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.     

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.     

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.     

High resolutionV-I characteristics measurements on highT c superconductors

V-I characteristics of sintered superconducting pellets of YBa₂Cu₃O_7−δ was measured with a resolution one order of magnitude better than the usual dc techniques employing nanovoltmeters. For this purpose software-based lock-in-amplifier technique was developed and used. A square-wave excitation current was used. The voltage signal was digitized into a time series and Fourier-analysed on a computer in this technique. Our results show a very small frequency-dependent resistance in some samples which are nominally superconducting. These results help in checking the material quality and defining the critical current densities better.     

Bulk Josephson energies in multi-gap models of highT c superconductors

Multi-gap models of high temperature ceramic superconductors have been of considerable recent interest. For the case of a two-gap model (Δ₁, and Δ₂), there exist two separate electron pair condensate reservoirs, and a bulk Josephson free energy can be associated with electron pair transitions from one reservoir to the other. Thus, a “Josephson effect” can exist in two-gap models without the requirement of a weak link separating the two electron pair reservoirs in space. A single bulk sample Josephson electron pair free energy is here derived from the conventional BCS model of the superconducting state.     

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.     

Effects of iodine intercalation into Bi-based copper oxide superconductors

We have investigated the stage-1 iodine-intercalation compounds, whose host materials are Bi₂Sr₂Ca_1–x Y _x Cu₂O₈ (Bi-2212 phase) and Bi_1.6Pb_0.5Sr_1.9–x La _x Cu_1.05O₆ (Bi-2201 phase) with variousx values. In the Bi-2212 phase, it has been found that the change inT _c through the stage-1 iodine intercalation is due to two effects; a change of the two-dimensionality and an increase in the carrier number. The former effect decreasesT _c by about 10 K independent of the carrier number of the host sample. The latter effect shifts thex dependence ofT _c to largerx values by about 0.08, taking account of oxygen release from the host samples during the intercalation process. This result is clear evidence for the occurrence of both the change of two-dimensionality and the increase in carrier number about 0.04 per CuO₂ unit due to the charge transfer through the stage-1 iodine intercalation. In the Bi-2201 phase,T _c decreases by about 10 K through the stage-1 iodine intercalation.     

Dislocation configurations in highT c oxide BiSrCaCuO

A study of dislocation configurations in superconducting oxide BiSrCaCuO has been made by transmission electron microscopy. Evidence was found for the presence of dislocation pairs composed of two parallel single dislocations holding together with the same Burgers vectors. The pair can further dissociate into partials, giving rise to a four-fold ribbon. Networks consisting of dislocation pairs were also observed. These phenomena are attributed to the possible ordering of strontium and calcium ions and the existence of some oxygen sheet of the Aurivillius type in the oxide crystal. Schemes to illustrate the configurations are suggested and discussed.     

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.     

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.     

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 .     

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.     

Raman scattering by electronic excitations in semiconductors and in highT c superconductors

Raman scattering by electronic excitations in normal and superconducting metals with complex Fermi surfaces is discussed. The theoretical results are applied to the case of high T_c superconductors. The possible implications of measurements of this scattering concerning the symmetry of the gap function is dealt with.     

Relevance of electron-phonon coupling in high-T c superconductors

The experimental observation of phonon renormalization in high-T_c superconductors gives clear evidence for the action of electron-phonon coupling. But in spite of the estimated coupling strength 1–3, these effects do not allow for a unique identification of the coupling mechanism. This situation is contrasted with that for the low-T_c superconductors where tunneling spectroscopy provides a hard proof for electron pairing via phonons.     

An accurate method of iodometric titration to measure copper valence of high-T c superconductors

The double iodometric titration is the method widely used to determine copper valence of high-T _c superconductors. There are some problems that remain to be solved, and certain procedures are used: adding KSCN to release adsorbed iodine, adding HN₄HF₂ to eliminate the interference of some dopants, and using a new calculating equation to accurately calculate experiment results. Al- and Fe-doped samples were prepared and their copper valence, 2 +p, was determined by the improved double iodometric titration method. With these samples, how to use the above procedures to improve experiment results is discussed. We find that the iodometric titration method described here can be used to determine copper valence and oxygen content for doped and undoped samples with high accuracy.     

Magnetic flux noise in copper oxide superconductors

We report on the magnetic flux noise in thin films of YBa₂Cu₃O_7-x (YBCO), Tl₂Ca₂Ba₂Cu₃O_x, and TlCa₂Ba₂Cu₃O_x and in crystals of YBCO and Bi₂Sr₂CaCu₂O_8+x, measured with a Superconducting QUantum Interference Device (SQUID). We ascribe the noise to the motion of flux vortices. In the low magnetic fields in which the experiments are performed the average vortex spacing always exceeds the superconducting penetration depth. The spectral density of the noise usually scales as 1/f (f is frequency) from 1 Hz to 1 kHz and increases with temperature to a peak which is of the same magnitude in all samples, at the transition temperature. Furthermore, the noise power increases with the magnitude of the magnetic field in which the sample is cooled, with a power-law dependence over several decades, whereas a supercurrent well below the critical current density applied to YBCO films suppresses the noise power by an order of magnitude. Most of the measurements were made on YBCO films, and for this set of samples the noise decreases dramatically as the crystalline quality is improved. A model of thermally activated vortex motion is developed which explains the dependence of the noise on frequency, temperature, magnetic field, and current. The pinning potential is idealized as an ensemble of symmetrical double wells, each with a different activation energy separating the two states. From the noise measurements, this model yields the distribution of pinning energies, the vortex hopping distance, the number density of mobile vortices, and the restoring force on a vortex at a typical pinning site. The distribution of pinning energies in YBa₂Cu₃O_7-x shows a broad peak below 0.1 eV. Over narrow temperature intervals, most samples exhibit random telegraph signals in which the flux switches between two discrete levels, with activation energies and hopping distances much greater than those deduced from the 1/f noise measurements.     

Roles of oxygen defects in copper oxide superconductors

Oxygen vacancy and interstitial defects can have a profound effect on the superconducting properties of copper oxide compounds. Recent work on compounds such as La₂CuO_4+x and HgBa₂CuO_4+x has provided new insight into the role of interstitial oxygen defects as a doping mechanism. The number of carriers created by each interstitial defect depends on the local defect structure. Studies of (La, Sr, Ca)₃Cu₂O_6+x with various metal compositions and metalsite ordering show that interstitial oxygen defects that form between the CuO₂ layers in this structure systematically lowerT _c and eventually destroy superconductivity. Conversely, oxygen vacancies in the CuO₂ planes have surprisingly little effect at concentrations below 3%. The infinite-layer compounds, ACuO₂, where A=La, Sr, Ca, Nd, etc., in solid-solution combinations, could offer a similar environment for the formation of interstitial oxygen defects between the CuO₂ planes, allowing interstitial oxygen defects to contribute to the doping of these compounds. However, neutron diffraction experiments on Sr_0.9La_0.1CuO₂ (T _c = 42 K) have not found any interstitial oxygen.