YBCO Lattice Research Articles

Deoxygenation of Y-Ba-Cu-O thin films by reactive ion implantation

Thin films of YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO), grown by pulsed laser deposition, were implanted with Si/sup +/ ions at energies of 30,60, and 90 keV and at doses ranging from 1/spl times/10/sup 13/ cm/sup -2/ to 3/spl times/10/sup 11/ cm/sup -2/. X-ray diffraction techniques were used to investigate the structural dependence on implant parameters and annealing conditions, while d.c. magnetization was measured to characterize superconducting properties. By implanting only the upper portion of the film, implanted Si/sup +/ ions, near the surface, inhibit the superconductivity by removing oxygen from the bottom YBCO lattice which still retains its original crystal structure.

New materials for superconducting electronics: Epitaxial growth of LaSrGaO4 and PrSrGaO4 dielectric thin films by MOCVD

Communication: There is a critical need for improved dielectric materials as substrates, and as buffer and insulator layers for HTS devices. Here the first in situ MOCVD growth of phase‐pure, epitaxially oriented YBCO lattice matched, low dielectric constant perovskites LnSrGaO4 (Ln = La and Pr) is described. The exclusively c‐axis oriented films have an ultimate surface roughness of 3.0 and 1.0 nm for the Pr and La materials, respectively. These films exemplify the requirements which are essential for fabrication of HTS thin‐film devices.

Effect of structural and morphological imperfections on the microwave surface resistance of YBCO thin films

The average grain size, non-uniform lattice distortion and the orthorhombic lattice parameters have been measured by means of X-ray diffraction analysis for a series of YBCO thin films. These films exhibited a microwave surface resistance ranging from less than 0.05 to 0.8 Ω (measured at 60 GHz and 77 K) but similarly high dc critical parameters ( T c = 89–90 K, J c(77 K,0 T) > 2×10 6A/cm 2). It has been found for the examined films that the decrease of the orthorhombic splitting, which results from the lattice distortion in the a- b plane, correlates with the increase of the surface resistance. On the other hand, the perfection of the YBCO lattice along the c axis, i.e. the average grain size and the lattice distortion, does not affect the microwave losses in these films. The microwave losses in the YBCO films were also correlated to the film surface morphology.

The 2√2 a0 × √2 b0 × c0 herringbone phase of YBa 2Cu 3O 6 + x. No oxygen ordering but an alien oxide, BaCu 3O 4

A synchrotron-radiation study has been carried out on a crystal of YBa 2Cu 3O 6 + x with a low oxygen content ( x = 0.15–0.20) to investigate the so-called 2√2 a 0 × √2 b 0 × c 0 phase, attributed up to now to oxygen ordering in the YBCO CuO x layers. This crystal was grown from a flux and quenched from 900°C. It is shown that neither the ordering of CuOCu monomers in the herringbone-like scheme nor any other kind of oxygen ordering in the YBCO lattice can explain the observed reflections that actually correspond to the unit cell 2√2 a 0 × √2 b 0 × c ≈ c 0/3. A structural model is proposed that gives a very good description of the diffraction pattern; it corresponds to a parasitic phase of formula BaCu 3O 4 that is characterized by a stacking of Cu 3O 4 layers connected by barium atoms and that probably grows by epitaxy upon the CuO 2 planes of YBCO.

Effect of Mn inclusion in superconducting YBCO-based composites

The doping effect of Mn in polycrystalline YBa2Cu3O7 − δ material (YBCO) is carefully analyzed considering the transport properties of composites formed by small inclusion particles of Mn embedded whether in a YBCO matrix or in superconducting Fe-YBCO composites. In both cases, the improvement of their respective electrical resistivity was evidenced. The use of double element doping clearly shows that an a priori degradation of the YBCO electrical properties by Fe atoms can be greatly restored by appropriate Mn doping. This character can be very useful for technological applications. This improvement is mainly due to the reduction of the secondary and/or insulating phases formed at the grain boundaries, limiting the current transport. The involved mechanism is the probable existence of free charged Mn-based species which are responsible for an optimal oxygenation in the material contributing to the electrical response improvement. These species can be located in the composite, whether entering the CuO chains of the YBCO lattice or complexing and reducing the secondary phases formed during the sintering process, especially Fe2O3 and Y2BaCuO5 in the case of Fe-YBCO composites.

Detailed analysis of the zero critical temperature improvement in MnYBa 2Cu 3O 7−δ composites from electrical and structural studies

The study was aimed at investigating in a continuous manner the influence of the solid and/or liquid phase sintering on xMn-(1 − x)YBCO (where YBCO represents YBa 2Cu 3O 7−δ) composites, with 0.02 ≤ x ≤ 0.07. The main result is obtained from resistance vs. temperature measurements; in comparison with the pure YBCO, we show that an improvement in the zero resistance temperature is obtained at an optimal sintering temperature, whatever the Mn concentration x of the composite is. From the microstructural study, associated with energy-dispersive X-ray analysis, we detect that, in that case, particular inclusions are formed, with a constant Mn content probably responsible for the zero critical temperature improvement. The interpretation is based on the fact that the Mn substitution in the YBCO lattice cannot be fully responsible for this improvement and that the secondary phases play a role in explaining such a phenomenon.

Optical spectroscopy study of the role of Ag in laser ablated YBa2Cu3O7−δ thin films

We have found evidence through optical spectroscopy that AgO is indeed generated in the laser plume during pulsed laser deposition of YBa2Cu3O7−δ (YBCO) thin films using Ag-doped YBCO targets. This supports our earlier conjecture that formation of AgO in the plume and its subsequent dissociation at the elevated substrate temperature (since AgO is unstable above 350 °C) provides active oxygen to the YBCO lattice, thereby increasing oxygen incorporation during growth of YBCO thin films.

Microstructure and properties of artificial grain boundaries in epitaxial YBA 2Cu 3O 7−δ thin films grown on [001] tilt YZrO 2 bicrystals

The microstructure of artificial grain boundaries in YBa 2Cu 3O 7−δ (YBCO) thin films grown on [001] tilt YZrO 2 (YSZ) bicrystal substrates has been characterized using transmission electron microscopy and atomic force microscopy. Despite a relatively straight morphology of the substrate boundaries, the film boundaries were wavy. The waviness was a result of the combined effects of grooving at the substrate boundaries prior to the film deposition and an island-growth mechanism for YBCO on YSZ substrates. The dihedral angle of the groove walls varied with the misorientation angle and depended on the symmetry of the substrate boundary. The amplitudes of the film boundary waviness compared well with the widths of the grooves. In addition, the grooves induced local bending of the YBCO lattice planes and additional tilt components perpendicular to the c-axis close to the film boundaries.

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

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.

Surface modification of YBa2Cu3O7- delta thin films using the scanning tunneling microscope: five methods

We have investigated using the scanning tunneling microscope (STM) as a tool for surface modification of YBa2Cu3O7- delta (YBCO) thin films and have identified five distinct methods whereby the STM tip can modify the superconductor surface. (i) By lowering the tunneling resistance we make the tip scratch or 'mill' the sample surface mechanically. (ii) By increasing the bias voltage above about 4 V we can modify the surface by an apparent electron beam damaging process. (iii) By increasing the bias voltage above 10 V and raising the tunneling current, we can cause a more dramatic effect which is probably due to a thermal process. (iv) By operating the STM in a damp carbon dioxide atmosphere we can cause the STM tip to etch the surface electrochemically. (v) Finally, we have some preliminary data suggesting that the high field under an extremely sharp tip displaces the oxygen atoms in the YBCO lattice. Examples of each of these techniques are shown and discussed.

Role of gold in the enhancement of the critical temperature in melt-textured YBa 2Cu 3O 7−x

Gold was introduced by diffusion into melt-textured YBCO. This gold-enriched material exhibits a critical temperature of 97 K as measured by a transport method. Introduction of gold into melt-textured YBCO produces a pronounced deficiency in oxygen stoichiometry, and shortens the c-axis parameter. In spite of the latter changes in the YBCO lattice, the material remains superconducting and the critical temperature is enhanced. Gold seems to keep the c parameter of YBCO in the range of values where the system remains orthorhombic, although there is a considerable loss of oxygen.

Nonrandom gold-YBa2Cu3O7−x composites

In this article we present a new method for the preparation of nonrandom Au-YBa2Cu3O7−x (YBCO) composites. We describe the structural and the morphological properties of this novel system. Gold introduction into this system causes an increase in the c axis value of the unit cell upon sintering at 950 °C. A systematic increase of the zero resistance transition temperature, up to 96 K as measured by a dc transport method, is observed upon the introduction of gold into the YBCO lattice. Introduction of gold above ≊3% by weight causes weakening of the intergranular links of the composite material.