ИСТИНА

We report the experimental observation of vortices on the surface of a 50nmthick layer of Cu in the hybrid structure Cu/Nb with ultra-low temperature Scanning Tunneling Spectroscopy (STS). In the studied samples the non-superconducting Cu-layer acquires superconducting correlations due to the proximity effect with 100nm-thick superconducting Nb. To avoid the oxidation at Cu-surface and allow STS, the samples were ex-situ grown on SiO2/Si in the inversed order, i.e. Cu was deposited directly on the substrate, Nb was deposited on Cu. The samples were then introduced to the UHV STM chamber and cleaved in-situ. The structural analysis showed that, upon cleavage, the samples break at Cu-SiO2 interface, thus exposing fresh Cu surface. [1] The presence of the proximity effect at the surface of Cu was first evidenced by observation of a proximity gap in the tunneling conductance spectra dI(V)/dV, in clear relation to the value of the superconducting gap of bulk Nb. The evolution of the proximity spectra with temperature was also studied in the range (0.3-4.2) K (Fig. e). Upon application of an external magnetic field, spatial variations of the tunneling conductance spectra were observed (see Fig. a,b,c). These variations appear in the detailed STS maps as round nm-size spots, in the centers of which the proximity gap vanishes. The density of spots rises continuously with magnetic field; it corresponds perfectly to the expected density of Abrikosov vortices in Nb. We identify the observed spots as proximity induced vortices in the normal Cu. On the basis of our STS data, we have determined the size and shape of the proximity vortex cores, and evaluated the coherence length in Cu. Independent numerical calculations of the quasiparticle spectra in S-N hybrids within the Usadel equation formalism were performed and found in agreement with experimental findings. [1] V.S. Stolyarov, T. Cren, F. Debontridder, C. Brun, I.S. Veshchunov, O.V. Skryabina, A.Yu Rusanov, and D. Roditchev. Ex situ elaborated proximity mesoscopic structures for ultrahigh vacuum scanning tunneling spectroscopy. Applied Physics Letters, 104 (2014) 172604