Anatolii Polyanskii

Template engineering of Co-doped BaFe2As2 single-crystal thin films

Understanding new superconductors requires high-quality epitaxial thin films to explore intrinsic electromagnetic properties and evaluate device applications. So far, superconducting properties of ferropnictide thin films seem compromised by imperfect epitaxial growth and poor connectivity of the superconducting phase. Here we report new template engineering using single-crystal intermediate layers of (001) SrTiO(3) and BaTiO(3) grown on various perovskite substrates that enables genuine epitaxial films of Co-doped BaFe(2)As(2) with a high transition temperature (T(c,rho=0) of 21.5 K, where rho=resistivity), a small transition width (DeltaT(c)=1.3 K), a superior critical current density J(c) of 4.5 MA cm(-2) (4.2 K) and strong c-axis flux pinning. Implementing SrTiO(3) or BaTiO(3) templates to match the alkaline-earth layer in the Ba-122 with the alkaline-earth/oxygen layer in the templates opens new avenues for epitaxial growth of ferropnictides on multifunctional single-crystal substrates. Beyond superconductors, it provides a framework for growing heteroepitaxial intermetallic compounds on various substrates by matching interfacial layers between templates and thin-film overlayers.

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.

Intergrain current flow in a randomly oriented polycrystalline SmFeAsO0.85 oxypnictide

We report a direct current transport study of the local intergrain connections in a polycrystalline SmFeAsO0.85 (Sm1111) bulk, for which we earlier estimated significant intergranular critical current density Jc. Our combined low temperature laser scanning microscopy and scanning electron microscopy observations revealed only few grain-to-grain transport current paths, most of which switched off when a magnetic field was applied. These regions typically occur where current crosses Fe–As, which is a normal-metal wetting-phase that surrounds Sm1111 grains, producing a dense array of superconducting-normal-superconducting contacts. Our study points out the need to reduce the amount of grain boundary-wetting Fe–As phase, as well as the crack density within pnictide grains, as these defects produce a multiply connected current-blocking network.

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)>5 GN/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)>5 GN/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.

Evidence for electromagnetic granularity in the polycrystalline iron-based superconductor LaO0.89F0.11FeAs

The recently discovered oxypnictide superconductors are layered, low carrier density compounds with many similarities to the high-Tc cuprates. An important question is whether they also exhibit weak-coupling across randomly oriented grain boundaries. In this work we show considerable evidence for such weak coupling by study of the dependence of magnetization in bulk and powdered samples. Bulk sample magnetization curves show very little hysteresis while remanent magnetization shows almost no sample size dependence, even after powdering. We conclude that these samples exhibit substantial electromagnetic granularity on a scale approximating the grain size, though we cannot yet determine whether this is intrinsic or extrinsic.

Topological hysteresis in the intermediate state of type-I superconductors

Samples had thickness between d = 0.1 and 1.5 mm and were about 1.5 × 1.5 mm 2 in planar dimension. The topological features described here were thickness independent above d � 0.5mm, which indicates that they are not due to surface-related effects. We show data for samples which had demagnetization factors of about N = 0.5 (determined both from initial magnetisation and direct calculations [20].) Quantum Design MPMS magnetometer was used for DC magnetization measurements. MO imaging was performed in a pumped flow-type optical 4 He cryostat using Faraday rotation of polarized light in Bi-doped iron-garnet films with in-plane magnetization [15]. In all images the bright regions correspond to the normal state and dark regions to the superconducting state.

Anisotropic in-plane reversible strain effect in Y0.5Gd0.5Ba2Cu3O7 − δ coated conductors

Recent experiments have shown that reversible effects of strain on the critical current density and flux pinning strength in the high-temperature superconductor Bi2Sr2Ca2Cu3Ox can be explained entirely by the pressure dependence of its critical temperature. Such a correlation is less simple for RE–Ba2Cu3O7 − δ (RE = rare earth) superconductors, in part because the in-plane pressure dependence of its critical temperature is highly anisotropic. Here, we make a qualitative correlation between the uniaxial pressure dependence of the critical temperature and the reversible strain effect on the critical current of RE–Ba2Cu3O7 − δ coated conductors by taking the crystallography and texture of the superconducting film into account. The strain sensitivity of the critical current is highest when strain is oriented along the [100] and [010] directions of the superconducting film, whereas the critical current becomes almost independent of strain when strain is oriented along the [110] direction. The results confirm the important role of the anisotropic pressure dependence of the critical temperature on the reversible strain behavior of RE–Ba2Cu3O7 − δ. The reversible strain effect in RE–Ba2Cu3O7 − δ is expected to decrease the performance of the conductor in many applications, such as high-field magnets, but the effect may be only minor in coated conductor cables, where strain is generally not aligned with the tape axis.

Double disordered YBCO coated conductors of industrial scale: high currents in high magnetic field

A significant increase of critical current in high magnetic field, up to 31 T, was recorded in long tapes manufactured by employing a double-disorder route. In a double-disordered high-temperature superconductor (HTS), a superimposing of intrinsic and extrinsic disorder takes place in a way that (i) the intrinsic disorder is caused by local stoichiometry deviations that lead to defects of crystallinity that serve as pining centers in the YBa2Cu3O x−δ matrix and (ii) the extrinsic disorder is introduced via embedded atoms or particles of foreign material (e.g. barium zirconate), which create a set of lattice defects. We analyzed possible technological reasons for this current gain. The properties of these tapes over a wider field-temperature range as well as field anisotropy were also studied. Record values of critical current as high as 309 A at 31 T, 500 A at 18 Tm and 1200 A at 5 T were found in 4 mm wide tape at 4.2 K and B perpendicular to tape surface. HTS layers were processed in medium-scale equipment that allows a maximum batch length of 250 m while 22 m long batches were provided for investigation. Abnormally high ratios (up to 10) of critical current density measured at 4.2 K, 19 T to critical current density measured at 77 K, self-field were observed in tapes with the highest in-field critical current. Anisotropy of the critical current as well as angular dependences of n and α values were investigated. The temperature dependence of critical current is presented for temperatures between 4.2 and 40 K. Prospects for the suppression of the dog-bone effect by Cu plating and upscale of processing chain to >500 m piece length are discussed.

Significant reduction of AC losses in YBCO patterned coated conductors with transposed filaments

The concentration of internal gettering sites within a semiconductor wafer is controlled by two-step thermal processing. In a concentration reduction phase, the wafer is rapidly heated to an elevated temperature in the range from about 900 DEG to 1350 DEG C., resulting in the partial or total dissolution of precipitable impurities within the wafer. In a concentration enhancement step, the wafers are subjected to a relatively low temperature anneal process where the density of potential internal gettering sites is increased. By properly controlling the processing temperatures and treatment times, the two steps may be performed in either order to obtain wafers having internal gettering site concentrations within a desired range.

Neutron irradiation of SmFeAsO1−xFx

SmFeAsO1−x Fx was irradiated in a fission reactor by a fast (E > 0.1 MeV) neutron fluence of 4 × 10 21 m −2 . The introduced defects increased the normal state resistivity due to a reduction in the mean free path of the charge carriers. This leads to an enhancement of the upper critical field at low temperatures. The critical current density within the grains, Jc, increases upon irradiation. The second maximum in the field dependence of Jc disappears and the critical current density becomes a monotonically decreasing function of the applied magnetic field. (Some figures in this article are in colour only in the electronic version)