42 K,

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Three-dimensional topological insulators (TIs) are characterized by their nontrivial surface states, in which electrons have their spin locked at a right angle to their momentum under the protection of time-reversal symmetry. The topologically ordered phase in TIs does not break any symmetry. The interplay between topological order and symmetry breaking, such as that observed in superconductivity, can lead to new quantum phenomena and devices. We fabricated a superconducting TI/superconductor heterostructure by growing dibismuth triselenide (Bi(2)Se(3)) thin films on superconductor niobium diselenide substrate. Using scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we observed the superconducting gap at the Bi(2)Se(3) surface in the regime of Bi(2)Se(3) film thickness where topological surface states form. This observation lays the groundwork for experimentally realizing Majorana fermions in condensed matter physics.

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Recent studies show that two low-energy van Hove singularities (VHSs) seen as two pronounced peaks in the density of states could be induced in a twisted graphene bilayer. Here, we report angle-dependent VHSs of a slightly twisted graphene bilayer studied by scanning tunneling microscopy and spectroscopy. We show that energy difference of the two VHSs follows ΔE(vhs)∼ℏν(F)ΔK between 1.0° and 3.0° [here ν(F)∼1.1 × 10(6) m/s is the Fermi velocity of monolayer graphene, and ΔK = 2Ksin(θ/2) is the shift between the corresponding Dirac points of the twisted graphene bilayer]. This result indicates that the rotation angle between graphene sheets does not result in a significant reduction of the Fermi velocity, which quite differs from that predicted by band structure calculations. However, around a twisted angle θ∼1.3°, the observed ΔE(vhs)∼0.11 eV is much smaller than the expected value ℏν(F)ΔK∼0.28 eV at 1.3°. The origin of the reduction of ΔE(vhs) at 1.3° is discussed.

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