Full Wafer In Situ Deposition of Thallium Lead Superconductors

Thin films of (Tl,Pb)Sr₂Ca_0.8Y_0.2Cu₂O₇, and (Tl,Pb)Sr₂CuO₅ can be grown in a single step process which involves sputter deposition from a mixed oxide target and simultaneous thermal evaporation of Tl₂O. The use of a radiant heater has allowed extension of this in situ deposition process to full LaAlO₃ and NdGaO₃ wafers. Variations in the composition of the deposited film is < 4% across a 50 mm wafer while the thickness uniformity is 8%. The highest transition temperature for a (Tl,Pb)Sr₂Ca_0.8Y_0.2 Cu₂O₇ film thus far is 83 K. The RMS surface roughness of (Tl,Pb)Sr₂CuO₅ films is uniform across the wafer and approximately 1% of the film thickness for films 20 to 100 nm thick.     

Crystal growth of Bi-Sr-Ca-Cu-O from Sr-Ca-Cu-O thin films

Bi-Sr-Ca-Cu-O crystals have been prepared from amorphous Sr-Ca-Cu-O thin films by annealing at 890 °C in the presence of bismuth oxide vapour. Sr_0.8Ca_0.2CuO₂ (08021) and Bi₂(Sr, Ca)₂CuO₆ (2201) crystals initially appeared. On further annealing, needle crystals of Sr_14?xCa_xCu₂₄O₄₁ (14?x) grew. These 14?x needle crystals gradually changed to 2201 needle crystals, which further changed to crystals of the nominal composition Bi-Sr-Ca-Cu-O with ac-axis length of 1.69 nm.     

Phase Assemblage in PBSSCO OPIT High‐Temperature Superconductor Precursor Powders – Rietveld Analysis of X‐ray Diffractograms

The phase assemblage in PBSCCO‐2223, (Bi, Pb)₂Sr₂ Ca₂Cu₃O_{10 + x}, precursor powders used for the production of long lengths of superconducting tapes by the oxide powder in tube (OPIT) technology has been determined by Rietveld analysis of powder x‐ray diffractograms. The effects of grain alignment, structure modulation and anisotropic line broadening have been taken into account in the Rietveld analysis. The phase assemblage of the air‐annealed precursor is typically Pb‐free 2212 (Bi, Pb)₂Sr₂CaCu₂O_{8 + x}, Pb₃Sr₃Ca₂CuO₁₂, Ca₂PbO₄, Sr₆Ca₈Cu₂₄O₄₁, and Ca_0.79Sr_0.04CuO₂. If the annealing is done in N₂, the Pb⁴⁺ phases disappear and Pb²⁺ enters the 2212 phase, which is accompanied mainly by Ca₂CuO₃ and CuO. Pb‐2223 formation occurs above 830°C in air.     

Single electron tunneling in TlBa2(Ca0.8Y0.2)Cu2O7 thin films at 10K

Tunneling measurements on TlBa₂(Ca_0.8Y_0.2)Cu₂O₇ taken at low temperatures with a scanning tunneling microscope reveal clear evidence of single electron tunneling behavior. Topographical studies of the film with both scanning tunneling microscopy and atomic force microscopy provide a simple explanation of the source of the ultra small capacitors necessary to explain the single electron tunneling spectra as a natural result of the planar crystal structure of TlBa₂(Ca_0.8Y_0.2)Cu₂O₇.     

A new series of mercury-based high-T c superconductors (Hg,W)Sr2(Y1−x Ca x )Cu2

A new series of superconducting mercury-based layered cuprates has been synthesized. X-ray diffraction indicates that the compounds with formula Hg_0.8W_0.2Sr₂Y_1?x Ca _x Cu₂O_6+δ (0.3≤x≤0.6) exhibit the 1212 structure with space groupP _4/mmm . The investigation of superconductivity determined by electrical resistance measurement shows that the superconducting transition temperature (T _c , onset) of Hg_0.8W_0.2Sr₂Y_0.4Ca_0.6Cu₂O_6+δ is up to 94 K.     

Sequential imposed layer epitaxy of cuprate films

Layer-by-layer epitaxy has been used to grow cuprate films since the discovery of high-T _c compounds. This deposition technique is in principle suitable for the growth of layered crystalline structures. However, the sequential deposition of atomic layer by atomic layer of cuprate compounds has presently not been optimized. Among the difficulties to overcome are the need to control separately the deposition of three to five elements, the oxidation requirements, and the observed tendency toward the nucleation of three-dimensional aggregates. Nevertheless, this deposition process is the only one which allows one to build artificial cell structures such as Bi₂Sr₂Ca_(n–1)Cu _n O _y withn as large as 10. This process will also be the best one to grow films of the so-called infinite layer phase compounds belonging to the Sr_1–x Ca _x CuO₂ family, in order to improve the transport properties and the morphological properties of the cuprate films. When performed at high substrate temperature (typically more than 600°C), the layer-by-layer epitaxy of cuprates exhibits usually 3D aggregate nucleation. Then the growth of the film no longer obeys the layer-by-layer sequence imposed during the deposition. We present here two experimental situations of true 2D sequential imposed layer epitaxy: the growth at 500°C under atomic oxygen pressure of Bi₂Sr₂CuO₆ and of Sr_1–x Ca _x CuO₂ phases.     

Formation and growth studies of the (Bi,Pb)<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10</Subscript> phase in Ag sheathed tapes

The formation and the thickness growth of the (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ phase in Ag-sheathed tapes have been investigated by scanning electron microscopy on samples sintered at 840°C in a flow of 8.5% O₂ (rest N₂) and quenched in air after sintering for 1 to 50 h. The thickness of the (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ grains was measured statistically after different sintering times. The experimental results show that the average thickness of these grains increases during the entire sintering period, while the average thickness growth rate decreases exponentially with sintering time. The volume fractions of the various phases present during the heat treatment were also measured from micrographs. Detailed studies of the microstructure and phase formation kinetics support the view that the formation of the (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ phase proceeds via a nucleation and growth process. Based on the present observations, a model describing the microstructure evolution is presented.     

Microstructure change during crystallization of amorphous Bi(Pb)-Sr-Ca-Cu-O ceramics

Amorphous Bi(Pb)-Sr-Ca-Cu-O ceramics can be obtained by quenching from the liquid. Microstructure change during crystallization has been investigated in detail. The Bi₂Sr₂CuO _x (2 2 0 1) phase crystallized first, followed by Cu₂O. This crystallization process began at a lower temperature in the specimen with Pb additive. The 2 2 0 1 phase changed to the Br₂Sr₂Ca₁Cu₂O _x (2 2 1 2) phase during heating until partial melting began to occur. The 2 2 1 2 phase changed to the Bi₂Sr₂Ca₂Cu₃O _x (2 2 2 3) phase under an existence of (Sr, Ca)₃Cu₅O _x phase after a period of time.     

Superconducting Bi-Sr-Ca-Cu-O films formed diffusion of metallic bismuth on Sr2Ca2Cu4O y substrates

Superconducting Bi-Sr-Ca-Cu-O films have been prepared on Sr₂Ca₂Cu₄O _y substrates by thermal evaporation of metallic Bi layer and heat-treatment at 830°C for a few minutes in air. The zero resistance temperature of Bi₂Sr₂Ca₁Cu₂O _y film of 1–μm in thickness is about 70 K. The surface diffusion process of Bi on the granular structure of Sr₂Ca₂Cu₄O _y was observed by scanning electron microscope and thermogravimetry analysis. The stripe pattern of superconducting film, typically 2μm thick and 0·3 mm wide, is formed by using a mask.     

Preparation and Superconducting Properties of (Tl,Pb)-Based Compounds with 1234-Type Structure

(Tl_1?x Pb _x )(Sr_2?y Ba _y )Ca₃Cu₄O _z samples were synthesized by using a two-step solid state reaction method and examined with respect to the (Tl,Pb)-1234 phase formation and superconducting properties. Samples were prepared from various starting compositions of 0≤x≤0.5 and 0.4≤y≤2.0. Nearly single-phased samples were obtained for (Tl_0.8Pb_0.2)(Sr_2?y Ba _y )Ca₃Cu₄O _z (1.0≤y≤1.25) and (Tl_1?x Pb _x )(SrBa)Ca₃Cu₄O _z (0.15≤y≤0.35). We have found that both the partial substitution of Ba for Sr and the partial substitution of Pb for Tl are effective for the formation of the Tl-1234 phase. Bulk superconductivity with a T _c value of 106 K is observed in the (Tl_0.7Pb_0.3)(SrBa)Ca₃Cu₄O _z sample.     

A new high-T c superconductor containing thallium and its crystal structure: The “1212” phase (Tl1-x Bi x )1.33Sr1.33Ca1.33Cu2O6.667+δ

Joining YBa₂Cu₃O_6.5+δ (123 phase) and Bi₄Sr₄Ca₂Cu₄O_16+δ (4424 phase) as structurally characterized high-T _c , superconductors, the thallium-containing superconductor (Tl_.75Bi_.25)_1.33Sr_1.33Ca_1.33Cu₂O_6.667+δ with the ideal stoichiometry (Tl,Bi)₁Sr₂Ca₁Cu₂O_6.5+δ (1212 phase) is reported here. As prepared from the component oxides, 1212 has an initial deviation from resistance linearity at 120 K, a superconducting onset temperature of 92 K, and zero resistance at 75 K. The tetragonal unit cell (P4/mmm, a=3.800 Å;c=12.072 Å, deduced from powder data) contains double copper oxygen sheets (like 4424 and 123) that alternate withsingle thallium-bismuth oxygen sheets (in contrast to 4424, which containsdouble bismuth oxygen sheets), resulting in a total of three stacked perovskite-like cells (as in 123). The copper oxide sheets (with intersheet spacing 3.38 Å) are separated by Ca²⁺ and the Cu oxide sheets and (Tl,Bi) oxide sheets (with spacing 4.35 Å) are separated by Sr²⁺, Ca²⁺, and excess (Tl,Bi)³⁺. The 1212 cell constitutes the building block for the centered, more complex 4424 cell. The 1212 structure persists to Bi contents as low as 1% and can also be stabilized by Pb instead of Bi; Tl cuprates also form other superconductors with lowerT _c .     

Metal-insulator transition and possible superconductivity in Pb2.2Cu0.8Sr3.1La1.5Cu1.5Oy with hexagonal structure

A metal-insulator transition and possible superconductivity has been found in Pb_2.2Cu_0.8Sr_3.1La_1.5 Cu_1.5O_y. The structure of this compound belongs to the hexagonal symmetry with the lattice parameter a=10.11 ?, c=3.56 ? and space group P6ˉ2 m. In this structure, there are not the traditional square CuO₂ planes as known in most cuprate superconductors. The parent compound of this phase is Pb₃Sr₅CuO₁₂(3501) which is an isolator. By replacing Pb and Sr with Cu and La, respectively, a metal-insulator transition and strong structural modulations occur in this material. Received: 27 July 1999 / Reviewed and accepted: 10 August 1999     

Phase evolution and gas-phase particle size distributions during spray pyrolysis of (Bi,Pb)SrCaCuO and Ag(Bi,Pb)SrCaCuO powders

Phase evolution, gas-phase particle size distributions and lead loss were studied during formation of (Bi,Pb)SrCaCuO powders and their composites with silver by spray pyrolysis starting from nitrate solutions. The 10 wt% Ag/90 wt% Bi_1.8Pb_0.44Sr₂Ca_2.2Cu₃O_x composite powders made at 700°C consisted of 20–60 nm grains of silver and mixed-oxide phases with a fine dispersion of Ag grains within the particles. At 700°C, the primary phases present in (Bi,Pb)SrCaCuO powders were (Bi,Pb)₂Sr₂CuO_x (2201), Ca₂PbO₄ (plumbate), (Bi,Pb)₂Sr₂CaCu₂O_x(2212), and (Bi,Pb)₃Sr₂Ca₂Cu₁O_x(3221). For T≥800°C, the powders were considerably depleted in lead, and the plumbate and 3221 phases were absent. For T = 900°C, a large number of ultrafine particles (<30 nm) were formed, probably from the PbO vapor released from the reactor walls. Using spray pyrolysis, it is easy to control stoichiometry and limit the phase segregation at the nanometer-scale so that homogeneous and phase-pure materials can be obtained rapidly during subsequent processing.     

Effect of Ba Substitution on the Formation of (Tl,Bi)-1234 Superconducting Phase

(Tl_1−x Bi _x )(Sr_2−y Ba _y )Ca₃Cu₄O _z ((Tl,Bi)-1234) samples (x=0.1–0.3, y=0.4–1.5) were synthesized under ambient pressure by using a two-step solid-state reaction method to investigate the effect of Ba substitution on the formation and superconducting properties of the (Tl,Bi)-1234 phase. X-ray diffraction analysis shows that nearly single-phased samples are obtained for (Tl_0.8Bi_0.2)(Sr_2−y Ba _y )Ca₃Cu₄O _z (0.8≤y≤1.2), suggesting that control of the composition ratio of Sr and Ba in the bridging layer is a key to stabilizing the 1234-type structure. The critical temperature for the nearly single-phased samples remained between 106.9 K and 109.1 K and was not significantly affected by the change in the Ba content.     

The Study of Hole Distribution in High-Tc Tl- and Hg-based 1212 Type Cuprates via High-Resolution X-Ray Absorption Near Edge Structure

High resolution O K-edge and Cu L₃-edge X-ray absorption near-edge-structure (XANES) spectra of the high-T_c cuprates of (Tl_0.5Pb_0.5)Sr₂(Ca_1?xY_x)Cu₂O₇ (Tl-1212) and (Hg_0.5Pb_0.5)Sr₂(Ca_1?xY_x)Cu₂O₇ (Hg-1212) in powder form were measured using a bulk sensitive total-fluorescence-yield technique. Near the O 1s edge, the pre-edge peak with maxima at ～ 528.3 eV is ascribed to the transitions to O 2p holes located in the CuO₂ planes. The intensity of this pre-edge peak increases with increasing doping level of Ca²⁺ into the Y³⁺ sites in Tl-1212 and Hg-1212. In the Cu L-edge absorption spectra, high-energy shoulders at around 933 eV are attributed to the transitions to the Cu(2p_3/2)^?13d¹⁰L states in the CuO₂ layers, where L denotes the O 2p ligand hole. The behavior of these shoulders in Tl-1212 and Hg-1212 correlates with that of the pre-edge peak at ～ 528.3 eV in the O K-edge absorption spectra. The results can lead us to understand the hole distribution in high-T_c cuprates which will give a direction to find new high-T_c materials.     

Synthesis of high purity Tl1212 superconductor with ultrafine grains from coprecipitated Tl-free precursor route

In this study, high purity and fine-grained Tl1212 superconductor samples were successfully synthesized from Tl_0.8Bi_0.2Sr₂Ca_0.8Y_0.2Cu₂O₇ starting composition by a two-step solid-state reaction method using Tl-free precursor powder prepared from coprecipitation method. XRD pattern of the sample, which was sintered at 1,000 °C for 6 min essentially showed formation of high purity 1212 phase. The sample’s zero resistance critical temperature, T _{c zero} was 91 K and bulk critical current density, J _c measured at 40 K in zero magnetic field was 11.2 A/cm². SEM investigation on the sample revealed ultrafine homogeneous grains that are randomly orientated with grain sizes of approximately 0.5–1 μm. Resintering the sample did not affect its superconducting properties but induced clustering of grains as observed by SEM. It is suggested that the sample be used in specialized measurement techniques such as in ultrasonic studies where high purity and ultrafine-grained samples are required.     

Crystal and Electronic Structure of High-Temperature Superconductive Layered Cuprates in Temperature Interval 100–300 K

For two HTSC materials, Y_1?x Ca _x Ba₂Cu₃ O _6.7 and Tl_0.8Hg_0.2Ba₂Ca₂Cu₃ O _8.15, their crystal structure has been studied earlier in the temperature interval 100–300 K. It is shown that for both compounds, the height of CuO₅ pyramids has a minimum in temperature interval from ～ 150 to ～ 250 K. The electronic structure is calculated using first principles based on the obtained data of crystal structure. It is revealed that a decrease in the CuO₅ pyramid height leads to the appearance of the electronic state density peak with an energy of 0.3–0.4 eV under Fermi level. Localized states of barium and apical oxygen make the main contribution to this peak. By reduction the temperature below ～ 150 K, these localized electronic states disappear, and under continuous cooling, the superconductivity appears.     

Pair breaking as a probe of d-wave high-temperature superconductivity

A unified picture is obtained of the Cooper pair-breaking data by Cu-site Zn and Ni in Nd_2–z Ce _z CuO₄, La_2–SrCuO₄, Bi₂Sr₂CaCu₂O₈, Bi_1.8Pb_0.2Sr₂Ca₂Cu₃O₁₀, YBa₂Cu₃O₇, and YBa₂Cu₄O₈. The data are generally inconsistent with spin-fluctuation d-wave pairing mechanisms of superconductivity and with all two-dimensional cuprate-plane models. The data are consistent with superconductivity in the charge reservoirs.     

Subsolidus phase relations of the Y2O3-SrO-CuO system

The 960 °C subsolidus phase relations of Y₂O₃-SrO-CuO system have been determined by X-ray diffraction. The section at room temperature in the Y₂O₃-SrO-CuO ternary system can be divided into six three-phase and two-phase regions. In this system, there are five binary compounds: Y₂SrO₄, Y₂Cu₂O₅, Sr₂CuO₃, SrCuO₂ and Sr₁₄Cu₂₄O₄₁ as well as a solid solution Y_xSr_14–x Cu₂₄O₄₁ (0 <x 4). The structure of the solid solution is discussed. The variation of the lattice parameters of the solid solution with increasing yttrium content has been measured. Electrical resistivity measurements of the solid solution show that there is no superconductive phase in the Y-Sr-Cu-O system.     

Thickness scaling and electric properties of highly (111) oriented Nb-doped Pb(Zr,Ti)O<Subscript>3</Subscript> thin film prepared at low temperature by a sol–gel route

A series of highly (111) oriented Pb(Nb_0.01Zr_0.2Ti_0.8)O₃ (PNZT) thin films of variant thickness were successfully achieved on Pt/Ti/SiO₂/Si substrate by a sol–gel route. By introducing Pb_0.8Ca_0.1La_0.1Ti_0.975O₃ (PLCT) layer between the PNZT film and Pt electrode, the PNZT film could be crystallized at as low as 500 °C. When a maximum applied voltage is 3 V, it was found that the PNZT film with PLCT seed layer possessed higher remnant polarization (22 μC/cm²) as film thickness was scaled down to 50 nm. It was also found that enhanced pyroelectric properties could be observed in 50-nm thickness PNZT thin film due to its relatively low dielectric constant. The results demonstrated that the film thickness could be scaled down for low voltage operations using lattice matched interface between PNZT film and PLCT seed layer on Pt/Ti/SiO₂/Si substrate, and this interface optimization is the key technology for synthesizing thin PNZT films at low temperature with good insulating and electric properties.