Possibility of Exciton Mediated Superconductivity in Nano-Sized Sn/Si Core–Shell Clusters: A Process Technology towards Heterogeneous Material in Nano-Scale

Abstract

We have produced Sn/Si core-shell cluster assemblies by a plasma-gas-condensation cluster beam deposition apparatus. For the sample with Si content = 12 at.%, the temperature dependence of electrical resistivity exhibits a metallic behavior above 10K and the onset of superconducting transition below 6.1 K. With decreasing temperature, the thermomagnetic curve for the sample with Si content = 8 at.% begins to decrease steadily toward negative value below 7.7 K, indicating the Meissner effect. An 2 increase in the transition temperature, T C is attributable to exciton-type superconductivity. 3Assembling of clusters is a promising approach to produce nanostructurecontrolled-materials because their sizes are comparable to the elemental units of material properties. Several experimental techniques have been applied to prepare nano-sized clusters. 1-2)A plasma-gas-condensation cluster beam deposition (PGCCD) apparatus, [3][4][5][6] which is a combination of sputtering vaporization and inert gas condensation, is one of the promising candidates for this purpose. It is versatile to vaporize transition metals and refractive elements, and able to control the cluster size by adjusting the sputter yield, the gas pressure, and the volume of the cluster growth region. Using the PGCCD apparatus, we have succeeded in producing Co-or Fe-core/Si-shell, Fe-core/Al-shell, and Co-core/CoO-shell clusters assemblies, [7][8][9][10] in which the core parts are thermally and chemically stable, semiconducting and tunneling conductivities are observed.In this study, we have tried to make an exciton-type superconductor with the Sn/Si core-shell cluster assemblies. Ginzburg suggested that T C of exciton-type superconductors are higher than those of conventional phonon-type superconductors. 11,12) Superconducting granules embedded in dielectric and/or semiconducting matrices, 13) and a sandwich structure consisting of alternate layers of metal and semiconductor have been proposed to be exciton-type superconductors, 12) in which electron-pairing force, i.e., exciton attraction, 4 is mediated by virtual exciton in the semiconductor layers 12) . Indeed, superconductivity has been observed in Au/Ge and Au/Si multilayer systems.14,15)It is assumed there that the superconductivity is due to the metallic phase of semiconductor. 16,17) However, Ginzburg point out possibility which those multilayer films are exiciton-type superconductors. 13)In this work, we have synthesized Sn/Si core-shell cluster assemblies and investigated their structures and electrical properties. The Sn/Si core-shell cluster assemblies were first prepared by the PGCCD apparatus. The apparatus is composed of three main parts: a sputtering chamber, a cluster growth room, and a deposition chamber. According to the TEM observations, the average thickness of the Si shells on the Sn cores does not vary when the Si content changes from 3 to 37 at.%. It is plausible that the Si vapor condenses to the primary particles of about 2 nm in diameter. They are assembled on the Sn cores to for...

Electrical Resistivity and Morphology of Sn_1&minus;<i>_x</i>/Si<i>_x</i> Core&ndash;Shell Cluster Network Prepared by a Plasma-Gas-Condensation Cluster Source

“…The Si contents at the cores were typically 20 mol% by the EDX analyses and those at the shells 78 mol% (see ref. 15)). This result demonstrated the formation of coreshell clusters.…”

Two-component Bose gas trapped by harmonic and annular potentials: Supercurrent, vortex flow and instability of superfluidity by Rabi coupling

“…That is, the system exhibits interesting behavior depending on the strength of the magnetic field, inter and intrarepulsions, and the Rabi coupling. The present study was originally motivated by the findings of co-existing two kinds of superconductivity in the Sn/Si core-shell clusters 5 , i.e., we expected that a phenomenon similar to the double superconductivity of the Sn/Si core-shell cluster can be realized in a ultracold atomic system. However in due course, we have realized that the ultracold atomic system has strong versatility and controllability.…”