Donald E. Morris

Conversion of 124 into 123+CuO and 124, 123 and 247 phase regions in the Y- Ba-Cu-O system

Abstract The phase diagram of the Y-Ba-Cu-O system is given as a function of temperature and partial pressure of oxygen over the range between 500°C and 1000°C and 10-6

Crystal structure of DyBa2Cu4O8: A new 77 K bulk superconductor

Single‐crystal x‐ray diffraction data from DyBa2Cu4O8 are used to confirm the structure of RBa2Cu4O8 (R=Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, and Tm), all of which have been synthesized in bulk and display substantial Meissner diamagnetism. The structure is orthorhombic, space group Ammm, with a=3.846 3(3), b=3.872 6(3), and c=27.237(2) A. Copper‐oxygen layers parallel to (001) are dimpled as in the 1‐2‐3 structure. Unique copper‐oxygen strips parallel to (100) have copper in distorted square‐planar coordination. Magnitudes of oxygen anisotropic thermal vibrations are significantly less than in the 1‐2‐3 structure; thus the oxygen content of 1‐2‐4 is expected to be significantly more stable.

Ca substituted tetragonal 123 superconductor with Tc above 85 K synthesized in 1–15 atm oxygen overpressure

Abstract We report studies on Ca substitution in the Y-Ba-Cu-O system at elevated oxygen pressure. We find that elevated P (O 2 ) increases the solubility range of Ca, and that Ca stabilizes the 123 phase over 247 or 124 in which the oxygen content is less variable. In the 123 phase [Ca x Y( 1− x )Ba 2 Cu 3 O y ], the orthorhombic distortion decreases systematically as Ca increases , and a , b and c /3 become equal at x ≈0.25, as measured by X-ray diffraction. Then the structure remains tetragonal (CaT-123) throughout the extended range of Ca solubility beyond x ≈0.25. The synthesis of CaT-123 has been achieved in oxygen overpressure as small as 1 bar. Doping by Ca is compensated by a corresponding reduction in oxygen content, and T c remains consistently high in CaT-123 (80–86 K), comparable to orthorhombic 123, and in striking contrast to the well-known oxygen deficient tetragonal 123. This work establishes that the controlling factor in determining T c in this class of superconductors is the optimum oxidation (or formal average copper valency), and not any specific oxygen content, ordering into chains, or orthorhombic distortion.

A stable fullerene charge-transfer complex in the solid state: HMTTEF–C60

The first stable complex of C60 with an organic electron donor has been synthesised; the complex, having C60 and HMTTEF (hexamethylenetetratellurafulvalene) in a 1 : 1 ratio, is black, insulating, weakly magnetic, insoluble in most solvents and stable in air.

Low background‐rate detector for 40‐keV ions using a conversion dynode and a microchannel‐plate electron multiplier to reject low‐energy ions, electrons, and photons

We have developed and tested an inexpensive ion detector with dark‐count rates below 10−4 s−1 and excellent background rejection. Each 40‐keV ion that strikes a grazing‐incidence Al2O3 conversion dynode ejects a few tens of secondary electrons, which enter separate pores of a microchannel plate and are independently multiplied to saturated pulse height. The pulses are summed to produce an output signal that is proportional to the number of secondary electrons. This permits pulse‐height discrimination against backgrounds of low‐energy ions, electrons, and photons. We have successfully tested the detector with C+, Na+, K+, Rb+, and Cs+ at 5–40 keV, and with 36‐keV C− and CH−. It should detect ions and neutrals of all species, at energies above 5–10 keV, with good efficiency and excellent background discrimination. Counting efficiency and background discrimination improve with higher ion energy. The detector can be operated at pressures up to at least 2×10−7 Torr and be exposed to air repeatedly without requ...

Cause of Tc depression in Sr substituted YBaSrCu3O7−y examined by isotope studies

Abstract We have measured oxygen isotope shifts in YBa 2− x Sr x Cu 3 O 7− y . With increasing Sr substitution, T c decreases monotonically, while the oxygen isotope exponent α 0 remains very small. This indicates that the hole concentration in the CuO 2 planes is still optimal, since it is known that high temperature superconductors with non-optimum hole concentration have reduced T c , but large isotope shifts. To confirm this, we carried out hole doping experiments on YBaSrCu 3 O 7− y using high pressure O 2 annealing, and also by Ca substitution for Y. In both cases, with the increased holes (overdoping), T c further decreased as expected. Thus, the significant decrease in T c with Sr substitution is not caused by reduced charge transfer into the CuO 2 sheets, but rather is due to decreased ion size, polarizability and/or changes in the phonon spectrum.

Lanthanum-substituted Y-Ba-Cu-O superconductors synthesized at elevated oxygen pressures

Lanthanum substitution in the Y‐Ba‐Cu‐O system at elevated P(O2) results in superconducting compositions which throw interesting light on the effects of nonisovalent substitution in a Y‐Ba‐Cu‐O system. Like Ca substitution, La substitution also shifts the phase formation boundary between 123, 247, and 124 phases to higher P(O2), and stabilizes a tetragonal 123 structure at moderately elevated P(O2). However, in contrast with Ca substitution, Tc decreases continuously and rapidly with increasing La content. This can be only partly explained by a decrease of the hole concentration.

Magnetic and resistive determination of the oxygen isotope effect in La1.85Sr0.15CuO4

Abstract In polycrystalline samples of the superconducting oxide La1.85Sr0.15CuO4, the substantial substitution of 18O for 16O causes a small downward shift in the superconducting transition temperature Tc. Although Tc(onset) measured resistively differs by as much as two Kelvin from that measured magnetically, the two measurement methods yield comparable Tc shifts upon isotope substitution. Samples with different superconducting phases (as evidenced by resistive fine structure in the transition region) show similar isotope shifts for different phases. We determine an overall oxygen isotope shift in La1.85Sr0.15CuO4 corresponding to α = 0.140±.008, where Tc∼M−α and M is the oxygen mass.

High‐pressure oxygen‐induced bulk superconductivity in 1222 structure Tl‐Pb‐Sr‐Eu(Ce)‐Cu‐O

A primarily single phase compound of composition (Tl0.5Pb0.5)Sr2(Eu2−xCex)Cu2O9 (where x ranges from ≪0.1 to 0.17) has been synthesized in the so‐called 1222 structure. Bulk superconductivity near 40 K is induced via oxygen doping in 100 bar of oxygen at 550 °C. The material has been structurally characterized by diffraction and high‐resolution lattice imaging techniques. The lattice images reveal the presence of some intergrowths of a 1212 structure nonsuperconducting (Tl0.5Pb0.5)Sr2EuCu2O7 phase in the bulk 1222 structure of the parent compound and indications of periodicity that suggest the possible existence of a new, composite structure 2434 phase.

A 40 keV cyclotron for radioisotope dating

We have built and begun testing a small low energy negative ion cyclotron for direct detection of 14C. At present, the cyclotron is operated in a high resolution mode at the 31st harmonic, with 1–2 kV on the dees. The high harmonic and a minimum number of turns of approximately 100, should give a fwhm mass resolution of about 130000 — sufficient to suppress the background from molecular ions such as 13CH−. Background such as scattered ions of 12C− and 13C− should be totally suppressed by the cyclotron acceleration process. (At the 88″ cyclotron at LBL we found that ions only 1% off-resonance are suppressed by more than a factor of 1017.) A miniature Cs sputter source located at the center of the cyclotron is expected to provide more than 1 μA of negative carbon ions. Negative ions are used in order to eliminate the interference from 14N. Unlike high energy cyclotrons, focussing is obtained solely from the axial components of the accelerating electric field. The magnetic field is kept flat to within 1 part in 104 in order to maintain exact isochronism throughout the several thousand accelerating rf cycles. The low final energy of 40 keV eliminates any danger from radiation or need for shielding, and the final orbit radius of only 10.5 cm, reduce the size and cost of the machine to that of conventional mass spectrometers.