A single atomic slice of α -tin-stanene-has been predicted to host the quantum spin Hall effect at room temperature, offering an ideal platform to study low-dimensional and topological physics. Although recent research has focused on monolayer stanene, the quantum size effect in few-layer stanene could profoundly change material properties, but remains unexplored. By exploring the layer degree of freedom, we discover superconductivity in few-layer stanene down to a bilayer grown on PbTe, while bulk α -tin is not superconductive. Through substrate engineering, we further realize a transition from a single-band to a two-band superconductor with a doubling of the transition temperature. In situ angleresolved photoemission spectroscopy (ARPES) together with first-principles calculations elucidate the corresponding band structure. The theory also indicates the existence of a topologically non-trivial band. Our experimental findings open up novel strategies for constructing two-dimensional topological superconductors.Confining superconductivity to a two-dimensional (2D) plane engenders a variety of quantum phenomena 1,2 . Of late, the realization of highly crystalline and atomically thin superconductors has triggered a flurry of discoveries, including the Griffiths singularity behavior 3 and a quantum metallic phase 4,5 , as well as an extremely large critical magnetic field in the plane 6,7 . One strategy for achieving 2D superconductors is to epitaxially grow superconductive single elements, such as Pb, In and Ga, for just one or two atomic layers 3,8,9 . Among the single elements, tin (Sn) is the very material in which the Meissner effect was first discovered 10 , but realizing ultrathin Sn in the superconductive β -phase, known as white tin 11 , remains challenging. The epitaxially grown Sn in the ultrathin limit tends to fall instead in the α -phase 12 , whose bulk is semi-metallic and non-superconductive.Recently, however, intensive research has been devoted to investigate the thinnest possible slice of α -tin (111) ) is the focus of current research. On the other hand, few-layer stanene is expected to show significant thicknessdependent properties due to the strong quantum confinement 20 , but its exploration is still lacking.In this Letter, by going from monolayer to few-layer stanene, surprisingly, we discover superconductivity. We report the stable superconducting properties of uncapped few-layer stanene films on PbTe (111)/Bi 2 Te 3 substrates. The superconducting transition temperature (T c ) can be effectively enhanced by varying the thickness of the PbTe buffer layer. Concomitantly with a doubling of T c , we observe a single-band to two-band transition, which is further elucidated by photoemission spectroscopy and theoretical calculations. The calculated band structure further indicates the existence of inverted bands in our system. Our results therefore underscore the potential of an in-plane integration of 2Dtopological insulator and superconductor-of the same material. The heterostructure, vertically...

Identifying the essence of doped Mott insulators is one of the major outstanding problems in condensed matter physics and the key to understanding the high-temperature superconductivity in cuprates. We report real space visualization of Mott transition in Sr1-xLaxCuO2+y cuprate films that cover the entire electron-and hole-doped regimes. Tunneling conductance measurements directly on the cooper-oxide (CuO2) planes reveal a systematic shift in the Fermi level, while the fundamental Mott-Hubbard band structure remains unchanged. This is further demonstrated by exploring atomic-scale electronic response of CuO2 to substitutional dopants and intrinsic defects in a sister compound Sr0.92Nd0.08CuO2.The results could be better explained in the framework of self-modulation doping, similar to that in semiconductor heterostructures, and form a basis for developing any microscopic theories for cuprate superconductivity.

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We report high temperature superconductivity in one unit-cell (1-UC) FeSe films grown on SrTiO3 (STO) (110) substrate by molecular beam epitaxy. By in-situ scanning tunneling microscopy measurement, we observe a superconducting gap as large as 17 meV on the 1-UC FeSe films. Transport measurements on 1-UC FeSe/STO(110) capped with FeTe layers reveal superconductivity with an onset transition temperature (TC) of 31.6 K and an upper critical magnetic field of 30.2 T. We also find that TC can be further increased by external electric field although the effect is weaker than that on STO(001) substrate.