Ruben Hühne

Formation of nanosized BaIrO3 precipitates and their contribution to flux pinning in Ir-doped YBa2Cu3O7−δ quasi-multilayers

Structural and transport measurements for quasimultilayers of Ir-doped YBa2Cu3O7−δ prepared by pulsed-laser deposition are presented. Due to metallic Ir doping, BaIrO3 particles form during film growth. These nanosized particles, having a perovskite structure, grow epitaxially in cube-on-cube relationship inside the film. A strong increase in pinning force density and, hence, Jc was found.

Strong Tc dependence for strained epitaxial Ba(Fe1−xCox)2As2 thin films

Ba(Fe1−xCox)2As2 superconducting thin films have been grown on SrTiO3, (La,Sr)(Al,Ta)O3, LaAlO3, and YAlO3 (YAO) single crystal substrates by pulsed laser deposition. All the films, except on YAO, have been grown epitaxially without buffer layers. The films deposited on YAO contained 45° in-plane rotated grains and showed a broad superconducting transition. The onset Tc of the films is observed to increase from 16.2 to 24.5 K with increasing c/a, mainly due to a slight distortion of the AsFe4 tetrahedron. From this correlation, we expect that higher superconducting transition temperatures than 24.5 K in a strained epitaxial film may be possible.

Enhanced flux pinning in YBa2Cu3O7 layers by the formation of nanosized BaHfO3 precipitates using the chemical deposition method

Both a high critical current density and a cost-competitive, easily upscalable procedure are among the most critical requirements for coated conductors. In this work, a low-cost method based on the trifluoroacetic acid process was used to prepare nanosized BaHfO3 particles inside a YBa2Cu3O7 film matrix. The inclusions have a perovskite structure and grow epitaxially in a cube-on-cube relationship. A drastically improved pinning force density and high irreversibility fields in the YBa2Cu3O7 layers were found and are discussed in respect to the Hf doping level in the precursor solutions, which resulted in an increased nanoinclusion content.

An all chemical solution deposition approach for the growth of highly textured CeO2 cap layers on La2Zr2O7-buffered long lengths of biaxially textured Ni–W substrates for YBCO-coated conductors

A reel-to-reel, dip coating process has been developed to continuously deposit epitaxial La2Zr2O7 (LZO) and CeO2 on 5 m long cube-textured {100} (001)Ni tapes. Recent results for La2Zr2O7 and CeO2 buffer layers deposited on long lengths of Ni substrate for the realization of YBa2Cu3O7−x (YBCO)-coated conductors are presented. The major achievement is the development of a new all chemical solution deposition (CSD) process leading to the formation of highly textured buffer layers at moderate annealing temperatures. Reproducible highly textured, dense and crack-free LZO buffer layers and CeO2 cap layers were obtained for annealing temperatures as low as 900 °C in a reducing atmosphere (Ar–5 at.%-H2). The thickness of the LZO buffer layers was determined to be (200 ± 10) nm per single coating; prepared cerium oxide layers showed a thickness of 60 nm ± 10 nm. Pulsed laser deposition (PLD) was used to grow YBCO films on these substrates. A Tc 0 of T = 90.5 K and ΔTc = 1.4 K was obtained on PLD-YBCO/CSD-CeO2 /CSD-LZO/Ni–5 at.% W, which shows the outstanding features of this new buffer layer architecture processed by CSD. The large layer thickness combined with low annealing temperatures is the main advantage of this new process for low-cost buffer layer deposition on Ni-RABiTS (rolling-assisted biaxially textured substrates).

Highly textured La2Zr2O7 buffer layers for YBCO-coated conductors prepared by chemical solution deposition

Recent results of La2Zr2O7 (LZO) buffer layer development for YBa2Cu3O7−x (YBCO-) coated conductors are presented. The major achievement is the development of a new precursor solution starting from 2,4-pentanedionates of lanthanum and zirconium leading to the formation of highly textured LZO buffer layers at low annealing temperatures. The preparation of the precursor solution using only a carboxylic acid as the solvent is simple and can be carried out at room temperature under atmospheric conditions. Reproducible highly textured buffer layers were obtained at annealing temperatures as low as 900 °C. The simplicity of the precursor solution preparation and the low annealing temperature (similar to the processing temperature of the YBCO layer) for preparation of an LZO buffer layer are the main advantages of this new process for cost-effective buffer layer deposition on Ni-RABiTS (rolling-assisted biaxially textured substrates).

Growth and anisotropy of La(O, F)FeAs thin films deposited by pulsed laser deposition

LaFeAsO1?xFx thin films were deposited successfully on (001)-oriented LaAlO3 and MgO substrates from stoichiometric LaFeAsO1?xFx polycrystalline targets with fluorine concentrations up to x = 0.25 by pulsed laser deposition (PLD). Room temperature deposition and post annealing of the films yield films with a pronounced c-axis texture and a strong biaxial in-plane orientation. Transport measurements show metallic resistance and the onset of superconductivity at 11?K. ?0Hc 2(T) was determined by resistive measurements and yield ?0Hc 2 values of 3?T at 3.6?K for and 6?T at 6.4?K for .

Scaling behavior of the critical current in clean epitaxial Ba(Fe1-xCox)2As2 thin films

The angular-dependent critical current density, Jc(theta), and the upper critical field, Hc2(theta), of epitaxial Ba(Fe1-xCox)2As2 thin films have been investigated. No Jc(theta) peaks for H || c were observed regardless of temperatures and magnetic fields. In contrast, Jc(theta) showed a broad maximum at theta=90 degree, which arises from intrinsic pinning. All data except at theta=90 degree can be scaled by the Blatter plot. Hc2(theta) near Tc follows the anisotropic Ginzburg-Landau expression. The mass anisotropy increased from 1.5 to 2 with increasing temperature, which is an evidence for multi-band superconductivity.

Experimental signatures of the mixed axial–gravitational anomaly in the Weyl semimetal NbP

The conservation laws, such as those of charge, energy and momentum, have a central role in physics. In some special cases, classical conservation laws are broken at the quantum level by quantum fluctuations, in which case the theory is said to have quantum anomalies. One of the most prominent examples is the chiral anomaly, which involves massless chiral fermions. These particles have their spin, or internal angular momentum, aligned either parallel or antiparallel with their linear momentum, labelled as left and right chirality, respectively. In three spatial dimensions, the chiral anomaly is the breakdown (as a result of externally applied parallel electric and magnetic fields) of the classical conservation law that dictates that the number of massless fermions of each chirality are separately conserved. The current that measures the difference between left- and right-handed particles is called the axial current and is not conserved at the quantum level. In addition, an underlying curved space-time provides a distinct contribution to a chiral imbalance, an effect known as the mixed axial–gravitational anomaly, but this anomaly has yet to be confirmed experimentally. However, the presence of a mixed gauge–gravitational anomaly has recently been tied to thermoelectrical transport in a magnetic field, even in flat space-time, suggesting that such types of mixed anomaly could be experimentally probed in condensed matter systems known as Weyl semimetals. Here, using a temperature gradient, we observe experimentally a positive magneto-thermoelectric conductance in the Weyl semimetal niobium phosphide (NbP) for collinear temperature gradients and magnetic fields that vanishes in the ultra-quantum limit, when only a single Landau level is occupied. This observation is consistent with the presence of a mixed axial–gravitational anomaly, providing clear evidence for a theoretical concept that has so far eluded experimental detection.

Coherent interfacial bonding on the FeAs tetrahedron in Fe/Ba(Fe1-xCox)2As2 bilayers

We demonstrate the growth of epitaxial Fe/Ba(Fe1−xCox)2As2 bilayers on MgO(001) and (La,Sr)(Al,Ta)O3(001) single crystal substrates using pulsed laser deposition (PLD). This architecture produces a coherent interfacial bond between body-centered cubic iron and Ba(Fe1−xCox)2As2 (Ba–122). The superconducting transition temperatures for both bilayers remain close to that of the PLD target. Direct observation of interfacial bonding between Fe and the Ba–122 FeAs sublattice by atomic resolution transmission electron microscopy implies that this bilayer architecture may work for other iron pnictide systems and pave the way for their fabrication into superconducting/ferromagnetic heterostructures.

Domain structure of epitaxial Co films with perpendicular anisotropy

Epitaxial hcp Cobalt films with pronounced c-axis texture have been prepared by pulsed lased deposition (PLD) either directly onto Al2O3 (0001) single crystal substrates or with an intermediate Ruthenium buffer layer. The crystal structure and epitaxial growth relation was studied by XRD, pole figure measurements and reciprocal space mapping. Detailed VSM analysis shows that the perpendicular anisotropy of these highly textured Co films reaches the magnetocrystalline anisotropy of hcp-Co single crystal material. Films were prepared with thickness t of 20 nm < t < 100 nm to study the crossover from in-plane magnetization to out-of-plane magnetization in detail. The analysis of the periodic domain pattern observed by magnetic force microscopy allows to determine the critical minimum thickness below which the domains adopt a pure in-plane orientation. Above the critical thickness the width of the stripe domains is evaluated as a function of the film thickness and compared with domain theory. Especially the discrepancies at smallest film thicknesses show that the system is in an intermediate state between in-plane and out-of-plane domains, which is not described by existing analytical domain models.