Large-grain Nb has become a viable alternative to fine-grain Nb for the fabrication of superconducting radio-frequency cavities. In this contribution we report the results from a heat treatment study of a large-grain 1.5 GHz single-cell cavity made of "medium purity" Nb. The baseline surface preparation prior to heat treatment consisted of standard buffered chemical polishing. The heat treatment in the range 800 -1400 • C was done in a newly designed vacuum induction furnace. Q0 values of the order of 2 × 10 10 at 2.0 K and peak surface magnetic field (Bp) of 90 mT were achieved reproducibly. A Q0-value of (5 ± 1) × 10 10 at 2.0 K and Bp = 90 mT was obtained after heat treatment at 1400 • C. This is the highest value ever reported at this temperature, frequency and field. Samples heat treated with the cavity at 1400 • C were analyzed by secondary ion mass spectrometry, secondary electron microscopy, energy dispersive X-ray, point contact tunneling and X-ray diffraction and revealed a complex surface composition which includes titanium oxide, increased carbon and nitrogen content but reduced hydrogen concentration compared to a non heat-treated sample.

Three decades after the prediction of charge-vortex duality in the critical vicinity of the two-dimensional superconductor-insulator transition (SIT), one of the fundamental implications of this duality—the charge Berezinskii-Kosterlitz-Thouless (BKT) transition that should occur on the insulating side of the SIT—has remained unobserved. The dual picture of the process points to the existence of a superinsulating state endowed with zero conductance at finite temperature. Here, we report the observation of the charge BKT transition on the insulating side of the SIT in 10 nm thick NbTiN films, identified by the BKT critical behavior of the temperature and magnetic field dependent resistance, and map out the magnetic-field dependence of the critical temperature of the charge BKT transition. Finally, we ascertain the effects of the finite electrostatic screening length and its divergence at the magnetic field-tuned approach to the superconductor-insulator transition.

Surfaces decorated with dense arrays of microscopic fibres exhibit unique materials properties, including superhydrophobicity and low friction. Nature relies on 'hairy' surfaces to protect blood capillaries from wear and infection (endothelial glycocalyx). Here we report on the discovery of self-assembled tunable networks of microscopic polymer fibres ranging from wavy colloidal 'fur' to highly interconnected networks. The networks emerge via dynamic selfassembly in an alternating electric field from a non-aqueous suspension of 'sticky' polymeric colloidal particles with a controlled degree of polymerization. The resulting architectures are tuned by the frequency and amplitude of the electric field and surface properties of the particles. We demonstrate, using atomic layer deposition, that the networks can serve as a template for a transparent conductor. These self-assembled tunable materials are promising candidates for large surface area electrodes in batteries and organic photovoltaic cells, as well as for microfluidic sensors and filters.

Anomalously high and sharp peaks in the conductance of intrinsic Josephson junctions in Bi 2 Sr 2 CaCu 2 O 8+␦ ͑Bi2212͒ mesas have been commonly interpreted as superconducting energy gaps but here we show they are a result of strong self-heating. This conclusion follows directly from a comparison to the equilibrium gap measured by tunneling in single break junctions on equivalent crystals. As the number of junctions in the mesa, N, and thus heating increase, the peak voltages decrease and the peak width abruptly sharpens for N Ն 12. Clearly these widely variable features vs N cannot all represent the equilibrium properties. Our data imply that the sharp peaks represent a transition to the normal state. That it occurs at the same dissipated power for N = 12-30 strongly implicates heating as the cause. Although peak sharpening due to heating is counterintuitive, as tunneling spectra usually broaden at higher temperatures, a lateral temperature gradient, leading to coexistence of normal hot spots and superconductive regions, qualitatively explains the behavior. However, a more uniform temperature profile cannot be ruled out. As the peak's width and voltage in our shortest mesa ͑N =6͒ are more consistent with the break junction data, we propose a figure of merit for Bi2212 mesas, the relative conductance peak width, such that small values signal a crossover into the strong self-heating regime.

Tunneling spectroscopy was performed on Nb pieces prepared by the same processes used to etch and clean superconducting radio frequency (SRF) cavities. Air exposed, electropolished Nb exhibited a surface superconducting gap Δ=1.55 meV, characteristic of clean, bulk Nb. However the tunneling density of states (DOS) was broadened significantly. The Nb pieces treated with the same mild baking used to improve the Q-slope in SRF cavities, reveal a sharper DOS. Good fits to the DOS were obtained using Shiba theory, suggesting that magnetic scattering of quasiparticles is the origin of the gapless surface superconductivity and a heretofore unrecognized contributor to the Q-slope problem of Nb SRF cavities.Elemental niobium (Nb) is the material of choice in two of the most important commercial superconducting devices: i) the Josephson tunnel junction (JTJ) 1 and ii) the superconducting radio frequency (SRF) cavity. The complex surface oxides of air-exposed Nb 2 have played an important role in the development of both devices over the past 30 years. For JTJs the deleterious effects of the surface oxides (reduced gaps and Josephson currents, large sub-gap quasiparticle currents, etc.) were eliminated by introducing an ultra-thin capping layer of Al, a technique first successfully established on Nb foils 3 . Prevention of oxygen exposure to the underlying Nb is the key to the construction of JTJ devices such as mixers and analog-digital converters, all of which now utilize Nb/Al bi-layer technology 4 .For SRF cavities the Nb oxide layers are relevant because they occupy a significant fraction of the region where electric and magnetic fields E and B are confined, within one magnetic penetration depth (~45 nm) from the surface. The reduction of the SRF cavity quality factor Q with increasing E field, (Q-slope problem) and its mitigation by a mild annealing procedure 5 (baking effect), are not well understood. There is evidence the baking effect is related to a decrease of Nb 2 O 5 and an increase of NbO 2 from surface probes such as X-ray photoemission spectroscopy measurements applied before and after the baking 6 . However, the particular mechanism by which oxygen affects Q-slope is still elusive.

We present an analysis of Nb 3 Sn surface layers grown on a bulk Niobium (Nb) coupon prepared at the same time and by the same vapor diffusion process used to make Nb 3 Sn coatings on 1.3 GHz Nb cavities. Tunneling spectroscopy reveals a well-developed, homogeneous superconducting density of states at the surface with a gap value distribution centered around 2.7 ±0.4 meV and superconducting critical temperatures (T c ) up to 16.3K. Scanning transmission electron microscopy (STEM) performed on cross sections of the sample's surface region shows a ∼ 2 microns thick Nb 3 Sn surface layer. The elemental composition map exhibits a Nb:Sn ratio of 3:1 and reveals the presence of buried sub-stoichiometric regions that have a ratio of 5:1. Synchrotron x-ray diffraction experiments indicate a polycrystalline Nb 3 Sn film and confirm the presence of Nb rich regions that occupy about a third of the coating volume. These low T c regions could play an important role in the dissipation mechanisms occurring during RF tests of Nb 3 Sn -coated Nb cavities and open the way for further improving a very promising alternative to pure Nb cavities for particle accelerators.Discovered in 1954 1 , the A-15 compound Nb 3 Sn is a Type II (κ∼20) strong coupling s-wave superconductor 2,3 with a maximum T c of 18 K 4 and superconducting order parameter ∆ of 3.4 meV 5 . Due to its relatively high T c and ability to carry high current densities, Nb 3 Sn is an ideal candidate for replacing NbTi for superconducting wire applications and Nb for superconducting radio frequency (SRF) resonators operating from a few hundred MHz up to several GHz. Early work into developing Nb 3 Sn for SRF applications started in the 1970s 6-9 . In particular, researchers from Wuppertal University optimized a coating recipe 8 based on the diffusion of Sn vapor into elemental Nb at temperatures between 1000 • C to 1200 • C. This approach has the unique advantage of being scalable to applications for which a coating process without a direct line of sight is required. State-of-the-art RF performance tests then 10 showed an extremely high quality factor ∼ 10 11 at 2K and ∼ 10 10 at 4.2K (about 20 times higher than pure Nb) with a strong decrease of the quality factor (Q -slope) above an accelerating field of 5 MV/m. The origin of this Q -slope remains unclear, however it was postulated that the onset of the Q decrease at 5 MV/m (peak surface magnetic field of 22 mT) was due to early vortex penetration above the Nb 3 Sn first critical field B C1 , and therefore was an intrinsic material limitation. A regain of interest was stimulated by recent RF tests done at Cornell University 11 that reproducibly exhibit a similar Q factor ∼ 2 × 10 10 at 4.2K (and 3 × 10 10 at 2K), but a very moderate Q -slope up to a quenching field of 12-17 MV/m, corresponding to a a) Electronic mail: prolier@anl.gov peak surface magnetic field of 50-70 mT, which is significantly higher than the B C1 of 25±7 mT measured on this cavity 11 . Another striking and reproducible feature is the very moderate ...

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