C. Traeholt

Stacking sequence of YBa2Cu3O7 thin film on SrTiO3 substrate

Abstract The interface between substrate SrTiO 3 and thin film YBa 2 Cu 3 O 7 was studied by high resolution electron microscopy. The atomic stacking sequence was found to be of only one type: bulk-SrO-TiO 2 -BaO-CuO 2 -Y-CuO 2 -BaO-CuO-bulk. Misfit dislocations in the YBa 2 Cu 3 O 7 film were only found to extend up to the first few unit cells of YBa 2 Cu 3 O 7 for a straight interface. Frequently, the YBa 2 Cu 3 O 7 lattice was observed to be shifted parallel to the interface with respect to the lattice of the substrate, which was accompanied by an elongation of the distance between YBa 2 Cu 3 O 7 and SrTiO 3 . This shift results from inserting a misfit dislocation down to the interface near the step of the substrate. On a single unit cell step of SrTiO 3 , the YBa 2 Cu 3 O 7 grows on both the flat surfaces with the same stacking sequence, resulting in a c /3 antiphase boundary at the step. On a large substrate step the YBa 2 Cu 3 O 7 will grow to form a perfect YBa 2 Cu 3 O 7 structure, one or two unit cells of YBa 2 Cu 3 O 7 away from the interface.

TEM investigation of YBa2Cu3O7 thin films on SrTiO3 bicrystals

YBa2Cu3O7 films in c-axis orientation on bicrystalline SrTiO3 substrates are investigated by TEM. The films and the substrates are examined in cross-section and in plane view. The grain boundary of the bicrystal substrate contains (110) faceted voids, but is otherwise straight on a nanometer scale. Contrary to this, the film grain boundary is not straight grain boundary can be up to 100 nm for a 100 nm thick film. The deviation from the intended position of the YBCO grain boundary can already occur at the film/substrate interface where it can be as much as ±50 nm.

High resolution electron microscopy of heavy-ion induced defects in superconducting Bi-2212 thin films in relation to their effect on Jc

Abstract Bi 2 Sr 2 CaCu 2 O 8+δ thin films were irradiated with 200 MeV 40 Ca, 1162 MeV and 2640 MeV 197 Au and 1404 MeV 238 U ions. The radiation-induced defects in the films were investigated with high resolution electron microscopy (HREM) both in cross-section and in-plane view. The HREM study shows that swift heavy ions produce well defined latent columnar tracks consisting of totally amorphized material divided by a sharp transition from the surrounding crystalline material. In-situ measurements of the transport critical current density J c on a single sample irradiated first with Ca then with Au ions were performed. Correlations of the transport measurements with HREM data show clearly that continuous columnar defects ( 197 Au, 238 U) are more efficient pinning centers than scattered defect cascades ( 40 Ca).

Direct high‐resolution electron microscopy observation of nonunit‐cell nucleation in the initial stage of high‐Tc superconducting film growth

The relationship between the starting and ending atomic layers of YBa2Cu3O7−x and Bi2Sr2CaCu2O8 thin films is studied by high‐resolution electron microscopy. Experimental results show that the starting atomic layer of the thin film is determined by surface chemistry and the interaction between the thin film and the substrate, while the ending layer of the thin film is thermodynamically determined by the high‐Tc cuprate itself. Therefore, nonunit cell nucleation is often observed in certain cases. The surface atomic layers of YBa2Cu3O7−x and Bi2Sr2CaCu2O8 were found to be the CuO layer and single BiO layer, respectively.

Enhanced flux pinning in Bi-2212 single crystals by planar defects introduced via Ti-substitution

Abstract Ti-doped Bi-2212 single crystals were grown by the travelling solvent floating zone technique. Concentrations of about 1 at% and 2 at% (with respect to Cu) were shown to be incorporated in the structure which leads to lowering of T c to about 73 K from 85 K. High resolution microscopy revealed high densities of planar defects parallel to the a , c -planes. The flux pinning properties at elevated temperatures are significantly improved with respect to undoped single crystals.

Thin-film growth of the charge-density-wave oxide Rb0.30MoO3

We report on the thin‐film fabrication of a charge‐density wave (CDW) compound. Single‐phase epitaxial films of the model CDW oxide Rb0.30MoO3 have been grown by pulsed‐laser deposition. Detailed analyses show that the Rb0.30MoO3 films have μm‐size grains with the CDW chains oriented parallel to the substrate. On SrTiO3 (510), the CDW chains align into a single direction within the film plane. The electrical resistance of the films demonstrates a CDW state below about 182 K. Structures patterned in films will permit unprecedented studies of phase‐coherent CDW transport, as well as the exploration of devices based on CDWs.

HREM study of the YBCO/MgO interface on an atomic scale

Abstract The film-substrate interface of c oriented YBCO thin films grown by sputtering or laser ablation on (001) MgO substrate has been investigated with high-resolution electron microscopy. The first atomic plane of the YBCO lattice is a CuO chain layer. Two interface configurations occur: (1) the YBCO lattice and the MgO lattice continue up to the interface (this configuration is occasionally associated with some periodic strain in the MgO lattice; (2) the YBCO lattice and the MgO lattice are separated by an (almost) amorphous layer with a thickness of the order of two atomic layers. This amorphous layer is found to lead to the absence of strain. In some cases the surface roughness coincided with misoriented grains but most of the steps in the MgO substrate were accommodated by steps in the YBCO of one or more complete unit cells in height and some lattice bending in the YBCO film.

Microstructures of ramp-edge YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox Josephson junctions on different substrates

Ramp-edge YBa2Cu3/PrBa2Cu3Ox/YBa2Cu3Ox Josephson junctions with PrBa2Cu3Ox (PrBCO) or SrTiO3 as a separating layer on different kinds of substrate have been studied by high-resolution electron microscopy. The bottom YBa2Cu3Ox (YBCO) layer and the separating layer (PrBCO or SrTiO3) were epitaxially c oriented, irrespective of the substrate (yttria stabilized zirconia (YSZ), SrTiO3 or NdGaO3, all in (001) orientation). The use of ion milling in the manufacturing of Josephson junctions was found to yield smooth slopes with an angle of about 20°. The Josephson junction was facing away from the beam direction was found to have a dimple in the substrate near the base of the junction. The barrier layers were observed to have a homogeneous thickness. These layers were as the top YBCO layers were oriented with their c-axis perpendicular to (001) plane of the substrate for perovskite substrates and perpendicular to the surface for YSZ substrates. In the case of a YSZ substrate, the dimple in the substrate as well as the slope of the substrate close to the base of the junction were found to lead to small angle grain boundaries in the YBCO film as well as randomly oriented YBCO grains, which results in a poor ramp-edge junction. In the case of SrTiO3 or NdGaO3 substrate, all components of the device were fully epitaxial, thus resulting in good ramp-edge junctions.

Molecular beam epitaxy growth and microstructure of thin superconducting Bi2Sr2CaCu2Ox films

Abstract The microstructure of molecular beam epitaxy (MBE) grown Bi 2 Sr 2 CaCu 2 O x films has been studied by reflection high-energy electron diffraction (RHEED), high-resolution electron microscopy (HREM) and X-ray diffraction (XRD). In situ recorded RHEED images of as-grown films show two-dimensional growth, 90° oriented domains (twist domains) and an incommensurate superstructure. For the first time, the presence of both 2201 and 2223 stacking faults in Bi 2 Sr 2 CaCu 2 O x thin films is reported. These local structural defects are not easily observed by standard XRD techniques. To examine the structure of the films quantitatively, a general one-dimensional XRD model was fitted to the experimental XRD data. The model considered changes in peak intensities, positions and line-widths, and thus allowed a quantitative determination of the structural properties of the high- T c superconducting thin films.

TEM analysis of planar defects induced by Ti doping in Bi-2212 single crystals

Abstract An electron microscopy study is performed on Ti doped Bi-2212 single crystals. High resolution electron microscopy reveals that the Ti doping leads to incorporation of (010) planar defects in the Bi-2212 lattice. These defects consist of pairs of antiphase boundaries, which are separated along the b-axis by one modulation distance. The atomic shift at the boundary is such that one of the two CuO2 planes is continuous throughout the defect. Energy dispersive X-ray analysis clearly reveals the presence of Ti in the planar defects, whereas no Ti could be detected in defect free areas. This evidences the fact that these defects are induced by the Ti doping.