Proximity effects and vortex dynamics in mesoscopic superconductor-normal metal-superconductor arrays.

摘要

Anderson's scaling theory of localization has proven invaluable in characterizing the behavior of real systems, that is, those possessing any amount of disorder. The theory predicts that, at zero temperature in 1D and 2D systems, the diffusive motion of electrons scattering off impurities ceases, and there is no long range electron transport. In other words, there are no metallic states at T = 0 in 1D and 2D systems. Although this theory has accurately described the low-temperature behavior of many materials, systems ranging from 2D semiconductors 1 to disordered superconductors2,3 have in fact shown evidence of a "forbidden" zero-temperature metallic state. To reconcile these experimental results with Anderson localization, it has been proposed that these observations do not pertain to conventional metals, but rather to spatially inhomogeneous correlated states4--6. Determining the origin and characteristics of such states has attracted intense theoretical and experimental interest over the past two decades. Contributing to these efforts, we engineer a tunable, intrinsically phase-separated system. Our research focuses on novel model systems of 2D superconductors, systems which have been predicted to exhibit unusual metallic states as the temperature approaches zero. In particular, we created triangular arrays of physically separated mesoscopic superconducting islands placed on normal metal films, and measured the temperature-dependent transition to the superconducting state as a function of the island separation. We found two surprising results: first, the long-range communication between the islands occurs in a way that cannot be explained by current theories. Second, the progressive weakening of superconductivity with increasing island spacing suggests that arrays with even further spacing would be metallic at T = 0. This is the first systematic study of an inhomogeneous superconducting system that systematically approaches a zero-temperature metallic state. Finally, the sparsest arrays studied show evidence of a 2D metallic state.;The results suggest that such superconductor-normal-metal systems may be an ideal medium for tunably controlling the properties of this strange metal. To further understand these systems, we characterize the vortex dynamics intrinsic to the 2D superconducting ground state, as well as that in response to an externally applied current and magnetic field. We provide evidence that the superconducting state is characterized by bound vortex-antivortex pairs. Additionally, we study the current-voltage characteristics; applying a current induces a Lorentz force on vortices that competes with pinning in the arrays. Lastly, in response to sweeping the field, we observe resistance oscillations, manifestations of competing magnetic ground states and correlated vortex motion.