Richard B. Flippen

A New High-Temperature Superconductor: Bi2Sr3-x Cax Cu2O8+y

A new superconductor that displays onset behavior near 120 K has been identified as Bi2Sr3-xCaxCu2O8+y, with x ranging from about 0.4 to 0.9. Single crystal x-ray diffraction data were used to determine a pseudo-tetragonal structure based on an A-centered orthorhombic subcell with a = 5.399 �, b= 5.414�, and c = 30.904 �. The structure contains copper-oxygen sheets as in La2CuO4 and YBa2Cu3O7, but the copper-oxygen chains present in YBa2Cu3O7 do not occur in Bi2Sr3-xCaxCu2O8+y. The structure is made up of alternating double copper-oxygen sheets and double bismuth-oxygen sheets. There are Ca2+ and Sr2+ cations between the adjacent Cu-O sheets; Sr2+ cations are also found between the Cu-O and Bi-O sheets. Electron microscopy studies show an incommensurate superstructure along the a axis that can be approximated by an increase of a factor of 5 over the subcell dimension. This superstructure is also observed by x-ray diffraction on single crystals, but twinning can make it appear that the superstructure is along both a and b axes. Flux exclusion begins in our samples at about 116 K and is very strong by 95 K. Electrical measurements on a single crystal of Bi2Sr3-xCaxCu2O8+y show a resistivity drop at about 116 K and apparent zero resistivity at 91 K.

Crystal Structure of Tl2Ba2Ca2Cu3O10, a 125 K Superconductor.

There is now a new series of high-temperature superconductors that may be represented as (AIIIO)2A2IICan-1CunO2+2n where AIII is Bi or Tl, AII is Ba or Sr, and n is the number of Cu-O sheets stacked consecutively. There is a general trend toward higher transition temperatures as n increases. The highest n value for a bulk phase is three and is found when AIII is Tl. This compound, Tl2Ba2Ca2Cu3O10, has the highest transition temperature(∼125 K) of any presently known bulk superconductor. The structure of Tl2Ba2Ca2Cu3O10 has been determined from single-crystal x-ray diffraction data and is tetragonal, with a = 3.85 � and c = 35.9 �. No superstructure is observed, and the material is essentially twin-free. Electron microscopy in the Tl/Ba/Ca/Cu/O system has revealed intergrowths where n = 5; such regions may well be responsible for the superconducting onset behavior observed in this system at about 140 K.

Bulk Superconductivity up to 122 K in the Tl-Pb-Sr-Ca-Cu-O System

New high-temperature superconductors based on oxides of thallium and copper, but not containing barium, have been prepared. A transition temperature (Tc) of about 85 K is found for (Tl0.5Pb0.5) Sr2CaCu2O7 whereas (Tl0.5Pb0.5)Sr2Ca2Cu3O9 has a Tc of about 120 K. Both materials possess tetragonal symmetry with a = 3.80 �, c = 12.05 � for (Tl0.5Pb0.5)Sr2CaCu2O7, and a = 3.81 �, c = 15.23 � for (Tl0.5Pb0.5)Sr2Ca2Cu3O9. A structure refinement of the latter phase has been carried out with single-crystal x-ray diffraction data.

Superconductivity near liquid nitrogen temperature in the PbSrRCaCuO system (R=Y or rare earth)

Abstract Superconductivity close to the boiling point of liquid nitrogen has been discovered in the P-Sr-R-Ca-Cu-O system where R is Y or rare earth. The superconducting compound has been identified as Pb 2 Sr 2 (R,Ca) 1 Cu 3 O 8+ y and the structure of the R=Y compound has been solved by single-crystal X-ray diffraction methods. The structure consists of double CuO 2 sheets interleaved by (Ca,R), a unit which is common to most of the high- T c copper oxide superconductors. A new structural feature found in this system is a double PbO layer separated by a sheet of copper atoms. A non-superconducting oxide of approximate composition (Y,Ca)Sr 2 (Cu,Pb) 3 O 7− y formed as a second phase in some preparations has been also characterized; its structure is related YBa 2 Cu 3 O 7 .

Influence of oxygen stoichiometry on the structure and superconducting transition temperature of YBa2Cu3Ox

A detailed study of the superconducting properties and the crystal symmetry of YBa2Cu3Ox as a function of oxygen content (x) is presented. We correlate the oxygen content, structure and superconducting transition temperature for YBa2Cu3Ox (6<×<7) series of materials formed by the topotactic intercalation/deintercalation of oxygen. It is shown that the orthorhombic to tetragonal phase transition coincides with a loss in superconductivity for samples prepared both by quenching from high temperature and samples prepared by deoxygenation at low temperature. For the orthorhombic phase, Tc monotonically decreases as x goes from 7.0 to 6.4 along with a complementary decrease in the extent of orthorhombic distortion. The decrease in Tc, however, is not uniform. For quenched samples it shows a plateau for x ∼ 6.75 to 6.55 and then a rather abrupt drop around x ∼ 6.5. Comparison of our data with the literature indicates that the dependence of superconducting properties and crystal structure on the oxygen content can be a complex function of sample processing history. Samples with the same oxygen content but prepared in different ways may have x-ray powder patterns that are indistinguishable, but significantly different electrical properties.

Superconducting and Magnetic Behavior in La2-xNaxCuO4

New phases of the type La2-xAxl+CUO4-y have been prepared where Al+ is sodium or potassium. The sodium phases are superconducting for x values from 0.2 to 0.5 at temperatures up to about 40 K. In addition, there are unusual magnetic properties below about 10 K that may be indicative of spin glass behavior. Phases of the type La2-xKxCuO4-y could only be prepared with x values up to about 0.1, and these phases are not superconducting above 4.2 K

Local magnetic field distribution in polycrystalline YBa 2 Cu 3 O 7 and its influence on bulk critical currents

The transport critical current density J c has been measured in high quality polycristalline YBa 2 Cu 3 O 7 samples made from extruded preforms. The hysteretic response of J c to applied magnetic fields was studied as a function of temperature (77-87 K) and sample morphology for various field sweeps in the 0-5 KOe range. A variety of samples show a basic J c ∼H −n behavior where n depends on temperature, but is independent of the other variables. A simple model of the local magnetic field distribution is presented and compared to te data, and the theoretical implications of the power law behavior are considered

χ″ peak measurements in HTSC single crystals

Abstract The use of AC inductance techniques to measure AC absorption (χ″) in high-temperature superconducting materials is discussed. A single sharp peak in χ″ versus temperature is typically seen for good crystals of YBa 2 Cu 3 O 7− x , However, some crystals of this material show multiple χ″ peaks which have different magnetic field and frequency dependences. Crystals of Bi 2 Sr 2 Ca 1 Cu 2 O 8+ y and Tl 2 Ba 2 Ca 1 Cu 2 O 8+ y show only single χ″ peaks. Magnetic field, temperature, and measurement frequency dependences of χ″ of these materials are presented. A critical state model is used to interpret these results. Multiple peak behavior is explained as the presence of different superconducting domains in the crystals. Attempts to alter these domains by modifying the oxygen content and distribution in crystals of YBa 2 Cu 3 O 7− x by oxygen annealing were unsuccessful. Etching experiments did not show any surface contribution to the magnetic properties of these samples.

Degradation of sinterability and superconducting properties of fine particle YBa2Cu3O7−y by exposure to moisture

The effect at room temperature of ambient level humidity on fine particle YBa2Cu3O7−y powder has been studied. Reaction with moisture, as detected by x-ray diffraction and scanning electron microscopy, was found to proceed rapidly initially (<48 hours), but then became quite slow. The sinterability and the superconducting properties (magnetic flux exclusion) of the powder also degraded rapidly at first while subsequent degradation was relatively slow, suggesting that surface reaction products are responsible for property degradation. The lower sinterability was the result of two contributions: (1) lower green density due to increasing agglomeration and (2) lower surface reactivity with regard to sintering. Suggestions are presented for minimizing the presence of moisture by processing the superconducting oxide in an appropriate non-aqueous solvent.

New oxide superconductors

Abstract Crystals of the new high temperature superconductor Bi 2 Sr 3−x Ca x Cu 2 O 8+y have been grown and studied. Single crystal x-ray diffraction data were used to determine a pseudo tetragonal structure based on an A-centered orthorhombic cell with a = 5.399A,b= 5.414A,and c= 30.904A. The structure contains the copper-oxygen sheets as in La 2 CuO 4 and YBa 2 Cu 3 O 7 , but the copper-oxygen chains present in YBa 2 Cu 3 O 7 do not occur in Bi 2 Sr 3−x Ca x Cu 2 O 8+y . Electrical measurements on our single crystals show a resistivity drop at about 116 K and apparent zero resistivity at 91 K. We find superconductivity at 120 K in the Tl/Ca/Ba/Cu/O system. Also, we have for the first time prepared phases of the type La 2−x Na x CuO 4 . These materials are superconducting with T c s as high as 40 K, and they have some unusual magnetic behavior starting at about 10 K.