David H.A. Blank

Preparation of YBa2Cu3O7- by citrate synthesis and pyrolysis

A recent preparation method by citrate synthesis in combination with pyrolysis has been applied to the new high-Tc superconductors. A step-by-step procedure is given to obtain the ultra-fine and homogeneous powder. X-ray diffraction analyses were performed to characterise the powder and to study the influence of the calcination temperature. Resistance measurements exhibit complete superconductivity at 91.2 K.

High-Temperature Magnetic Insulating Phase in Ultrathin La0.67Sr0.33MnO3 Films

We present a study of the thickness dependence of magnetism and electrical conductivity in ultrathin La0.67Sr0.33MnO3 films grown on SrTiO3 (110) substrates. We found a critical thickness of 10 unit cells below which the conductivity of the films disappeared and simultaneously the Curie temperature increased, indicating a magnetic insulating phase at room temperature. These samples have a Curie temperature of about 560 K with a significant saturation magnetization of 1.2±0.2μ(B)/Mn. The canted antiferromagnetic insulating phase in ultra thin films of n<10 coincides with the occurrence of a higher symmetry structural phase with a different oxygen octahedra rotation pattern. Such a strain engineered phase is an interesting candidate for an insulating tunneling barrier in room temperature spin polarized tunneling devices.

Preventing the Reconstruction of the Polar Discontinuity at Oxide Heterointerfaces

When comparing a set of La0.67Sr0.33MnO3 (LSMO) samples, the Curie temperature (TC) of the samples is an important figure of merit for the sample quality. Therefore, a reliable method to determine TC is required. Here, a method based on the analysis of the magnetization loops is proposed.

High Tc thin films prepared by laser ablation: material distribution and droplet problem

The lateral material distribution of laser-deposited YBa2Cu3O7?? films and the density of droplets coming from the target were studied by varying the laser pulse energy, the laser spot size and the target-to-substrate distance. Silicon wafers at ambient temperature were used as substrates to guarantee a large sticking coefficient of the particles. The deposition rate is found to depend linearly on the laser energy density E and quadratically on the spot size S at the target, whereas the droplet density is slightly dependent on E and increases linearly with 1/S, yielding a threshold energy of 0.9 J cm?2. With a laser spot size of 7.15 mm2 and a laser energy density of 1.2 J cm?2, we were able to reduce the number of droplets to one to two per 500 ?m2 for a high quality high Tc film with a typical thickness of 100 nm.

Hard x-ray photoemission and density functional theory study of the internal electric field in SrTiO3/LaAlO3 oxide heterostructures

A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO3 on SrTiO3 is presented. High-resolution, hard x-ray photoemission is used to uncover the occupation of Ti 3d states and the relative energetic alignment—and hence internal electric fields—within the LaAlO3 layer. First, the Ti 2p core-level spectra clearly show occupation of Ti 3d states already for two unit cells of LaAlO3. Second, the LaAlO3 core levels were seen to shift to lower binding energy as the LaAlO3 overlayer thickness, n, was increased, agreeing with the expectations from the canonical electron transfer model for the emergence of conductivity at the interface. However, not only is the energy offset of only ∼300 meV between n=2 (insulating interface) and n=6 (metallic interface) an order of magnitude smaller than the simple expectation, but it is also clearly not the sum of a series of unit-cell-by-unit-cell shifts within the LaAlO3 block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO3 valence bands above the SrTiO3 conduction bands, resulting in charge transfer only for n≥4. We discuss effects which could frustrate this elegant and simple charge-transfer model, concluding that although it cannot be ruled out, photodoping by the x-ray beam is unlikely to be the cause of the observed behavior. Turning to the theoretical data, our density functional simulations show that the presence of oxygen vacancies at the LaAlO3 surface at the 25% level reverses the direction of the internal field in the LaAlO3. Therefore, taking the experimental and theoretical results together, a consistent picture emerges for real-life samples in which nature does not wait until n=4 and already for n=2 mechanisms other than internal-electric-field-driven electron transfer from idealized LaAlO3 to near-interfacial states in the SrTiO3 substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.

Superconducting weak links in YBa2Cu3O7-δ an AC magnetic susceptibility study

AC magnetic susceptibility, X and X, and ac resistivity, ρ, of two different samples of superconducting YBa2Cu3O7-I´ have been measured. The results show a strong non-linear behaviour and are very sensitive to the exciting amplitudes. From the analysis of the data and comparison with appropriate models and other superconducting compounds, evidence for the existence of weak superconducting links is inferred. Finally, the utility of acX measurements for detection of different superconducting phases is made evident.

Thermogravimetry and neutron thermodiffractometry studies of the H-YBa2Cu3O7 system.

The high Tc superconducting oxide YBa2Cu3O7?x reacts with hydrogen gas. Thermogravimetric, X-ray and neutron scattering experiments allow us to propose a two-step type of hydrogen bonding. Firstly, a few hydrogen atoms fill some oxygen vacancies and may favourably modify the electron state, giving rise to a slight increase in the critical temperature. Secondly, after a prolonged heating period, the collapse of the YBa2Cu3O7?x type framework and of superconductivity were observed, and a new, highly hydrogenated material appeared.

La ( Ni , Fe ) O3 Stability in the Presence of Chromia—A Solid-State Reactivity Study

The perovskite

CeO2 as insulation layer in high Tc superconducting multilayer and crossover structures

La(Ni_{0.6}Fe_{0.4})O_3

Nucleation and growth of PrBa2Cu3O7−δ barrier layers on ramps in DyBa2Cu3O7−δ studied by atomic force microscopy

(LNF) is a candidate material for the electrochemically active cathode layer, the cathode current collecting layer, and/or the interconnect protective coating in intermediate temperature solid oxide fuel cells (IT-SOFCs) operated at . Since these operating temperatures enable the use of relatively cheap interconnect materials such as chromia-forming ferritic stainless steel, investigation of the chemical stability of LNF in the presence of chromium species is of importance. This study demonstrates that LNF is chemically unstable at when it is in direct contact with