O. Brunner

In situ thin films of YBa2Cu3O7 prepared by single target DC-magnetron sputtering

Abstract We have prepared in situ thin films of YBa 2 Cu 3 O 7 by single target dc reactive magnetron sputtering. By a delicate adjustment of the oxygen partial pressure as well as the total sputtering pressure, we can grow stoichiometric films using a stoichiometric target. This method gives highly reproducible results and the films have optimum superconducting properties without the need for any post-annealing treatment. The films are highly textured with the c -axis perpendicular to the plane of the substrate for films grown on unoriented ZrO 2 and a mixed c -axis and a -axis orientation on (100) SrTiO 3 . Zero resistances are obtained between 85 and 88 K with onsets between 90 and 91 K. The residual resistivity ratios R(RT) R(100 K) have values between 2.5 and 3 and the resistivity at 100 K is 60 μΩ cm for films on SrTiO 3 . SEM photos reveal much smoother surfaces than post-annealed films. We also obtained RHEED streaks, indicative of an oriented atomically smooth surface, on a 250–300 A thick layer on SrTio 3 .

Superconducting and magnetic properties of Y1Ba2Cu4O8±δ

Abstract We have carried out extensive characterization and measurements on a high purity Y 1 Ba 2 Cu 4 O 8±δ (124) sample. Metallographic investigations, X-ray diffraction and microprobe analysis have been performed and reveal a single phase with traces of CuO. Resistivity measurements give a resistivity ratio R (300 K)/ R (100 K) of ≈ 6.1, and a critical temperature T co of 82.5 K. The AC susceptibility shows a sharp transition at about same temperature. The Meissner effect, with a 55% fraction (at 4.2 K with H =20.3 Oe) confirms the bulk nature superconductivity. The specific heat difference C (0, T )− C ( H , T ) has been estimated from magnetization measurements in the vicinity of T c for μ 0 H =8 T and compared to the direct calorimetric measurement. Both methods indicate that the specific heat jump amounts to approximately 15 mJ/K 2 mol. Fluctuation contributions are found up to several degrees above T c . Inter- and intra-grain critical currents were determined at 4.2 K by AC inductive measurements and by measuring the irreversibility of the magnetization curve at a constant temperature (Beans method). The results shows that there is no significant difference between the critical currents of the 124 phase and those of the 123 phase (Y 1 Ba 2 Cu 3 O 7 ).

Microstructure of YBa2Cu3O7/PrBa2Cu3O7 superlattices deposited on (1 0 0) SrTiO3 single crystals

Abstract High-quality, c -oriented YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 superlattices have been grown on (1 0 0) SrTiO 3 substrates. TEM cross sections revealed that the sharpness of the (1 0 0) interfaces was of the order of the lattice parameter (1 nm). The films were deposited at 790°C and no evidence of Y/Pr interdiffusion could be detected. The orthorhombic (1 1 0) twins were 20 nm wide and the orthorhombic distortion in the YBa 2 Cu 3 O 7 layer yielded a ( b − a )/ b ratio of 1.6%. Surface steps and inclined surface areas at the SrTiO 3 YBa 2 Cu 3 O 7 interface were imaged by high-resolution TEM. Apart from a 2–6 nm wide layer at the interface, which contained extended defects accounting for the mismatch due to the imperfect surface, the film grew perfectly on top of this layer. (0 0 1) lattice planes passed continuously from regions with perfect substrate surfaces to regions containing a larger density of surface steps.

X‐ray photoelectron spectroscopy study of thin Ag overlayers on YBa2Cu3O7−x: Effects of oxygen annealing

X‐ray photoelectron spectroscopy was used to examine vapor deposited Ag overlayers on YBa2Cu3O7−x thin films and cleaved single‐crystal surfaces (x=1), including the effects of subsequent heat treatment in oxygen. The core level spectra of the present tetragonal crystals were found to closely resemble those previously reported for nominally orthorhombic superconducting samples. Minor differences, notably an increased fraction of Cu in the +1 oxidation state and a slightly higher binding energy of the Ba 3d levels, may be attributed to the lower oxygen content of the tetragonal semiconducting phase. Deposition of Ag on clean monocrystal surfaces was found to cause some disruption of CuO bonds at the Ag/YBa2Cu3O7−x interface. We also find weak indications of surface band bending (i.e., Schottky barrier formation) from a uniform shift in the measured binding energies of the substrate core levels upon Ag deposition. Heat treatment in pure oxygen (1 h at 500 °C) of Ag‐coated superconducting thin films was foun...

Extended defects in YBa2Cu3O7/PrBa2Cu3O7 superlattices deposited on (1 0 0) MgO single crystals

Abstract Mixed a - and c -oriented YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 superlattices have been grown on (1 0 0) MgO substrates. These superlattices were analyzed by transmission electron microscopy. The interfaces between the various layers are sharp (sharpness ≈ 1 nm) and show no evidence for Y/Pr interdiffusion. In cross sections, a -oriented nuclei 25 nm in diameter were observed at the MgO interface. The volume fraction of a -oriented grains increases with the distance from the interface. At the surface the 115 nm thick film is almost completely a -oriented and consist of grains 150 nm in size. The planar defects in these grains were studied by high- resolution transmission electron microscopy and were due to additionally inserted CuO layers.

Artificially prepared YBa2Cu3O7 PrBa2Cu3O7 superlattices: Growth and superconducting properties

Abstract We report on the growth and superconducting properties of artificially prepared YBa 2 Cu 3 O 7 PrBa 2 Cu 3 O 7 ( YBCO PrBCO ) superlattices. We have grown these superlattices by single target dc planar magnetron sputtering using the high sputtering pressure technique to obtain the in situ growth with the correct stoichiometry. We find in these modulated structures that the superconducting critical temperature, Tc, depends markedly on the individual YBCO thickness in the multilayer and on the insulating PrBCO separation thickness. For series of samples with constant 12A or 24A layers of YBCO (one or two unit cells), Tc initially decreases with increasing thickness of PrBCO, d-PrBCO, and then saturates for d-PrBCO larger than 60–70A. We discuss this behavior in terms of a progressive decoupling of the ultrathin YBCO layers in the multilayer. Resistively measured parallel critical fields show, for structures with thick PrBCO layers, a transition from a flux flow regime for thick YBCO layers to a behavior with only very weak magnetic field induced broadening of the resistive transitions for thin YBCO layers. For multilayers with individual YBCO layers of 48A or 96A thickness we find a linear behavior of the resistively determined critical field with a slope of ≅7.5T/K. When the individual YBCO layer thickness is reduced to 24A practically no effect of a 9T magnetic field could be observed. These different regimes are discussed in terms of the particular vortex structures in the multilayers.

Epitaxial high Tc cuprate superlattices: A study of the thermal flux motion

Abstract Epitaxial c-axis YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 (YBCO/PrBCO) superlattices grown by single target dc planar magnetron sputtering are used as a model system to investigate the nature of flux motion in high T c superconductors (HTS). When a magnetic field H is applied parallel to the c-axis we find that, for sufficiently thick PrBCO layers, the activation energy for flux motion U is proportional to the YBCO thickness. This allows us to extract a lower limit, about 500A, for the correlation lenght along the vortices in pure YBCO. We find also for thin YBCO layers that U∼lnH which could be the signature that the dissipation process is related to the creation in the 2-D vortex lattice of dislocations pairs. When the field is applied in the a-b plane we find for thin YBCO layers separated by thick enough PrBCO layers that the resistive transition becomes field independent. Measurements on a 24A/96A YBCO/PrBCO multilayer up to 20T do not show any broadening of the resistive transition. We also find that the activation energy for flux motion decreases with decreasing the YBCO thickness and is insensitive to magnetic field below the H c1 of the individual YBCO layers.

Artificially prepared superlattices: Growth and superconducting properties

Abstract We report on the growth and superconducting properties of artificially prepared YBa 2 Cu 3 O 7 PrBa 2 Cu 3 O 7 ( YBCO PrBCO ) superlattices. We have grown these superlattices by single target dc planar magnetron sputtering using the high sputtering pressure technique to obtain the in situ growth with the correct stoichiometry. We find in these modulated structures that the superconducting critical temperature, Tc, depends markedly on the individual YBCO thickness in the multilayer and on the insulating PrBCO separation thickness. For series of samples with constant 12A or 24A layers of YBCO (one or two unit cells), Tc initially decreases with increasing thickness of PrBCO, d-PrBCO, and then saturates for d-PrBCO larger than 60–70A. We discuss this behavior in terms of a progressive decoupling of the ultrathin YBCO layers in the multilayer. Resistively measured parallel critical fields show, for structures with thick PrBCO layers, a transition from a flux flow regime for thick YBCO layers to a behavior with only very weak magnetic field induced broadening of the resistive transitions for thin YBCO layers. For multilayers with individual YBCO layers of 48A or 96A thickness we find a linear behavior of the resistively determined critical field with a slope of ≅7.5T/K. When the individual YBCO layer thickness is reduced to 24A practically no effect of a 9T magnetic field could be observed. These different regimes are discussed in terms of the particular vortex structures in the multilayers.

Studies of the surface structure of YBa2Cu3O7 thin films using STM

Abstract The surface structure of magnetron-sputtered YBa 2 Cu 3 O 7 thin films has been investigated using an STM operated in air. Both c -axis and a -axis oriented films have been investigated. In the case of c -axis thin films, different types of step and plateau structures are observed depending on the deposition conditions and substrates used (SrTiO 3 and MgO), suggesting different growth behavior. In some cases, very regularly spaced parallel plateaus separated by steps are found, suggesting directional step flow; whereas in other cases, more complex surface structures with circular plateaus and screw dislocations are seen. In the case of a -axis growth, the substrate determines the grain orientation and results in a cobblestone appearance of the film surface. The tunneling conditions necessary for obtaining stable and reproducible images of the surface are discussed as well as attempts to correlate the deposition parameters with the resulting film surface structure.

Anisotropic magnetotransport in high temperature superconductor multilayers

The high temperature superconducting cuprates are layered materials in which the CuO2 layers are found to play an important role for the superconducting properties. When produced in thin film form, these materials can be grown in a layer by layer sequence and this has allowed us to make superlattices of various combinations of these compounds. In particular, in the YBa2Cu3O7/PrBa2Cu3O7 system it is possible to produce superlattices where the individual layers are as thin as one c-axis unit cell. This allows us to modify the anisotropy of these materials in a controlled manner. In this contribution we present a study of the resistive transition in a magnetic field perpendicular or parallel to the layers, and how the results can be used to gain insight into the role of the anisotropy for the dissipative behaviour of these high temperature superconductors.