The As-prepared surfaces of C-axis-oriented Nd1Ba2Cu3Oy thin films characterized using scanning probe microscopes

Recently, superconducting Nd₁Ba₂Cu₃O_y (Ndl23) thin films with high superconducting transition temperature (T _c) have been successfully fabricated at our institute employing the standard laser ablation method. In this paper, we report the results of surface characterization of the Nd123 thin films using an ultrahigh vacuum scanning tunneling microscope/spectroscopy (UHV-STM/STS) and an atomic force microscope (AFM) system operated in air. Clear spiral pattern is observed on the surfaces of Nd123 thin films by STM and AFM, suggesting that films are formed by two-dimensional island growth mode. Contour plots of the spirals show that the step heights of the spirals are not always the integer or half-integer number of thec-axis parameter of the structure. This implies that the surface natural termination layer of the films may not be unique. This result is supported byI-V STS measurements. The surface morphology of the Nd123 thin films is compared with that of thec-axis-oriented Y₁Ba₂Cu₃S_y thin films. Surface atomic images of the as-prepared Nd123 thin films are obtained employing both STM and AFM. STS measurements show that most of the surfaces are semiconductive. The results of STS measurements together with the fact that we are able to see the surface atomic images using scanning probe microscopes suggest that exposure to air does not cause serious degradation to the as-prepared surfaces of Nd123 thin films.     

Growth and characterization of evaporated thin films of Y1Ba2Cu3Ox

We have prepared thin films of Y-Ba-Cu-O by coevaporation in the presence of oxygen onto room-temperature substrates, followed by postannealing in oxygen. Films have been grown on (100) SrTiO₃, (100) MgO, and LiNbO₃. The films have been characterized by electrical resistivity, X-ray diffraction, and elastic backscattering (EBS) measurements.     

Enhanced Superconductivity in the Tl1?XBiXBa2Ca2Cu3Oy System

Compounds of the system, Tl_1–xBi_xBa₂Ca₂Cu₃O₉, were synthesized and studied. Bi substitutes at the Tl-site for values of x up to x=0.2. The superconducting transition temperature, T_c, increases from 110K for x=0.0 to 116K for x=0.2. SEM images reveal a plate-like morphology which is highly suitable for powder-in-tube processing. M-H loops show that the Bi-substituted phase has significantly better low temperature flux pinning properties than the pristine Tl-cuprate, and exhibits only a weak dependence on temperature and field when compared to that of unsubstituted phase. The J_c values calculated using the Bean model are: 3 × 10⁷ A/cm² at 10K,1T and 2 × 10⁴ A/cm² at 77K.1T. Substitution of 20% of Bi in TlBa₂Ca₂Cu₃O₉ makes the compound an excellent choice for magnet wire applications at relatively low temperatures.     

Microwave characterization of laser ablated Y1Ba2Cu3O7−x thin films

In the present study we report the measurements of microwave surface resistance (R_s) of YBCO thin films on LaAlO₃ substrate as a function of temperature, thickness and magnetic field by microstrip resonator technique. The T_c(R = 0) of the films is 90 K and J_c > 10⁶ A/cm² at 77 K. The microwave surface resistance has been measured for films of various thicknesses. The value of R_s has been found to be initially decreased with increasing film thickness due to increase in number of defects. A minimum microwave surface resistance has been obtained for film thickness of about 300 nm. The increase of R_s with film thickness above 300 nm is possibly due to degradation of the film microstructure as observed with Atomic Force Microscopy. Temperature dependence of surface resistance has been studied for best quality films. The field induced variations of surface resistance are also investigated by applying dc magnetic field perpendicular to stripline structure and surface of the film. A general linear and square field dependence of R_s at low and high value of fields has been observed with critical field value of 0.4 T which confirms the microwave dissipation induced by flux flow in these resonators at 10 GHz frequency. The hysteresis of R_s in dc field observed for field value above critical field shows the higher value of surface resistance in decreasing field than in increasing field which is in agreement with one state critical model and is a characteristic of homogeneous superconductors.     

Investigations of Bi substitution in NdBa2Cu3Oy

Attempts to substitute Bi for Nd in orthorhombic NdBa₂Cu₃O _y , prepared in air or oxygen at about 950°C led instead to formation of Ba₂NdBiO₆, a new cubic compound witha=0.8703 nm. The possibility was then explored of preparing superconducting (Nd_1–x Bi _x )Ba₂Cu₃O _y , by first forming the tetragonal phase at 880–950°C in nitrogen or argon followed by reheating in oxygen or air at 250–500°C in order to insert the additional oxygen required to yield the orthorhombic form while avoiding oxidation of Bi³⁺ to Bi⁵⁺. X-ray diffraction studies, electrical conductivity measurements, and thermogravimetric analysis of products indicate that Bi does not enter the NdBa₂Cu₃O _y , lattice in either the tetragonal or the orthorhombic phase. Ba₂NdBiO₆ clearly forms on reheating in oxygen or air even at low temperatures, and evidence is presented that a poorly crystallized oxygen-deficient form of this compound is already present prior to the reheating.     

Low-field magnetic properties of high-TcGdxY1?xBa2Cu3Oy superconductor

The magnetization of Gd_0.85Y_0.15Ba₂Cu₃O _y high-T _c superconductor was measured at low fields (<100 Oe) and at a constant temperature of 77 K. The magnetization curves deviate from linearity at 25 Oe, causing the destruction of the weak couplings of the superconducting grains. The hysteresis due to granular superconductivity was also observed.     

Observation of weakly adsorbed oxygen on Y5Ba6Cu11Oy

We observed, for the first time, adsorption of weakly bonded oxygen at low temperature (<-250°C) by a Y₅Ba₆Cu₁₁O _y sample, using thermogravimetric analysis. The resulting oxygen enriched phase in the surface layers may be attributed to the observation of a superconductivity-like transition at above 200 K.     

Evaporated YBa2Cu3O7 thin films and device technology

Epitaxial thin films of YBa₂Cu₃O₇ were grown by evaporation of the metals from electron-guns in the presence of atomic oxygen. The films were characterized structurally by X-ray diffraction and electrically by measurements of d.c. magnetization and flux creep. The progress in device technology is discussed. Results are presented on Josephson junctions, and a superconducting microwave filter with the ground plane grown on the back.     

Partial substitution of Li for Cu in superconducting YBa2Cu3Oy

0.33–33% of the Cu in superconducting YBa₂Cu₃O _y has been replaced by Li (i.e.,x=0.01–1 in single-phase or nominal YBa₂Cu_3–x Li_xO_y). X-ray diffraction powder patterns remain the same as for YBn₂Cu₃O _y , with identical patterns up to about 17% substitution (i.e.,x=0.5). At higher percentages an additional phase appears. Electrical conductivity measurements indicate a small elevation ofT _c at low Li content. Starting at about 5% Li (x=0.15),T _c declines progressively and its width increases asx is raised.     

Twinning of YBa2Cu3O7 thin films on different substrates

The twinning of YBa₂Cu₃O₇ (YBCO) thin films with c-axis orientation on (001) MgO, (001) SrTiO₃, (012) LaAlO₃, (110) NdGaO₃ and (001) NdGaO₃ substrates, prepared by laser ablation, has been examined using a combination of and θ/2θ scans at a four-circle diffractometer. On all substrates, except for (001) NdGaO₃, the tetragonal to orthorhombal phase transition results in four different orientations of YBCO twins relating to the substrate. On (001) NdGaO₃ only two different twin orientations, accompanied by a slight lattice monoclinization, has been observed.     

High-capacity superconducting current leads of Y1Ba2Cu3O7?x

We have fabricated and measured a high-capacity superconducting current lead composed of a Y₁Ba₂Cu₃O_7–x cylinder, 20 cm long and 0.9 cm² cross section. A steady-state, d.c., critical current of 225 A at a temperature of 77 K was measured in this sample, using a voltage criterion of 2×10^–7 V/cm (p = 8×10^–10 ohm-cm). This current was limited by the currentinduced, self magnetic field. To our knowledge this is the largest d.c. critical current so far reported in a Y₁Ba₂Cu₃O_7–x sample and demonstrates the possibility of using hightemperature superconducting HTS materials for current leads to low-temperature superconducting LTS magnets or in power distribution systems.     

Preparation and properties of Tl2Ba2CaCu2O8 thin films

Anex situ process has been developed to produce thin superconducting Tl₂Ba₂CaCu₂O₈ films. The properties of films grown on different substrates using different annealing regimes were studied. Critical temperatures of 103–107 K were measured on films prepared in a broad range of annealing temperatures on SrTiO₃, LaAlO₃, and Y-ZrO₂ substrates. A critical current density,J _c, of 2×10⁶ A/cm² at 77 K was measured on LaAlO₃. Film morphology was studied by SEM, AFM, and STM.     

Synthesis and Processing of Doped Hg1Ba2Ca2Cu3Oy Superconductors

A series of quenching experiments were conducted to understand the sequence of reactions that occur during the synthesis of doped Hg1223, (Hg, A)Ba₂Ca₂Cu₃O _y , A = Re, Bi, and Pb (HgA1223). The formation and decomposition of the intermediate phases during the high-temperature reaction were followed as a function of temperature. HgA1223 phase forms over a wide range of temperatures, 750–950°C, 750–880°C, and 840–880°C for A = Re, Pb, and Bi, respectively. At T<750°C, HgA1212 phase forms for A = Re and Pb. Based on the results of quenching experiments, heat treatment conditions were optimized for the synthesis of pure HgA1223 phase using commercial BaCaCuO precursor powders. A reduced-temperature annealing stage after the high-temperature reaction helps in grain growth and improves the microstructural characteristics of HgA1223 samples. Control of Hg pressure during the reaction is crucial for achieving phase purity, grain growth, and texture in the final products. A novel approach for the control of Hg pressure during the synthesis of HgA1223, which consists of using CaHgO₂ as an external Hg source, is reported. HgA1223 samples synthesized using the new synthesis protocol exhibit improved microstructural and superconducting properties.     

The Structural, Superconducting and Transport Properties of the Compounds Y3Ba5Cu8O18 and Y3Ba5Ca2Cu8O18

This work is related to the structural, superconducting, and transport properties of the compounds Y₃Ba₅Cu₈O₁₈ and Y₃Ba₅Ca₂Cu₈O₁₈ prepared by the sol-gel method. The influence of the doping of Ca atoms into the compound Y₃Ba₅Cu₈O₁₈ was studied by employing XRD, SEM, AFM, EDX, DTA, TGA, and the electrical resistivity (ρ), Hall coefficient (R _H), Hall mobility (μ _H), and magnetoresistance measurements. The XRD spectra showed that Y₃Ba₅Ca₂Cu₈O₁₈ almost has the same crystal structure as that of Y₃Ba₅Cu₈O₁₈, except with some impurity peaks. The resistivity measurements have pointed out that the compounds Y₃Ba₅Cu₈O₁₈ and Y₃Ba₅Ca₂Cu₈O₁₈ have their T _c-onset temperatures at approximately 92.7 and 86.6 K, respectively. The Hall coefficients R _H and Hall mobilities μ _H have been measured at the 10–300 K temperature interval in a magnetic field of 0.55 T. The signs of R _H and μ _H are found to be positive for both samples, which indicate that the conduction is p-type in our samples. As expected, the magnetoresistance results clearly demonstrate a considerable decrease of the offset temperatures with increasing magnetic field.     

Investigations on TlBa2Ca2Cu3Ox superconducting thin films formation on LaAlO3 substrate

Investigations on pure superconducting phase TlBa₂Ca₂Cu₃O_x (Tl-1223) thin films formation, of about 100-125 nm in thickness, on (001) LaAlO₃ single crystal substrate, were made using radio-frequency sputtering deposition of Ba₂Ca₂Cu₃O_x precursor films and ex-situ thallination in sealed quartz tube. The precursor films were thallinated under different conditions of partial oxygen pressure, temperature, time and y thallium source content using unreacted pellets of composition Tl_yBa₂Ca₂Cu₃O_x. In all cases, strongly c-oriented multiphase films were obtained. A correlation between the Tl-1223 phase purity and the precursor film conditions of thallination is established. Temperature and time of thallination as well as the thallium source content and the partial pressure of oxygen play a key role in the quality of the obtained film. The films' onset temperature of the superconducting transition ranges between 90 and 103 K. It is shown that the best samples can be obtained from a dense precursor film and relatively medium thallination time.     

Surface planarization and masked ion-beam structuring of YBa2Cu3O7 thin films

Surface planarization and masked ion-beam structuring (MIBS) of high-T_c superconducting (HTS) YBa₂Cu₃O_7-δ (YBCO) thin films grown by pulsed-laser deposition (PLD) method is reported. Chemical-mechanical polishing, plasma etching, and oxygen annealing of YBCO films strongly reduce the particulate density (~ 10^-2 ×) and surface roughness (~ 10^-1 ×) of as-grown PLD layers. The resistivity, critical temperature T_c ≈ 90 K and critical current density J_c (77 K) > 1 MA/cm² of films are not deteriorated by the planarization procedure. The YBCO films are modified and patterned by irradiation with He⁺ ions of 75 keV energy. Superconducting tracks patterned by MIBS without removal of HTS material and, for comparison, by wet-chemical etching show same T_c and J_c(T) values. Different micro- and nano-patterns are produced in parallel on planarized films. The size of irradiated pattern depends on the mask employed for beam shaping and features smaller than 70 nm are achieved.     

Process monitoring during post-annealing of YBa2Cu3O7 thin films containing fluorine

The post-annealing method of producing thin films of YBa₂Cu₃O₇ (YBCO) has taken on a new impetus due to the recent work showing that films of the highest quality can be made by using low partial pressures of oxygen during the annealing cycle. Here it is shown that for films produced by using BaF₂ as a source material, the post-annealing procedure can be closely controlled by monitoring the F that evolves due to the water vapor reaction with BaF₂. The use of an ion-sensitive electrode allowed small F evolution rates (about 1 ng s^–1) to be detected above background, sufficient to measure the F evolution rate from even the smallest samples used. The time interval during which F evolves was found to increase with increasing YBCO film area being annealed.     

High-resolution and analytical transmission electron microscopy of epitaxial YBa2Cu3O7−x thin films and YBa2Cu3O7/PrBa2Cu3O7 superlattices

Transmission electron microscopy (TEM) studies of epitaxial YBa₂Cu₃O_7−x thin films and YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices are summarized. High-resolution imaging of cross-sections and plan views and energy-dispersive X-ray microanalysis and electron energy loss spectroscopy in the transmission electron microscope were the methods applied. In the first section results on YBa₂Cu₃O_7−x thin films With varying oxygen stoichiometry deposited onto SrTiO₃ are discussed. Then, YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices deposited onto SrTiO₃ and MgO are investigated. Finally, an interface analysis of high-quality YBa₂Cu₃O_7−x thin films deposited onto sapphire with yttrium-stabilized zirconia buffer layers is presented.     

Growth and structure characterization of epitaxial Bi2Sr2Co2Oy thermoelectric thin films on LaAlO3 (001)

Epitaxial Bi₂Sr₂Co₂O_y thin films with excellent c-axis and ab-plane alignments have been grown on (001) LaAlO₃ substrates by chemical solution deposition using metal acetates as starting materials. Microstructure studies show that the resulting Bi₂Sr₂Co₂O_y films have a well-ordered layer structure with a flat and clear interface with the substrate. Scanning electron microscopy of the films reveals a step-terrace surface structure without any microcracks and pores. At room temperature, the epitaxial Bi₂Sr₂Co₂O_y films exhibit a resistivity of about 2 mΩ cm and a seebeck coefficient of about 115 μV/K comparable to those of single crystals.     

Pressure dependence of Tc in Tl0.2Hg0.8Ba2Ca2Cu3Ox

We report results of our study of the pressure dependence of T_c in Tl_0.2Hg_0.8Ba₂Ca₂Cu₃O_x to 6 GPa in a diamond anvil cell. The sample studied has an ambient T_c of 136.4 K and application of quasihydrostatic pressure raised T_c to 144.4 K at 6 GPa. We will also describe recent modifications of our ac susceptibility apparatus that have improved our sensitivity while simplifying the coil design and the balancing.