Fate of global superconductivity in arrays of long SNS junctions

Normal-metal films overlaid with arrays of superconducting islands undergo Berezinskii-Kosterlitz-Thouless (BKT) superconducting transitions at a temperature $T_{BKT}$. We present measurements of T$_{BKT}$ for arrays of mesoscopic Nb islands patterned on Au films for a range of island spacings $d$. We show that $T_{BKT} \sim 1/d^2$, and explain this dependence in terms of the quasiclassical prediction that the Thouless energy, rather than the superconducting gap, governs the inter-island coupling. We also find two deviations from the quasiclassical theory: (i) $T_{BKT}$ is sensitive to island height, because the islands are mesoscopic; and (ii) for widely spaced islands the transition appears to lead, not to a superconducting state, but to a finite-resistance "metallic" one.Comment: 12 pages, 4 figure

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

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 to disordered superconductors 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 states. 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

Vortex phases and glassy dynamics in the highly anisotropic superconductor HgBa2_{2}CuO4+δ_{4+δ}

We present an extensive study of vortex dynamics in a high-quality single crystal of HgBa$_{2}$CuO$_{4+δ}$, a highly anisotropic superconductor that is a model system for studying the effects of anisotropy. From magnetization M measurements over a wide range of temperatures T and fields H, we construct a detailed vortex phase diagram. We find that the temperature-dependent vortex penetration field H$_{p}$(T), second magnetization peak H$_{smp}$(T), and irreversibility field H$_{irr}$(T) all decay exponentially at low temperatures and exhibit an abrupt change in behavior at high temperatures T/Tc >~ 0.5. By measuring the rates of thermally activated vortex motion (creep) S(T, H) = |dlnM(T, H)/dlnt|, we reveal glassy behavior involving collective creep of bundles of 2D pancake vortices as well as temperature- and time-tuned crossovers from elastic (collective) dynamics to plastic flow. Based on the creep results, we show that the second magnetization peak coincides with the elastic-to-plastic crossover at low T, yet the mechanism changes at higher temperatures

Designing high-performance superconductors with nanoparticle inclusions: Comparisons to strong pinning theory

One of the most promising routes for achieving high critical currents in superconductors is to incorporate dispersed, non-superconducting nanoparticles to control the dissipative motion of vortices. However, these inclusions reduce the overall superconducting volume and can strain the interlaying superconducting matrix, which can detrimentally reduce T$_{c}$. Consequently, an optimal balance must be achieved between the nanoparticle density n$_{p}$ and size d. Determining this balance requires garnering a better understanding of vortex–nanoparticle interactions, described by strong pinning theory. Here, we map the dependence of the critical current on nanoparticle size and density in (Y$_{0.77}$, Gd$_{0.23}$)Ba$_{2}$Cu$_{3}$O$_{7−δ}$ films in magnetic fields of up to 35 T and compare the trends to recent results from time-dependent Ginzburg–Landau simulations. We identify consistency between the field-dependent critical current J$_{c}$ (B) and expectations from strong pinning theory. Specifically, we find that J$_{c}$ ∝ B$^{−α }$, where α decreases from 0.66 to 0.2 with increasing density of nanoparticles and increases roughly linearly with nanoparticle size d/ξ (normalized to the coherence length). At high fields, the critical current decays faster (∼B$^{Z-1}$), suggesting that each nanoparticle has captured a vortex. When nanoparticles capture more than one vortex, a small, high-field peak is expected in Jc(B). Due to a spread in defect sizes, this novel peak effect remains unresolved here. Finally, we reveal that the dependence of the vortex creep rate S on nanoparticle size and density roughly mirrors that of α, and we compare our results to low-T nonlinearities in S(T) that are predicted by strong pinning theory