L. W. Rupp

Structural anomalies, oxygen ordering and superconductivity in oxygen deficient Ba2YCu3Ox

Abstract We report the characterization of series of oxygen deficient Ba2YCu3Ox samples for 7 ≥ x ≥ 6 prepared by Zr gettered annealing at 440°C. Measurements include complete crystal structure analysis at 5 K by powder neutron diffraction, electron microscopy study of the oxygen ordering, and magnetic measurements of the superconducting transitions, with particular attention to the transition widths. The results show for the first time that the 90 K and 60 K plateaus in Tc as a function of oxygen stoichiometry are associated with plateaus in the effective valence of the plane coppers. We also correlate the disappearance of superconductivity for x

Superconductivity near 70 K in a new family of layered copper oxides

A new family of high-temperature superconductors is described, with the general formula Pb2Sr2ACu3O8+δ. Although they have the planes of CuO5 square pyramids characteristic of the other copper-oxide superconductors, the new compounds belong to a distinct structural series, with wide scope for elemental substitution. Their unusual electronic configuration also gives new insight into the role of charge distribution among the structural building blocks in controlling superconductivity.

Optical and electrical properties of Mn‐doped GaAs grown by molecular‐beam epitaxy

An isolated acceptor impurity is shown to be responsible for the ∼113‐meV deep luminescence band which is frequently seen in GaAs layers grown by molecular‐beam epitaxy. We establish that this center, which is also observed in melt‐grown and solution‐grown GaAs and which has often been associated with the presence of native defects, is due to Mn. The luminescence arises from donor‐acceptor and conduction‐band–acceptor recombination involving holes bound at the Mn acceptor. Electron paramagnetic resonance on Mn‐doped layers shows that Mn is incorporated as an isolated defect with cubic symmetry. This result, coupled with the suppression of Mn‐associated luminescence and electrical activation under Ga‐rich growth conditions, indicates that Mn is incorporated primarily as a substitutional acceptor on Ga sites. Hole concentrations scale with Mn source temperature in a manner consistent with arrival‐rate‐controlled incorporation from a Knudsen effusion source. Room‐temperature hole concentrations up to the 101...

A systematic study of an isomorphous series of organic solid state conductors based on tetrathiafulvalene

The solid state physical properties of an isomorphous series of TTF salts (TTF11 (SCN)6, TTF11 (SeCN)6, and TTF7I5) were examined. While there was no noticeable trend in the conductivity (as a function of temperature and anion), the effective Fermi energy (?F) and the magnetic susceptibility transition temperature exhibited a definite trend as a function of anion. There is a correlation between the above properties and subtle variations in solid state structure of each of the above salts.

A straightforward synthetic route to the bulk form of the LnBa2Cu4O8 superconductors (Ln=Er, Ho) at one atmosphere oxygen pressure

The LnBa 2 Cu 4 O 8 superconductors are similar in crystal structure to LnBa 2 Cu 3 O 7 but contain double rather than single copper-oxygen chains. They have previously been synthesized in bulk form only at high oxygen pressures, and, recently, through the use of reaction rate enhancers. Here we describe for the first time the direct synthesis of ErBa 2 Cu 4 O 8 and HoBa 2 Cu 4 O 8 in bulk form at one atmosphere oxygen pressure from simple starting materials.

Giant magnetoresistance in pyrochlore Tl2−xInxMn2O7

Abstract We report the giant magnetoresistance (GMR) effect in the first non-perovskite compound Tl 2 Mn 2 O 7 forming in the pyrochlore structure. Our results, furthermore, show that the GMR is greatly enhanced in the miscibility gap region of the quasi-binary phase of Tl 2 Mn 2 O 7 In 2 Mn 2 O 7 . Our findings should invigorate the exploration of new GMR materials as well as possible utilization of phase mixing to enhance the GMR effect.

Structural and chemical analysis of ion beam produced conductive regions on highly resistive organic films

Thin films of both polymeric and nonpolymeric organic solids turn optically dense and electrically conductive on irradiation with high energy ion beams (e.g., 2 MeV Ar+). The structural and chemical properties of these films were investigated by ultraviolet (UV) visible, infrared (IR), Raman spectroscopic techniques, electron spin resonance (ESR), electron spectroscopy for chemical analysis (ESCA), and Rutherford backscattering (RBS) measurements. Specifically, in the case of 3,4,9,10‐perylenetetracarboxylic dianhydride (PTCDA) and nickel phthalocyanine (NiPc), and UV visible, IR, and Raman spectra show the loss of the initial molecular structure at low irradiation doses (1013–1014 cm−2) followed by the appearance, at high doses, of a spectrum similar to that observed for amorphous carbon. The Raman spectra indicate the absence of any long range graphitic microcrystalline structure and suggest that the films are nearly amorphous at higher doses. The RBS spectra indicate gradual loss of oxygen in PTCDA with increasing irradiation dose. There is negligible oxygen left in the film at high doses and a maximum loss of ∼35% (∼15%) of the carbon atoms in PTCDA (NiPc) is observed. The resistivity of the films decreases with increasing dose, reaching a minimum of ∼5×10−4 Ω cm at a dose of ∼1017 Ar+/cm2. Surprisingly, the resistivity of these films at high doses (∼1017 Ar+/cm2) is considerably lower than that of any amorphous phase of carbon. In the case of NiPc, such a low resistivity is obtained even though 60% of the N and 100% of the Ni originally contained in the films are retained. In situ measurements of the evolution rate of molecular fragments during the bombardment indicate a decrease with dose, suggestive of an irreversible modification of the material with ion bombardment.Thin films of both polymeric and nonpolymeric organic solids turn optically dense and electrically conductive on irradiation with high energy ion beams (e.g., 2 MeV Ar+). The structural and chemical properties of these films were investigated by ultraviolet (UV) visible, infrared (IR), Raman spectroscopic techniques, electron spin resonance (ESR), electron spectroscopy for chemical analysis (ESCA), and Rutherford backscattering (RBS) measurements. Specifically, in the case of 3,4,9,10‐perylenetetracarboxylic dianhydride (PTCDA) and nickel phthalocyanine (NiPc), and UV visible, IR, and Raman spectra show the loss of the initial molecular structure at low irradiation doses (1013–1014 cm−2) followed by the appearance, at high doses, of a spectrum similar to that observed for amorphous carbon. The Raman spectra indicate the absence of any long range graphitic microcrystalline structure and suggest that the films are nearly amorphous at higher doses. The RBS spectra indicate gradual loss of oxygen in PTCDA wit...

A comparison of electron-spin resonance spectra in some organic charge-transfer salts

Abstract Studies of multiline ESR spectra in ten organic charge-transfer salts give g-tensors which are characteristic of individual molecular ions containing sulphur, nitrogen and smaller atoms. Such nearly universal g-tensors are not found in three salts with molecules containing selenium. These conclusions are supported by some results at variable temperature and hydrostatic pressure.

Pb3Sr3Cu3O8+δCl: A new layered copper oxychloride

Abstract The crystal structure and basic properties of a new layered copper oxychloride, Pb 3 Sr 3 Cu 3 O 8+δ Cl, are described. The new structure type is related to that of Pb 2 Sr 2 RECu 3 O 8+δ , with the RE (rare-earth) layer replaced by an (Sr, Pb)-Cl-(Sr, Pb) layer. A wide range of oxygen stoichiometry is observed. The new compound has the planes of CuO 5 pyramids required for high- T c superconductivity, but we have been unable to introduce enough (positive or negative) charge to drive it superconducting.

Superconductivity in multiple phase Sr2Ln1−xCaxGaCu2O7 and characterization of La2−xSrxCaCu2O6+δ

Abstract We have observed the occurrance of superconductivity at temperatures between 40 and 50K for multiple phase samples of Sr 2 Ln 1−x Ca x GaCu 2 O 7 treated at 950–1000°C at 25 atmospheres oxygen pressure. We have not been able to find conditions at oxygen pressures of 25 atmospheres or below which make single phase samples superconducting. We also report the characterization of the properties of the simple double-layer 60K superconductor La 2−x Sr x CaCu 2 O 6±δ . The optimal properties occur at x = 0.35 and δ = 0.0.