Oxidative Nucleobase Modifications Leading to Strand Scission

THE recent discovery1 of superconductivity below a transition temperature (Tc) of 94 K in HgBa2CuO4+δ has extended the repertoire of high-Tc superconductors containing copper oxide planes embedded in suitably structured (layered) materials. Previous experience with similar compounds containing bismuth and thallium instead of mercury suggested that even higher transition temperatures might be achieved in mercury-based compounds with more than one CuO2 layer per unit cell. Here we provide support for this conjecture, with the discovery of superconductivity above 130 K in a material containing HgBa2Ca2Cu3O1+x (with three CuO2 layers per unit cell), HgBa2CaCu2O6+x (with two CuO2 layers) and an ordered superstructure comprising a defined sequence of the unit cells of these phases. Both magnetic and resistivity measurements confirm a maximum transition temperature of ∼ 133 K, distinctly higher than the previous established record value of 125–127 K observed in Tl2Ba2Ca2Cu3O10 (refs 2,3). The discovery in 1986 of the first copper oxide superconductor stimulated an explosion of research activity that continues to the present day. The early years of high-temperature superconductivity were characterized by the rapid discovery of many new materials with increasingly high transition temperatures. The record now stands at ~133 K, attributed to a mercury-containing compound reported by Schilling et al. in 1993, although the dream of achieving room-temperature superconductivity has yet to be fulfilled.

Superconductivity above 130 K in the Hg–Ba–Ca–Cu–O system

Since the discovery of superconductivity above 30 K by Bednorz and M\"uller in the La copper oxide system, the critical temperature has been raised to 90 K in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and to 110 and 125 K in Bi-based and Tl-based copper oxides, respectively. In the two years since this Nobel-prize-winning discovery, a large number of electronic structure calculations have been carried out as a first step in understanding the electronic properties of these materials. In this paper these calculations (mostly of the density-functional type) are gathered and reviewed, and their results are compared with the relevant experimental data. The picture that emerges is one in which the important electronic states are dominated by the copper $d$ and oxygen $p$ orbitals, with strong hybridization between them. Photon, electron, and positron spectroscopies provide important information about the electronic states, and comparison with electronic structure calculations indicates that, while many features can be interpreted in terms of existing calculations, self-energy corrections ("correlations") are important for a more detailed understanding. The antiferromagnetism that occurs in some regions of the phase diagram poses a particularly challenging problem for any detailed theory. The study of structural stability, lattice dynamics, and electron-phonon coupling in the copper oxides is also discussed. Finally, a brief review is given of the attempts so far to identify interaction constants appropriate for a model Hamiltonian treatment of many-body interactions in these materials.

Electronic structure of the high-temperature oxide superconductors

We have previously shown that Tc for Bi2 (Sr, Ca)n+1CunO2n+4+δ (n=1, 2 and 3) varies with oxygen stoichiometry δ determined by annealing in a variety of oxygen partial pressures and temperatures. Annealing results are now also presented for several Tl-superconductors in the series Tlm(Ba, Sr)2Can−1CunO2n+m+2+δ, for both m=1 and 2, revealing two general results for this class of Bi or TI superconductors: (1) effects on Tc due to labile oxygen occur for all m=2 compounds but are absent in m = 1 compounds with Sr. m=1 compounds with Ba do have variable oxygen but the effects on the c-axis are the opposite to m=2 compounds. This suggests for the latter that the labile oxygen resides in interstitial sites within the Bi2O2 or Tl2O2 bi layers; (2) the hole concentration per Cu decreases in progressing from n = 1 to 2 to 3 in either class so that the parent n = 3 and n = 2 compounds lie respectively on the low- and high-hole concentration sides of the peak in Tc while the n = 1 compounds extend out into the non-superconducting domain at very high hole concentration.

/pdf/general-trends-in-oxygen-stoichiometry-effects-on-tc-in-bi-4gmly6ibdk.pdf

General trends in oxygen stoichiometry effects on Tc in Bi and Tl superconductors

http://www.its.caltech.edu/~jkbgrp/abstracts/A267.pdf

Metallo-intercalators and metallo-insertors

Bulk superconductivity is reported in T12Ca2 Ba2 Cu3 Ox with a transition to zero resistance at ≃ 125 K, the highest transition temperature (Tc) yet found. Transmission electron spectroscopy shows that the unit cell is body-centered tetragonal and contains trilayer Cu perovskitelike units separated by bilayer TI-O units. (T c in this material varies from 118 to 125 K depending on preparation conditions. A high density of perovskitelike bilayer intergrowths are observed in the 118-K material, which we speculate result in the decreased (T c. A second structure containing only bilayer perovskitelike units—TI2 Ca1 Ba2Cu2Ox—is a bulk superconductor with Tv ranging from 95 to 108 K.

Bulk superconductivity at 125 K in Tl2Ca2Ba2Cu3Ox.

New class of high Tc structures: Intergrowth of multiple copper oxide perovskite-like layers with double sheets of BiO

Over the last decade, the search for high-temperature superconducting materials remained virtually stagnant. This situation changed radically with the discovery of Bednorz and Mueller of superconductivity above 30 K in a layered perovskite oxide composed of La, Ba, and Cu. Improvements in the superconducting transition temperature (T/sub c/) to approx. 45 K by Sr substitution and identification of the phase responsible for superconductivity (La/sub 2-x/Ba(or Sr)/sub x/CuO/sub y/ where x is typically between 0.1-0.3) followed rapidly. The next major advance was immediate and dramatic. Wu, Chu, and co-workers reported a new material based on the starting composition Y/sub 1.2/Ba/sub 0.8/CuO/sub y/ with T/sub c/ well above 90 K. Nearly simultaneous reports by other groups confirmed these results. The Y-Ba-Cu material was a mixture of several unidentified phases and only a small fraction of the sample actually was superconducting. Recently, the structure of this new superconductor was identified as an oxygen-defect perovskite corresponding to the composition Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub y/. In this paper, they report on the synthesis of single-phase Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub y/ and show how the preparation conditions play a dramatic role in determining the superconducting properties. Also, based on their knowledge of the structure, they havemore » prepared a variety of new, single-phase high-temperature superconducting compounds in which Y and Ba are substituted by related elements, demonstrating that 90 + K superconductivity is a more general property of this structure.« less

Superconductivity above liquid nitrogen temperature: preparation and properties of a family of perovskite-based superconductors

Compositional properties and thermoelectric power of the superconducting ceramic. Nd2−xCexCuO4−y

We report the development of a reaction technique which yields ceramic Nd 1.85 Ce 0.15 CuO 4−y with macroscopically uniform concentrations of Ce and O. Under appropriate argon annealing conditions, samples exhibiting bulk superconductivity as gauged by resistive transition width and diamagnetic shielding are obtained. In addition. metallic behavior, indicated by positive dp/dT above T c , is obtained for the normal state.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400011675

R. J. Savoy

Papers

Bulk superconductivity at 125 K in Tl2Ca2Ba2Cu3Ox.

New class of high Tc structures: Intergrowth of multiple copper oxide perovskite-like layers with double sheets of BiO

Superconductivity above liquid nitrogen temperature: preparation and properties of a family of perovskite-based superconductors

Compositional properties and thermoelectric power of the superconducting ceramic. Nd2−xCexCuO4−y

The preparation and processing of bulk superconducting ceramic Nd 1.85 Ce 0.15 CuO 4 - y