Jeremy D. Weiss

Weak-link behavior of grain boundaries in superconducting Ba(Fe1−xCox)2As2 bicrystals

We show that despite the low anisotropy, strong vortex pinning, and high irreversibility field Hirr close to the upper critical field Hc2 of Ba(Fe1−xCox)2As2, the critical current density Jgb across [001] tilt grain boundaries (GBs) of thin film Ba(Fe1−xCox)2As2 bicrystals is strongly depressed, similar to high-Tc cuprates. Our results suggest that weak-linked GBs are characteristic of both cuprates and pnictides because of competing orders, low carrier density, and unconventional pairing symmetry.

Strong vortex pinning in Co-doped BaFe2As2 single crystal thin films

We report the field and angular dependences of Jc of truly epitaxial Co-doped BaFe2As2 thin films grown on SrTiO3/(La,Sr)(Al,Ta)O3 with different SrTiO3 template thicknesses. The films show Jc comparable to single crystals and a maximum pinning force Fp(0.6Tc)>5u2002GN/m3 at H/Hirr∼0.5 indicative of strong high-field vortex pinning. Due to the strong correlated c-axis pinning, Jc for field along the c-axis exceeds Jc for H∥ab plane, inverting the expectation of the Hc2 anisotropy. High resolution transmission electron microscopy reveals that the strong vortex pinning is due to a high density of nanosize columnar defects.We report the field and angular dependences of Jc of truly epitaxial Co-doped BaFe2As2 thin films grown on SrTiO3/(La,Sr)(Al,Ta)O3 with different SrTiO3 template thicknesses. The films show Jc comparable to single crystals and a maximum pinning force Fp(0.6Tc)>5u2002GN/m3 at H/Hirr∼0.5 indicative of strong high-field vortex pinning. Due to the strong correlated c-axis pinning, Jc for field along the c-axis exceeds Jc for H∥ab plane, inverting the expectation of the Hc2 anisotropy. High resolution transmission electron microscopy reveals that the strong vortex pinning is due to a high density of nanosize columnar defects.

Self-assembled oxide nanopillars in epitaxial BaFe2As2 thin films for vortex pinning

We report the structure and chemistry of the self-assembled oxide nanopillars that form in superconducting Co-doped BaFe2As2 thin film grown by pulsed laser deposition. The oxide nanopillars consist of a BaFeO2 phase, form epitaxially on the SrTiO3 template, and grow coherently with the BaFe2As2 film. The nanopillars are square with a uniform size of 4–5 nm, which is close to twice the superconducting coherence length. Despite a volume content of ∼5%, the nanopillars do not degrade the structural quality of the BaFe2As2 matrix. Indeed the nanopillars provide exceptionally strong vortex pinning and high critical current density due to the very close correlation of pillar and vortex core diameters.

Mössbauer studies of the superconducting cobalt/nickel-doped BaFe2As2. Whither go the injected electron(s)?

Mössbauer studies of cobalt- and nickel-doped BaFe(2)As(2) show that the s-electron density at the (57)Fe nuclei, as measured by the isomer shift, is the same as that for the parent BaFe(2)As(2). Apparently, the electron population of the d shell, which shields the s-electron density at the nuclei, remains unchanged. We invoke the involvement of p-orbital hybridization with the d orbital in Fe-As bonding. Furthermore, the shrinkage of the lattice on substitution enhances the As-As sp hybridization, providing a path for the migration of additional electrons. The proposed mechanism is consistent with Hall coefficient and thermoelectric effect measurements.

Introduction of CORC ® wires: highly flexible, round high-temperature superconducting wires for magnet and power transmission applications

Conductor on Round Core (CORC®) technology has achieved a long sought-after benchmark by enabling the production of round, multifilament, (RE)Ba2Ca3O7−x coated conductors with practical current densities for use in magnets and power applications. Recent progress, including the demonstration of engineering current density beyond 300 Amm−2 at 4.2 K and 20 T, indicates that CORC® cables are a viable conductor for next generation high field magnets. Tapes with 30 μm substrate thickness and tape widths down to 2 mm have improved the capabilities of CORC® technology by allowing the production of CORC® wires as thin as 3 mm in diameter with the potential to enhance the engineering current density further. An important benefit of the thin CORC® wires is their improved flexibility compared to thicker (7–8 mm diameter) CORC® cables. Critical current measurements were carried out on tapes extracted from CORC® wires made using 2 and 3 mm wide tape after bending the wires to various diameters from 10 to 3.5 cm. These thin wires are highly flexible and retain close to 90% of their original critical current even after bending to a diameter of 3.5 cm. A small 5-turn solenoid was constructed and measured as a function of applied magnetic field, exhibiting an engineering current density of 233 Amm−2 at 4.2 K and 10 T. CORC® wires thus form an attractive solution for applications between 4.2 and 77 K, including high-field magnets that require high current densities with small bending diameters, benefiting from a ready-to-use form (similar to NbTi and contrary to Nb3Sn wires) that does not require additional processing following coil construction.

Record current density of 344 A mm−2 at 4.2 K and 17 T in CORC® accelerator magnet cables

One of the biggest challenges in developing conductor on round core (CORC®) magnet cables for use in the next generation of accelerator magnets is raising their engineering current density J E to approach 600 A mm−2 at 20 T, while maintaining their flexibility. One route to increase J E could be to add more RE-Ba2Cu3O7−δ coated conductors to the cable, but this would increase the cable size and reduce its flexibility. The preferred route to higher J E is a reduction in diameter of the CORC® cable, while maintaining the number of tapes wound into the cable. The availability of very thin tapes containing substrates of 30 μm thickness enabled us to wind a 5.1 mm diameter CORC® cable from 50 coated conductors, while maintaining a tape critical current I c of about 97% after cabling. The cable I c was 7030 A at 4.2 K in a background field of 17 T, corresponding to a J E of 344 A mm−2, which is the highest performance of any CORC® cable so far. The magnetic field dependence allowed us to extrapolate the cable performance to 20 T to predict an I c of 5654 A and a J E of 309 A mm−2. The results clearly show that rapid progress is being made on overcoming the J E hurdle for use of CORC® cables in the next generation of accelerator magnets. Further optimization of the cable layout will likely increase J E towards 600 A mm−2 at 20 T in the near future, while further reduction in cable size will also make them even more flexible.

Structural, electro-magnetic, and optical properties of Ba(Fe,Ni)2As2 single-crystal thin film

We investigated the superconducting transition temperature (T c), critical current density (J c) and optical properties of optimally doped Ba(Fe0.95Ni0.05)2As2 (Ni-Ba122) single-crystalline epitaxial thin films grown by pulsed laser deposition for the first time. The T c at zero resistivity was about 20.5 K and the J c at self-field and 4.2 K was 2.8 MA cm−2 calculated by the Bean model. The superconducting properties such as T c and J c of thin films are comparable to those of bulk single-crystal samples. The superfluid plasma frequency (λ p,S) of Ni-Ba122 thin film is ~7033 cm−1 obtained by optical spectroscopic technique. Based on this plasma frequency, we obtained the London penetration depth (λ L), ~226 nm at 8 K, which is comparable to those of optimally Co- and K-doped BaFe2As2 single crystals.

Evidence for composition variations and impurity segregation at grain boundaries in high current-density polycrystalline K- and Co-doped BaFe2As2 superconductors

Some polycrystalline forms of the K- and Co-doped BaFe2As2 and SrFe2As2 superconductors now have a critical current density (Jc ) within a factor of ∼5 of that required for real applications, even though it is known that some grain boundaries (GBs) block current, thus, raising the question of whether this blocking is intrinsic or extrinsically limited by artefacts amenable to improvement by better processing. Herein, we utilize atom-probe tomography (APT) to study the grain and GB composition in high Jc K- and Co-doped BaFe2As2 polycrystals. We find that all GBs studied show significant compositional variations on the scale of a few coherence lengths (ξ), as well as strong segregation of oxygen impurities, which we believe are largely introduced in the starting materials. Importantly, these findings demonstrate that APT enables quantitative analysis of the highest Jc K-doped BaFe2As2 samples, where analytical transmission electron microscopy (TEM) fails because of the great reactivity of thin TEM samples. The observations of major chemical perturbations at GBs make us cautiously optimistic that there is a large extrinsic component to the GB current blocking, which will be ameliorated by better processing, for which APT will likely be a crucial instrument.

Transmittance and reflectance measurements at terahertz frequencies on a superconducting BaFe 1.84 Co 0.16 As 2 ultrathin film: an analysis of the optical gaps in the Co-doped BaFe 2 As 2 pnictide

AbstractnHere we report an optical investigation in the terahertz region of axa040xa0nm ultrathinnBaFe1.84Co0.16As2 superconducting film withnsuperconducting transition temperaturenTcxa0=xa017.5xa0K. A detailed analysis of thencombined reflectance and transmittance measurements showed that the optical properties ofnthe superconducting system can be described in terms of a two-band, two-gap model. Thenzero temperature value of the large gap ΔB, which seems tonfollow a BCS-like behavior, results to benΔB(0)xa0=xa017xa0cm-1. For the small gap, for whichnΔA(0)xa0=xa08xa0cm-1, the temperature dependencencannot be clearly established. These gap values and those reported in the literature fornthe BaFe2−xCoxAs2nsystem by using infrared spectroscopy, when put together as a function ofnTc, show a tendency to cluster along twonmain curves, providing a unified perspective of the measured optical gaps. Below antemperature aroundxa020xa0K, the gap-sizes as a function ofnTc seem to have a BCS-like linearnbehavior, but with different slopes. Above this temperature, both gaps show differentnsupra-linear behaviors.n

Irreversibility Line Measurement and Vortex Dynamics in High Magnetic Fields in Ni- and Co-Doped Iron Pnictide Bulk Superconductors

The de-pinning or irreversibility lines were determined by ac susceptibility, magnetization, radio-frequency proximity detector oscillator (PDO), and resistivity methods in Ba(Fe0.92Co0.08)2As2 ( Tc = 23.2 K), Ba(Fe0.95Ni0.05)2As2 ( Tc = 20.4 K), and Ba(Fe0.94Ni0.06)2As2 ( Tc = 18.5 K) bulk superconductors in ac, dc, and pulsed magnetic fields up to 65 T. A new method of extracting the irreversibility fields from the radio-frequency proximity detector oscillator induction technique is described. Wide temperature broadening of the irreversibility lines, for any given combination of ac and dc fields, is dependent on the time frame of measurement. Increasing the magnetic field sweep rate (dH/dt) shifts the irreversibility lines to higher temperatures up to about dH/d t = 40,000 Oe/s; for higher dH/dt, there is little impact on the irreversibility line. There is an excellent data match between the irreversibility fields obtained from magnetization hysteresis loops, PDO, and ac susceptibility measurements, but not from resistivity measurements in these materials. Lower critical field vs. temperature phase diagrams are measured. Their very low values near 0 T indicate that these materials are in mixed state in nonzero magnetic fields, and yet the strength of the vortex pinning enables very high irreversibility fields, as high as 51 T at 1.5 K for the Ba(Fe0.92Co0.08)2As2 polycrystalline sample, showing a promise for liquid helium temperature applications.