The initial discovery by Bednorz and Muller1 of 35-K superconductivity in the La-Ba-Cu-O system has stimulated worldwide activity in searching for higher-temperature superconductors. Elemental substitution has proved to be most effective in raising transition temperature. Substitution of Sr for Ba has produced 40-K superconductivity2–5and substitution of Y for La has produced a new high-temperature superconductor with transition temperature above liquid-nitrogen temperature6. A class of superconducting compounds of the form RBa2Cu307-x has been explored by further substitutions of other rare earths (Y is considered in the rare-earth [RI category here) for Y7-13. To date, a rare earth, an alkaline earth, copper and oxygen have been required for all high-temperature superconductors14,15. (Zhanget al 14reported 90-K superconductivity in the Th-Ba-Pb(Zr)-Cu-O system. Panetal15reported 50-K superconductivity in the Y-Ba-Ag-O system. As Th is a member of the actinide series which belongs to the same Group 3B in the periodic table as the lanthanide series and Ag belongs to the same Group 1B as Cu, high-temperature supercon-ductors are still thought to be closed in the Group 3B—Group 2A-Group 1B—oxygen system. ) Only partial substitutions ha. e led to superconductors, but with no significant rise of transition tem-perature (the only exception is 40-K superconductivity in La2CuO4-x , refs 16, 17). Here we report superconductivity in the rare-earth-free TI-Ba-Cu-O system. We have obsened sharp drops of resistance starting above 90 K with zero resistance at 81 K in this system. Magnetic measurements have confirmed that these sharp drops of resistance in the TI-Ba-Cu-O samples origi-nate from superconductivity. The samples are stable in air for at least two months, and their preparation is easily reproduced.

Superconductivity in the rare-earth-free Tl–Ba–Cu–O system above liquid-nitrogen temperature

Oxygen stoichiometry in Ba2YCu3Ox

BiFeO3 is one of the most studied multiferroic materials. Both its magnetic and ferroelectric properties can be influenced by doping. A large body of work on the doped material has been presented in the past couple of years. In this paper we provide a perspective on general doping concepts and their impact on the material's functionality.

Doping BiFeO3: approaches and enhanced functionality.

STARTING with the pioneering work of Bednorz and Muller1, many copper-oxide-based superconductors with high transition temperatures (Tc) have been discovered. All contain layers of copper-oxygen squares, pyramids or octahedra as their electronically active structural components2,3. One structure type, first reported for La2SrCu2O6 and La2CaCu2O6 (ref. 4), has stood as an enigma since the beginning of high-Tc research. This crystal structure4-7 (Fig. 1) is the least complex of all the structures with the double layers of copper oxide pyramids common to the compounds with highest Tc, yet despite considerable effort, both published8,9 and unpublished, it has not until now been made superconducting. Here we report the successful synthesis and preliminary physical characterization of superconducting (La, Sr)2CaCu2O6. The highest transition temperature observed is 60 K at the composition La1.6Sr0.4CaCu2O6. This is a uniquely simple double-layer superconductor, which, like its single-layer analogue (La, Sr)2CuO4, becomes superconducting through the introduction of carriers in an unambiguous manner—by straightforward atomic substitution without the intervention of charge reservoir layers with flexible valence states.

Superconductivity at 60 K in La2-xSrxCaCu2O6: the simplest double-layer cuprate

From ab initio calculations on various clusters representing the La_(2-x)Sr_xCu_1O_4 and Y_1Ba_2Cu_3O_7 classes of high-temperature superconductors, it is shown that (i) all copper sites have a Cu^(II)(d^9)oxidation state with one unpaired spin that is coupled antiferromagnetically to the spins of adjacent Cu^(II) sites; (ii) oxidation beyond the cupric (Cu^(II)) state leads not to Cu^(III) but rather to oxidized oxygen atoms, with an oxygen pπ hole bridging two Cu^(II) sites; (iii) the oxygen pπhole at these oxidized sites is ferromagnetically coupled to the adjacent Cu^(II)d electrons despite the fact that this is opposed by the direct dd exchange; and (iv) the hopping of these oxygen pπ holes (in CuO sheets or chains) from site to site is responsible for the conductivity in these systems (N-electron band structures are reported for the migration of these localized charges).

https://science.sciencemag.org/content/sci/239/4842/896.full.pdf

Electronic structure and valence-bond band structure of cuprate superconducting materials.

Structural, magnetic, and electronic properties of compounds in the series La 2- x Sr x CuO 4- y for 0.05 ≤ x ≤ 1.1, with δ x = 0.025, were studied. Resistance, susceptibility, Meissner, and shielding measurements have revealed superconductivity among several members of the series. For x = 0.15, the transition temperature T c , measured at the midpoint of the resistive transition, is a maximum at 39.3 K with a width of 2 K. At other compositions the transition is broader and occurs at a lower temperature. Annealing the x = 0.15 sample in oxygen at 500°C increases T c to 40.3 K, while annealing at the same temperature under vacuum suppresses the superconductivity. These changes in oxygen content and T c are reversible.

https://science.sciencemag.org/content/sci/235/4794/1373.full.pdf

T. H. Geballe

Papers

Superconductivity at 40 K in the Oxygen-Defect Perovskites La2-xSrxCuO4-y