Crystal structure imperfections in solids often act as efficient carrier trapping centres, which, when suitably isolated, act as sources of single photon emission. The best known examples of such attractive imperfections are well-width or composition fluctuations in semiconductor heterostructures (resulting in the formation of quantum dots) and coloured centres in wide-bandgap materials such as diamond. In the recently investigated thin films of layered compounds, the crystal imperfections may logically be expected to appear at the edges of commonly investigated few-layer flakes of these materials exfoliated on alien substrates. Here, we report comprehensive optical micro-spectroscopy studies of thin layers of tungsten diselenide (WSe2), a representative semiconducting dichalcogenide with a bandgap in the visible spectral range. At the edges of WSe2 flakes (transferred onto Si/SiO2 substrates) we discover centres that, at low temperatures, give rise to sharp emission lines (100 μeV linewidth). These narrow emission lines reveal the effect of photon antibunching, the unambiguous attribute of single photon emitters. The optical response of these emitters is inherently linked to the two-dimensional properties of the WSe2 monolayer, as they both give rise to luminescence in the same energy range, have nearly identical excitation spectra and have very similar, characteristically large Zeeman effects. With advances in the structural control of edge imperfections, thin films of WSe2 may provide added functionalities that are relevant for the domain of quantum optoelectronics.

In bulk materials superconductivity is remarkably robust with respect to nonmagnetic disorder [1]. In the two-dimensional limit however, the quantum condensate suffers from the effects produced by disorder and electron correlations which both tend to destroy superconductivity [2][3][4][5][6][7]. The recent discovery of superconductivity in single atomic layers of Pb, the striped incommensurate (SIC) and √ 7 × √ 3 Pb/Si(111) [8,9], opened an unique opportunity to probe the influence of well-identified structural disorder on two-dimensional superconductivity at the atomic and mesoscopic scale [10,11]. In these two ultimate condensates we reveal how the superconducting spectra loose their conventional character, by mapping the local tunneling density of states. We report variations of the spectral properties even at scales significantly shorter than the coherence length. Furthermore, fine structural differences between the two monolayers, such as their atomic density, lead to very different superconducting behaviour. The denser SIC remains globally robust to disorder, as are thicker Pb films [12-17], whereas in the slighly more diluted √ 7 × √ 3 system superconductivity is strongly fragilized. A consequence of this weakness is revealed at monoatomic steps of √ 7 × √ 3, which disrupt superconductivity at the atomic scale. This effect witnesses that each individual step edge is a Josephson barrier. At a mesoscopic scale the weakly linked superconducting atomic terraces of √ 7 × √ 3 form a native network of Josephson junctions. We anticipate the Pb/Si(111) system to offer the unique opportunity to tune the superconducting coupling between adjacent terraces [18], paving a new way of designing atomic scale quantum devices compatible with silicon technology.

No abstract

International audienceWe present a combined experimental and theoretical study of the proximity effect in an atomic-scale controlled junction between two different superconductors. Elaborated on a Si(111) surface, the junction comprises a Pb nanocrystal with an energy gap Delta(1) = 1.2 meV, connected to a crystalline atomic monolayer of lead with Delta(2) = 0.23 meV. Using in situ scanning tunneling spectroscopy, we probe the local density of states of this hybrid system both in space and in energy, at temperatures below and above the critical temperature of the superconducting monolayer. Direct and inverse proximity effects are revealed with high resolution. Our observations are precisely explained with the help of a self-consistent solution of the Usadel equations. In particular, our results demonstrate that in the vicinity of the Pb islands, the Pb monolayer locally develops a finite proximity-induced superconducting order parameter, well above its own bulk critical temperature. This leads to a giant proximity effect where the superconducting correlations penetrate inside the monolayer a distance much larger than in a nonsuperconducting metal

The proximity effect between a superconductor and a highly diffusive two-dimensional metal is revealed in a scanning tunneling spectroscopy experiment. The in situ elaborated samples consist of superconducting single crystalline Pb islands interconnected by a nonsuperconducting atomically thin disordered Pb wetting layer. In the vicinity of each superconducting island the wetting layer acquires specific tunneling characteristics which reflect the interplay between the proximity-induced superconductivity and the inherent electron correlations of this ultimate diffusive two-dimensional metal. The observed spatial evolution of the tunneling spectra is accounted for theoretically by combining the Usadel equations with the theory of dynamical Coulomb blockade; the relevant length and energy scales are extracted and found in agreement with available experimental data.

We have studied 2H-NbSe2 by scanning tunneling spectroscopy with two different orientations, along the c and the a/b axis. The results can be understood in the framework of a two-gap model: along the c-axis, the large gap is dominant in the tunneling spectra, while a smaller gap is measured along the a/b axis. Our measurement thus shows unambiguously the existence of two gaps, where the orientation preferentially selects one gap or the other. Similarly as for MgB2, the tunneling spectra are well described by the McMillan equations for a two-band superconductor, showing that interband coupling originates from quasiparticle scattering from one band to the other. The electronic structure of 2H-NbSe2 is further studied theoretically by means of DFT calculations. Examining the different contributions to the Fermi level DOS, we conclude that the large gap observed in tunneling originates from states associated with the Fermi surface cylinders around K, whereas the small gap originates from the cylinders around Γ. In addition, we show that the tunneling current at large distance from the surface is dominated by the selenium orbitals. This finding suggests that the third component of the Fermi surface, the Se-based pancake around Γ, is strongly coupled to the cylinders around K, possibly due to the charge density wave state.

We have investigated the electronic properties of two-dimensional (2D) transition metal dichalcogenides (TMDs), namely trilayer WSe 2 and monolayer MoSe 2 , deposited on epitaxial graphene on silicon carbide, by using scanning tunneling microscopy and spectroscopy (STM/STS) in ultra-high vacuum. Depending on the number of graphene layers below the TMD flakes, we identified variations in the electronic dI/dV(V) spectra measured by the STM tip: the most salient feature is a rigid shift of the TMD spectra (i.e. of the different band onset positions) towards occupied states by about 120 mV when passing from bilayer to monolayer underlying graphene. Since both graphene phases are metallic and present a work function difference in the same energy range, our measurements point towards the absence of Fermi-level pinning for such van der Waals 2D TMD/ Metal heterojunctions, following the prediction of the Schottky-Mott model. PAPEROriginal content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.

International audienceThe effect of electron doping on the van Hove singularities (vHs) which develop in twisted graphene bilayers (tBLs) is studied for a broad range of rotation angles θ(1.5°<θ<15°) by means of scanning tunneling microscopy and spectroscopy. Bilayer and trilayer graphene islands were grown on the 6H-SiC(000-1) (3×3) surface, which results in tBLs doped in the 10 E12 cm−2 range by charge transfer from the substrate. For large angles, doping manifests in a strong asymmetry of the positions of the upper (in empty states) and lower (in occupied states) vHs with respect to the Fermi level. The splitting of these vHs energies is found essentially independent of doping for the whole range of θ values, but the center of theses vHs shifts towards negative energies with increasing electron doping. Consequently, the upper vHs crosses the Fermi level for smaller angles (around 3°). The analysis of the data performed using tight-binding calculations and simple electrostatic considerations shows that the interlayer bias remains small (<100mV) for the doping level resulting from the interfacial charge transfer (≃5×10E12 cm−2)