high-Tc Oxide Superconductors Research Articles

Materials Science of High-Temperature Superconductors

For thirteen years, the highest recorded superconductivity transition temperature (Tc) stayed at 23.2K (in Nb3Ge). This record was broken a little over a year ago with the discovery of the mixed La−Ba−Cu−O compound with a TC above 30K.1 The superconducting phase was identified to be a single layered K2 Ni F4 structure. On January 29, 1987, superconductivity above liquid nitrogen was first accomplished by Chu and coworkers.2 The crystal structure of this high Tc oxide superconductor with composition Y1Ba2Cu3O7 has been determined from x-ray3 and neutron diffraction4 by several workers. It is now clear that the superconducting phase is an orthorhombic oxygen deficient perovskite structure with a space group Pmmm with a = 0.386nm b = 0.387nm and C = 1.170nm (Figure 1). The structure thus has two-dimensional CuO2 planes, and there are linear chains of Cu−O. The role of these features in determining Tc (and indeed the possible mechanism of Tc) is currently a matter of controversy.

DEPENDENCE OF THE SUPERCONDUCTING Tc UPON THE ATOM MASS IN AMORPHOUS SUPERCONDUCTORS

The correlation between the superconducting Tc and the atom mass (M) for amorphous superconductors has been studied. It is found that the superconducting Tc is inversely proportional to the cubic root of the atom mass, i.e. Tc∞1/M1/3 for the elements of the same group in which they have the same number of valence electrons. The Tc of crystalline superconductors are discussed too, including the superionductivity of high Tc oxide superconductors.

Macrostructure of High <i>T</i><sub>c</sub> Oxide Superconductors

Microscopic observation has been conducted on newly discovered high Tc oxide superconductors using the polarized optical microscope. It was revealed that all the superconductors exhibited almost the same microstructure irrepective of the kind of the rare earth site. In the samples prepared in the usual sintering process, the grains are oriented at random and have rectangular shapes. In the sample with the orthorhombic structure, the grains contain twins, whereas no twin is observed in the tetragonal sample.

Mean Field Theory of Normal and Superconducting Heavy Fermion Systems and Oxide Superconductors

Normal heavy fermion systems may be assumed to have infinitely strong short range electron-electron interaction U within the f-orbital. Formulating the Anderson lattice model in terms of slave bosons and working in mean field approximation the very high masses, high specific heats and high susceptibilities of these materials may be simply understood. Allowing U to be large but finite, a generalisation of the theory allows for the possibility of superconductivity. In an isotropic model estimates of Tc give values close to experiment. Gap and Hc take BCS values in the weak coupling limit. In a more realistic model, the gap is found to be anisotropic with zeros and other nonuniversal features developing. The model is also applied to high-Tc oxide superconductors.

Electronic structure of high-Tc oxide superconductors studied by photoemission spectroscopy.

Photoemission studies on the electronic structure of high-Tc superconducting Cu oxides have been made, and the results can be summarized as follows : 1) The Cu 3 d elertrons are localized as a spin-1/2 d9 configuration due to strong intra-atomic Coulomb interaction. 2) However, hybridization between the localized Cu 3 d states and the band-like O 2p states is strong in the ground state, and thus resuts in a net d occupation of about 9.5 and a magnetic moment of about 0.5 μB per Cu atom. This is in good agreement with neutron experiment. 3) Doped holes are predominantly O 2p-like rather than Cu 3d-like. 4) From these, attractive interaction between O 2p holes mediated by fluctuations of the Cu 3d spins would be a most likely candidate for the cooper pairing mechanism.

Present and Perspective of High-Tc Oxide Superconductors(Papers presented at the 34th Annual Meeting, April 1988, Tokyo)

Present and Perspective of High-Tc Oxide Superconductors(Papers presented at the 34th Annual Meeting, April 1988, Tokyo)

酸化物高温超電導体とマイスナー効果による釣り上げ現象

酸化物高温超電導体とマイスナー効果による釣り上げ現象

Synthesis and properties of thin film high-Tc oxide superconductors

Recent results of the synthesis of thin films of the high-Tc oxide superconductors are described. The methods employed include electron-beam co-evaporation and co-sputtering in the case of YBa2Cu3O7 and molecular beam epitaxy in the case of DyBa2Cu3O7. The films exhibit high, sharp transition temperatures (∼90 K and < 2 K wide) and large critical current densities (106-107 A/cm2 at 4.2 K). Measurement of the optical transmission of some of these films shows no evidence for a charge transfer resonance indicating excitronic superconductivity. Measurement of the fluctuation conductivity above Tc appears to show a distinct 3D to 2D dimensional crossover.

Studies of high-Tc oxide superconductors

Recent results of experimental studies on high-Tc oxide superconductors are presented. Anisotropy and temperature dependence of the magnetic susceptibility χ of La2CuO4 do not seem to be easily explained by a simple antiferromagnetic spin ordering with the spin direction within the CuO layers. Moreover with increasing magnetic field perpendicular to the layers, χ exhibits a sudden increase below TN. The tunneling study revealed that the energy gap value of YBa2Cu3Oy is consistent with BCS theory and any additional structure has not been observed in the present tunneling spectra.

ESR and magnetic susceptibility of high Tc oxide superconductors

Abstract ESR measurements have been made on Y-Ba-Cu-O, La-(Sr, Ba)-Cu-O systems and La2CuO4, together with the magnetic susceptibility measurement. The ESR signal in a good superconductor of the Y-Ba-Cu-O system could not be observed except for a very weak signal associated with Cu2+ ions of impurity phases. No ESR signals were also observed in the La-(Sr, Ba)-Cu-O system. These results are consistent with the essentially temperature-independent susceptibility in Y-Ba-Cu-O and La-(Sr, Ba)-Cu-O.

Optical studies of high-Tc oxide superconductors

Abstract Oxygen deficiency effects on the superconductivity of YBa2Cu3O7-δ are investigated by the electrical resistivity and the infrared reflection spectroscopy. On increasing the quenching temperature the sample changes to a semiconductor from a metal. The superconducting gap of the slowly cooled sample is anisotropic and the minimum gap is 2Δ/kBTc∼1.6. The anisotropy of the sample quenched from 600°C is very small in the proximity-coupled superconducting region and the 2Δ/kBTc∼1.

Quasiparticle tunneling and Josephson tunneling of high-Tc oxide superconductors

Abstract We present the quasiparticle tunneling measurements between superconducting bulk ceramics (Y-Ba-Cu-O and RE-Ba-Cu-O: RE=Dy, Er, Ho) and a metal film (Pb, Sn). A series of gap structures and the value of the gap parameter of 20–25 meV are observed for all these compounds exhibiting bulk T c of 92–93 K. Josephson tunneling utilizing the crack-type bulk ceramic junctions is also studied. Macroscopic quantum interference effect is shown to be observable up to 90 K.

Strong-Coupling Theory of Superconductive Electron-Electron Interaction for High-Tc Oxide Superconductors

We discuss the role of the spin exchange interaction for high- Tc Y–Ba type copper oxide superconductors. We then formulate a strong-coupling theory for our recently proposed mechanism for high-Tc oxide superconductors by a two-band model of lighter and heavier electrons while taking into account an infrared divergent effect due to the Fermi surface of the lighter electrons. We show how to theoretically derive a very high experimental ratio, twice the energy gap at T=0, over the transition temperature, 2Δ(0)/kTc, to be in the range from 4.5 to 7.5.

OXIDE SUPERCONDUCTIVITY RESEARCH AT NORTHWESTERN UNIVERSITY

A short review was presented of a wide range of activities involving the high Tc oxide superconductors at the Northwestern University. Calculations of the charge density associated with the d9 shell of the copper atom in the Cu-O planes and the d8 shell of the copper atoms along the Cu-O lines of the unit cell of YBa2Cu3O7 performed by Freeman, Massida and Yu were presented. Results of a detailed study of the pseudoternery phase diagram involving CuO, BaO and YO1.5 were shown which verify the existence of three compounds having the Y:Ba:Cu ratios 123 (the high Tc superconductor), 211 (the so-called green phase) and 132 (a new insulating compound). This work, performed by Mason and coworkers, involved X-ray diffraction, optical spectroscopy, magnetic susceptibility, and electrical transport studies. Transport measurements by Poeppelmeier, Song, Hwu and coworkers on the assembledges YBaSrCu3O7 anf YBa2Cu3O5F2 were reported; no evidence of superconducting behavior at temperatures above 90K was seen in the latter. A detailed study of the magnetic and transport properties of the YBa2−xLaxCu3−xNixO7±δ system was reported. For small Ni concentration (x&lt;0.3) a continuous drop in Tc was observed. For x= 0.4 and 0.5 evidence of two separate transitions, near 75K and 25K, were seen in the resistance (with the resistance going to zero at about 15K); for x=0.7 semiconducting behavior was observed. A continuous drop in the normalized diamagnetic signal was seen with increasing x with the diamagnetic fraction being below 5 % for x=0.7 and above. X-ray diffraction showed a continuous decrease in the orthorhombic fraction and an increase in the tetragonal fraction with increasing x. Results on d.c. magnetron sputtering of thin films on MgO from a stoichiometric 123 target in an argon atmosphere were shown. With proper annealing films with onset temperature of 90K and zero resistance at 68K have been prepared; this work was done by Jin, Lee, Song and coworkers. A brief report on the effect of O-stoichiometry on EPR and susceptibility measurements in the 123 compound performed by Hoffman and coworkers and NMR in the La2−xSrxCuO4 compound by Halpern and coworkers was presented.

Microstructural irregularities in superconducting YBa2Cu3O7-y

Electron microscopy has been used to obtain ultrahigh-resolution images projected along the major high-symmetry directions of YBa2Cu3O7-y. It reveals an unexpected structural irregularity which could be of significance in influencing the superconducting properties of this high-Tc oxide superconductor.

Comparison of carbonate, citrate, and oxalate chemical routes to the high-Tc metal oxide superconductors, lanthanum strontium copper oxide, La2-xSrxCuO4

In this communication the authors compare the carbonate, citrate, and oxalate solution techniques for the synthesis of the high-T/sub c/ compounds La/sub 2-x/Sr/sub x/CuO/sub 4/. The superconducting properties of the compounds are determined as much by the oxygen stoichiometry as by the metal stoichiometry, and therefore, the sintering and annealing temperatures and O/sub 2/ partial pressures all influence the conductive properties of the materials.

Identification of the phase responsible for high-temperature superconductivity in Y–Ba–Cu oxides

The discovery of La–Ba(Sr)–Cu oxides of K2NiF4 structure exhibiting superconductivity in the 30–40 K region was reported early this year1. The recent announcement of superconductivity above the temperature of liquid nitrogen in Y–Ba–Cu oxides2 has belied all expectations. Unfortunately, all the Y–Ba–Cu oxide compositions exhibiting this remarkable feature were biphasic or multiphasic. We have studied various compositions of the Y3–xBa3+xCu6O14 and now report the isolation of the pure oxide phase responsible for high Tc superconductivity. This oxide has the composition Y0.33Ba0.67CuO2.33±δ, with an oxygen defect perovskite structure. The pure oxide shows the onset of superconductivity at 120 K, attaining zero resistance at 87 K. It also exhibits the highest Meissner effect of all the high-Tc oxide superconductors.

Tunneling Measurements of the Energy Gap in High Tc Oxide Superconductors

We have measured the current-voltage (I-V) characteristics for tunneling into the high critical temperature superconducting materials La-Sr-Cu-O and Y-Ba-Cu-O. We find superconducting energy gaps that are consistent with strong coupling superconductivity in both materials. We find no significant differences in the measured gaps when the tunneling tip approaches a single crystal of Y-Ba-Cu-O from a direction normal to the CuO planes, as opposed to parallel to them. There are structures in the conductance-voltage characteristics above the gap energy that can occasionally have very large amplitudes. The physical origin of this structure is not certain. However, its presence makes it difficult to obtain quantitative information on the electron-phonon coupling strength from such tunneling measurements.

Materials Aspects of Oxide Superconductors - Effect of Ambient Water on Superconductivity-

ABSTRACTIt has been found that the chemical instability of Ba2LnCu3O7 causes some problems in the measurements of critical temperature and creates a highly resistive grain boundary contact which is the major cause of the low critical current density at the present moment. Firstly, measurements of the superconductive critical temperature could be seriously misled if based solely on resistive transition because apparent resistivity anomaly is observed due to electrochemical EMF which appears when the electrode contact is attacked by water. Secondly, the barrier nature of the grain boundary against charge carriers is significantly enhanced by the chemical attack of the boundary region with water and water/carbon dioxide in the annealing atmosphere. This leads to a further weakening of the Josephson weak link between polycrystalline grains in the high Tc oxide superconductors.

High-Temperature Superconductivity; Past, Present and Future

The history of the research on high-Tc oxide superconductors is reviewed. Fundamental physical properties of high-Tc oxides and its implication are described. Based on the experimental data presented so far, the potential future of these materials is briefly discussed.