Systems of superconducting islands placed on normal metal films offer tunable realizations of twodimensional (2D) superconductivity 1, 2 ; they can thus elucidate open questions regarding the nature of 2D superconductors and competing states. In particular, island systems have been predicted to exhibit zero-temperature metallic states 3-5 . Although evidence exists for such metallic states in some 2D systems 6, 7 , their character is not well understood: the conventional theory of metals cannot explain them 8 , and their properties are difficult to tune 7,9 . Here, we characterize the superconducting transitions in mesoscopic island-array systems as a function of island thickness and spacing. We observe two transitions in the progression to superconductivity; both transition temperatures exhibit unexpectedly strong depression for widely spaced islands. These depressions are consistent with the system approaching zero-temperature metallic states. The nature of the transitions and the state between them is explained using a phenomenological model involving the stabilization of superconductivity on each island via a weak coupling to and feedback from its neighbors.Conventional zero-temperature ( 0  T ) metallic states do not exist in 2D systems possessing any disorder, because of Anderson localization 8,9 . To reconcile this fact with experimental evidence for 0  T metals in 2D, it has been proposed that the experimental observations do not pertain to conventional metals, but rather to spatially inhomogeneous superconducting (or, more generally, correlated) states 3, 4, 10 . Inhomogeneity is thought to arise in some of these systems because of phase

We demonstrate that 3.5-MeV oxygen irradiation can markedly enhance the in-field critical current of commercial 2 nd generation superconducting tapes with an exposure time of just one second per 0.8 cm 2 . The speed demonstrated here is now at the level required for an industrial reel-to-reel post-processing. The irradiation is made on production line samples through the protective silver coating and does not require any modification of the growth process. From TEM imaging, we identify small clusters as the main source of increased vortex pinning. 2Increasing the current carrying capacity of 2 nd generation (2G) YBCO high temperature superconducting (HTS) wires in the presence of high magnetic fields is critical for the commercialization of HTS based rotating machine applications such as lightweight and compact off-shore wind turbines and motors as well as various HTS magnet applications [1][2][3][4]. For these, operation in magnetic fields of several Tesla and at temperatures around 30K is envisioned. Although conductors of hundreds of meters in length with self-field critical current densities J c of more than 3 -4 MA/cm 2 (more than 300 -400 A/cm-width) at 77 K can now reliably be manufactured, the rapid suppression of J c in even modest applied magnetic fields continues to be a major challenge for HTS conductor development.In recent years, impressive advances in the in-field performance of short-length samples have been achieved [5][6][7][8][9][10], largely due to the strict control over the micro-and nanostructures. The formation of the desired pinning centers depends sensitively on the film deposition technique and substrate architecture. For instance, self-assembled nanorods can be engineered in films grown by pulsed laser deposition (PLD) or MOCVD from material containing excess metal oxides such as BaZrO 3 [11][12][13][14], BaSnO 3 [15] or BaHfO 3 [16], whereas the deposition of films with excess Zr using metal organic deposition (MOD) on single-crystal substrates [17] and on IBAD substrates [18] does not yield nanorods but nanoparticles. In general, the enhanced vortex pinning arises from the complex combined effects of the introduced second phases (nanorods or nanoparticles), additional structural disorder such as twin boundaries, stacking faults and point defects, as well as from isotropic pinning due to strain fields [5,17]. In short-length samples, critical current densities as high as ~ 7 MA/cm 2 at 30 K and 9 T applied parallel to caxis have been reported [9]. The translation of these advances into a reliable large-scale production process is a time consuming process currently under development.An alternative to increase the critical current density by modifying the chemical synthesis is afforded by particle irradiation, which may be applicable to all superconducting materials. Depending on the mass and energy of the ions and the properties of the superconducting material, irradiation enables the creation of defects with well-controlled density and topology, such as points, clusters or tracks. The...

The search for a universal description of vortex matter -one that is applicable to a range of systems and regimes -is a formidable challenge, complicated by the complexity of the interactions between vortices and the environment. Vortex motion that can be induced by Magnus and Lorentz forces or thermal activation can also be counteracted by pinning forces. Because vortex cores are normal (i.e., superfluidity or superconductivity is destroyed inside them), creating a vortex costs energy, and pinning can occur when it is energetically more favorable for a vortex to appear in one location than in another. In type-II superconductors at high enough magnetic fields, vortices are formed by the penetration of magnetic flux, and material disorder that locally reduces the vortex core energy can produce pinning forces that almost completely preclude vortex motion. This results in nearly zero resistance, as long as the current density (J) does not exceed the critical current density (Jc). The caveat is that, for J

YBa2Cu3O7-δ coated conductors (CCs) have achieved high critical current densities (Jc) that can be further increased through the introduction of additional defects using particle irradiation. However, these gains are accompanied by increases in the flux creep rate, a manifestation of competition between the different types of defects. Here, we study this competition to better understand how to design pinning landscapes that simultaneously increase Jc and reduce creep. CCs grown by metal organic deposition show non--monotonic changes in the temperature--dependent creep rate, S(T). Notably, in low fields, there is a conspicuous dip to low S as the temperature (T) increases from 20 K to 65 K. Oxygen--, proton--, and Au--irradiation substantially increase S in this temperature range. Focusing on an oxygen--irradiated CC, we investigate the contribution of different types of irradiation-induced defects to the flux creep rate. Specifically, we study S(T) as we tune the relative density of point defects to larger defects by annealing both an as--grown and an irradiated CC in O2 at temperatures TA = 250°C to 600°C. We observe a steady decrease in S(T > 20 K) with increasing TA, unveiling the role of pre--existing nanoparticle precipitates in creating the dip in S(T) and point defects and clusters in increasing S at intermediate temperatures.

We demonstrate a 2-fold increase of the in-field critical current of AMSC's standard 2G coil wire by irradiation with 18 MeV Au ions. The optimum pinning enhancement is achieved with a dose of 6x10 11 Au ions/cm 2 . Although the 77 K, self-field critical current is reduced by about 35%, the in-field critical current (H//c) shows a significant enhancement between 4 -50K in fields >1 T. The process was used for the roll-to-roll irradiation of AMSC's standard 46 mm wide production coated conductor strips which were further processed in to standard copper laminated coil wire. The long length wires show the same enhancement as attained with short static irradiated samples. The roll-to-roll irradiation process can be incorporated in the standard 2G wire manufacturing with no modifications to the current process. The enhanced performance of the wire will benefit rotating machine and magnet applications.