Slip processing of highT c superconductors

Slip-processing technique has been used to fabricate tapes and alumina-supported films of superconducting YBa₂Cu₃O_7−x . Good densification and connectivity are revealed by scanning electron microscopy. Both the tape and film show superconductivity well above the liquid nitrogen temperature with a transition range of 3° and 5°K respectively.     

New effects in highT c superconductors

We show that a proper interpretation of the critical field measurements can distinguish spin fluctuation mechanism from others in the context of the pairing hypothesis. We can predict the existence of a new mode in the mixed state of a superconductor.     

A microscopic theory of layered highT c superconductors

A generalized Gorkov formalism of superconductivity in a layered system with non-primitive lattice structure is given. This is used to justify theoretically a model of highT _c oxide superconductors proposed recently by one of us (SSJ). A numerical estimation of the critical temperature as a function of the number of superconducting layers is mentioned.     

Effect of annealing on properties of bismuth based highT c superconductors

A novel method to increase the amount of the 110 K phase in the Bi-Sr-Ca-Cu-O system was examined. After the sample with composition of Bi₂Sr₂CaCu₂O_x had been calcined, it was soaked in ethanol containing copper acetate and calcium acetate, the amounts of which were determined to give the composition of Bi₂Sr₂Ca₂Cu₃O_y after sintering. Subsequently, annealing at about 400 °C was conducted to examine the stability of the 110 K phase. The sample containing mainly the 80 K phase and a small amount of the 110 K phase showed an increase in the amount of the 110 K phase while that containing only the 80 K phase showed no change. Therefore, it is presumed that the 110 K phase may grow when the excess calcium and copper coexist adjacent to the 80 K phase at relatively low temperatures such as 400 °C.     

Electronic and vibrational excitations in layered highT c superconductors

In the series of the layered highT _c superconductors (A)_1,2 B₂Ca _n−1Cu _n O_2n+3,4 (A=Tl or Bi;B=Ba or Sr), withn=number of consecutive Cu-O layers, the electron energy states of some of the Tl-based systems have been investigated. The electron and phonon dispersion curves have been obtained. The electronic states near the Fermi level are dominated by the hybridized Cu(d) and the O(p) orbitals. The dispersion curves are highly two-dimensional with very small dispersion alongc-axis. The number of bands is enhanced with increase in the number of the consecutive Cu-O planes (n). The present results agree with those obtained earlier for other superconducting phases. The phonons are overall dominated by the vibrations of the light mass oxygen atom modes both lying in or outside the Cu-O planes. The low-frequency phonons involve the motions of the heavier Tl, Ba atoms etc.     

Relative thermodynamic stabilities of constituent oxides of highT c superconductors

A thermodynamic scale of the relative stabilities of constituent oxides can help the choice of stoichiometric multi-component compositions and can provide guidance for judicious selection of heat-treatment conditions for high-T _c superconductors. A thermodynamic analysis was undertaken to study Y₂O₃-BaO-CuO, BaO-K₂O-Bi₂O₃ and La₂O₃-SrO-NiO systems. The relative stability of the oxides was expressed in terms of two ratios,S _v andS _p computed using free-energy of formation and vapour pressure data. CuO, Bi₂O₃ and NiO were taken as reference oxides for YBa₂Cu₃O_7\t-\gd, Ba_0·6K_0·4BiO₃ and La_1·9Sr_0·1NiO₄ respectively. Thermodynamically, the reference oxide was found to be the least stable amongst the constituent oxides in each of these systems.     

A new look at the highT c superconductors using BCS-theory

Using the conventional phonon-exchange mechanism of superconductivity we have succeeded in reproducing the transition temperatureT _c of a large number of newly discovered highT _c superconductors by introducing a certain modification to the well-known BCS-formula forT _c .     

CESR in superconductors: Relevance of highT c copper oxide systems

CESR is a highly effective tool to study the interaction among conduction electrons in normal metals. In superconductors belowT _c it can reveal vital information concerning the pairing interactions. A comparative study of highT _c and conventional superconductors is presented and it is shown that the disappearance of CESR in the superconducting state is a feature common to both conventional and highT _c superconductors and establishes the importance of exchange interactions in pairing. This is supported by experimental observations.     

Crystal chemistry of Ruddlesden-Popper type structures in highT c ceramic superconductors

Similar structural patterns have been noticed in the systems La-Cu-O, La-Ni-O and Bi and Tl-containing superconducting oxides. The formation of Ruddlesden-Popper type layers (alternating slabs of rocksalt and perovskite structures) is seen in these oxides which is similar in many respects to what is seen in the system Sr-Ti-O. However, there are some significant differences, for example the rocksalt and perovskite blocks in new superconducting compounds are not necessarily electrically neutral, unlike in the Sr-Ti-O system. It, thus, becomes necessary to create oxygen vacancies in the basic perovskite (figure 1) structure of Bi-containing compounds, when the width of the perovskite slab changes on addition of extra Cu-O planes. Results of our atomistic simulations suggest that these missing oxygen ions allow the Cu-O planes to buckle in Bi-series of compounds. This is also supported by the absence of buckling in the Sr-Ti-O series of compounds and the first member of Bi-containing compounds in which there are no missing oxygen ions. We present additional results on the phase stability of polytypoid structures in La-Cu-O system and defect chemistry of compounds of La-Ni-O system.     

Effect of fluorine on the critical current density of thallium based highT c superconductors

(Tl_0.5, Pb_0.5)Sr_1.6Ba_0.4Ca₂Cu₃O_y/F_x high temperature superconductors are reported with transition temperatures above 125 K. Samples with different fluorine contents (0 x 3) have been prepared. The addition of fluorine has little effect on the transition temperature, but remarkably increases the width of the hysteresis loops (which is proportional to J_c). The width of the hysteresis loops at 4.2 K was found to increase with x reaching an optimum value at x = 2.2. The. maximum width of the hysteresis loops is about 3 times larger than that for fluorine free material. Upon further increases in fluorine content the width of the hysteresis loops starts decreasing.     

Forced flow He vapor cooled critical current testing facility for measurements of superconductors in a wide temperature and magnetic field range

As superconducting materials find their way into applications, there is increasing need to verify their performance at operating conditions. Testing of critical current with respect to temperature and magnetic field is of particular importance. However, testing facilities covering a range of temperatures and magnetic fields can be costly, especially when considering the cooling power required in the cryogenic system in the temperature range below 65 K (inaccessible for LN₂). Critical currents in excess of 500 A are common for commercial samples, making the testing of such samples difficult in setups cooled via a cryocooler, moreover it often does not represent the actual cooling conditions that the sample will experience in service. This work reports the design and operation of a low-cost critical current testing facility, capable of testing samples in a temperature range of 10–65 K, with magnetic field up to 1.6 T and measuring critical currents up to 900 A with variable cooling power.     

A.c. magnetic measurements on superconductors using two-channel dynamic analyser

A.c. magnetic measurements are powerful tools for characterizing superconducting materials. The a.c. susceptibility is the most used measurement and is commonly performed using a two-phase lock-in amplifier. This article describes a different method to carry out this measurement, employing a two-channel dynamic analyser. This instrument also allows us to measure further quantities, such as higher harmonics of the a.c. susceptibility, the so-called wide-band susceptibility and the magnetization loops. A remarkable feature of the dynamic analyser is its wide frequency range, from 244 μHz to 102 kHz, allowing simple, comprehensive and reliable characterization of superconductors.     

The effect of Ca doping on the properties of melt-processed Gd-based bulk superconductors

The effect of Ca doping to melt-processed Gd-based bulk superconductors fabricated by the oxygen-controlled melt growth (OCMG) method was studied. First, we simply added CaCO₃ to the starting materials to dope Ca and observed an increase in the trapped field and the critical current density (J_c) up to, at least, 0.31 wt.% CaCO₃. However, an additional oxygen annealing resulted in a decrease of J_c in magnetic fields and in a monotonous exponential-like field dependence ofJ_c. This suggests that oxygen deficiency had caused the enhancement of J_c, although the same annealing process as the non-doped sample was applied. We also prepared samples by adding not only CaCO₃ but also BaCO₃ and CuO in a molar ratio of Ca:Ba:Cu = 1:2:3, and observed a large peak effect of J_c up to at least 0.31 wt.% CaCO₃. It is likely that this large peak effect is also caused by oxygen defects, although the superconducting transition temperature was significantly large. These results indicate that J_c at 77 K can be significantly enhanced by introducing a proper amount of oxygen deficiency and compensating at the same time the decrease of carrier density by Ca doping.     

Precision submillimeter wave laser reflection measurements of high-temperature superconductors

Submillimeter wave laser reflection measurements of surface resistance can provide improved capability in the combination of sensitivity, spatial resolution, and frequency range. We have made reflectivity measurements on metals at 1630 GHz with an uncertainty of less than 0.3%. This sensitivity corresponds to a measurement sensitivity for surface resistance of 0.3 . Assuming anf ² frequency scaling of high-temperature superconductor surface resistance from the microwave to the terahertz frequency range, this sensitivity corresponds to about 1 ×10^–5 at 10 GHz. Capability for 10^–7 sensitivity could eventually be possible. Preliminary submillimeter wave reflection measurements of a YBCO thin film have been made with a sensitivity of 1%. Submillimeter wave reflectometry can make it possible to determine the spatial dependence of surface resistance in a wide range of material sizes and shapes. The spatial resolution could be on the order of 0.3–0.5 mm.     

An X-ray spectral study of the influence on the electronic structure of highT c superconductors of the parameters determining the superconducting transition temperature

CuL _α and OK _α X-ray fluorescence spectra, corresponding to the Cd3d-Cu2p and O2p-O1s transitions, and XANES and EXAFS spectra (CuK, YK, BiL, PbL) have been obtained for the following series of samples: (i) YBa₂Cu₃O_7−δ (0<δ<0·8), (ii) La_2−x Sr _x CuO₄ (0<x<0·3) and (iii) Bi₂(Sr, Ca) _n+1 Cu _n O_2n+4 (n=1,2,3), Bi_1·8Pb_0·3Sr_1·9Ca₂Cu₃O _y . A correlation has been found between the decrease in the superconducting transition temperature (T _c ) with variation in the stoichiometric parametersδ andx in series (i) and (ii), and the shift of the maximum in their CuL _α spectra and the relative increase in the number of occupied states Cu3d in the upper part of the valence band. The changes in the distribution of the Cu3d and O2 _p densities and in the spectral parameters of the samples are discussed.     

Automatic device for magnetization measurements on superconductors

A measuring system is described which is based on the amplification at low temperature of small dc voltages proportional to the derivative of the superconductive magnetization. The system provides full computer control of the magnetization process, the data acquisition and the evaluation of the magnetization curves.     

Ion beam and thermally-induced interface reaction between highT c superconductor thin film and metal overlayer

Ion beam and thermally-induced interface reactions between highT _c superconducting thin film of Y₁Ba₂Cu₃O_7−x and metal overlayer of Ag are studied with a view to control the interfacial property of contact resistance. The interface reaction is induced by 100 keV Ar⁺ ion beam with different ion dose values ranging from 5 × 10¹³ to 3×10¹⁴ ions/cm². The YBaCuO film-metal interface is characterized by using the small angle XRD to study the structural properties of the interfacial phases. The electrical property of the interface, specifically contact resistance, has been investigated for different dose values and thermal treatments. Three-probe vs four-probe configuration has been adopted to measure the contact resistance.     

Vibrating reed studies on high-T c superconductors

We present vibrating reed (VR) measurements on single crystal and ceramic “1-2-3” and melt-processed polycrystalline Bi-based compounds in a wide range of temperature (4·2–100 K) and magnetic field (B=0–4 T). The “depinning line” (DL) determined by the VR technique is equivalent to the “irreversibility line” determined by magnetization and susceptibility measurements. A comparison of the results on single crystal and polycrystalline 1-2-3 compounds indicates that the VR technique is sensitive to the intragranular properties of the polycrystalline reed. It is found that the DL for 1-2-3 compounds is much steeper than that for Bi-based compounds, reflecting an intrinsically different pinning in both the materials, in agreement with the measured elastic coupling (Labusch constant α(B, T)).     

Influence of sintering time and quenching on the formation of highT c phase

Influence of sintering time and quenching in Bi_{2 −x} Pb _x Ca₂Sr₂Cu₃O _y (x=0.0, 0.1, 0.2, 0.25, 0.3 and 0.4) samples have been studied by resistance and XRD measurements. In samples sintered at 850°C for 4 days,T _c(0) increases with Pb concentration.T _c(0) increased from 81 K forx=0.0 to 109 K inx=0.30 sample and then decreased. Increasing the sintering time to 10 days decreased theT _c Quenching further decreased theT _c(0). From X-ray diffraction patterns, the intensity peaks of low and highT _c phases have been measured. The addition of Pb promotes highT _c-phase. Sintering time, slow cooling and rapid quenching studies show that there is an optimum sintering time and cooling rate to produce a highT _c-phase.     

The effect of frequency-dependent attenuation and the frequency characteristic of the converters on the error of measurements in ultrasonic flaw detection

The error in measuring the distance to a defect, due to white noise, is estimated. Distortions of the signal spectrum due to frequency-dependent attenuation of ultrasound and the nonuniformity of the frequency characteristics of the converters are taken into account. Translated from Izmeritel'naya Tekhnika, No. 11, pp. 64–66, November, 2008.