57 research outputs found
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Effect of pressure on the Fermi surface of Nb3Sb
The pressure dependences of three cross sections of the Fermi surface of Nb3Sb are determined from de Haas van Alphen measurements in solid He to several kbar. A large negative derivative is observed for the smallest cross section of the hole ellipsoid at M. © 1981 The American Physical Society
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Superconducting La2CuO4+x prepared by oxygenation at high pressure: A Raman-scattering study.
Superconducting crystals of La2CuO4+x prepared by high-pressure oxygenation have been analyzed by Raman spectroscopy. A direct comparison of the role of excess oxygen was made by examining the same crystals with and without excess oxygen. La2CuO4+x, like nonsuperconducting La2CuO4.0, is found to have a soft phonon associated with a tetragonal-to-orthorhombic phase transition. The Ag phonons of La2CuO4.0 and La2CuO4+x occur at essentially the same frequency. At room temperature, La2CuO4+x has no well-defined peak from two-magnon scattering, unlike La2CuO4.0. However, in its phase-separated form, La2CuO4+x exhibits well-defined, two-magnon scattering. This establishes that the La2CuO4.0 phase present in La2CuO4+x at low temperatures is antiferromagnetic. La2CuO4+x samples prepared by slightly different methods are found to have differing amounts of excess oxygen, as indicated by variations in the intensity of the phonon and magnetic scattering. Certain samples of La2CuO4+x had a phonon peak at 630 cm-1 that is absent in La2CuO4.0. While a definitive assignment is not possible, the frequency of this peak is consistent with a peroxidelike species in La2CuO4+x. © 1991 The American Physical Society
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Superconducting La2CuO4+x prepared by oxygenation at high pressure: A Raman-scattering study.
Superconducting crystals of La2CuO4+x prepared by high-pressure oxygenation have been analyzed by Raman spectroscopy. A direct comparison of the role of excess oxygen was made by examining the same crystals with and without excess oxygen. La2CuO4+x, like nonsuperconducting La2CuO4.0, is found to have a soft phonon associated with a tetragonal-to-orthorhombic phase transition. The Ag phonons of La2CuO4.0 and La2CuO4+x occur at essentially the same frequency. At room temperature, La2CuO4+x has no well-defined peak from two-magnon scattering, unlike La2CuO4.0. However, in its phase-separated form, La2CuO4+x exhibits well-defined, two-magnon scattering. This establishes that the La2CuO4.0 phase present in La2CuO4+x at low temperatures is antiferromagnetic. La2CuO4+x samples prepared by slightly different methods are found to have differing amounts of excess oxygen, as indicated by variations in the intensity of the phonon and magnetic scattering. Certain samples of La2CuO4+x had a phonon peak at 630 cm-1 that is absent in La2CuO4.0. While a definitive assignment is not possible, the frequency of this peak is consistent with a peroxidelike species in La2CuO4+x. © 1991 The American Physical Society
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Heat capacity (150-300 K) and anisotropic magnetic susceptibility (5-300 K) of single-crystal La2CuO4+x
Heat capacity measurements from 150 to 300 K were carried out on a single crystal of La2CuO4+x synthesized by subjecting an La2CuO4 crystal to 3 kbar oxygen pressure at 575 °C. The data reveal three small (about 1%) anomalies at temperatures (T) of 206, 222 and 259 K. The first two are tentatively attributed to CuO inclusions in the crystal. The third is observed on warming, but not on cooling, and is attributed to the previously documented first-order transition from the orthorhombically distorted K2NiF4 structure to a low T mixture of nearly stoichiometric La2CuO4 and oxygen-rich superconducting La2CuO4+y (y>x). The size of the anomaly at 259 K is about one-seventh of that observed previously for a single crystal of La2CuO4 at the second-order tetragonal-to-orthorhombic phase transition temperature of about 530 K. Magnetization measurements from 5 to 300 K and from 50 G to 50 kG are also reported for the La2CuO4+x crystal. The normal state magnetic susceptibility χ(T) is quite anisotropic, with χ(T) for H perpendicular to the CuO2 layers (χc) in good agreement with previous data on a different, but similarly prepared, crystal. The anisotropy in χ is nearly independent of T from 40 to 300 K and the magnitude of χc-χab per CuO2 layer is very similar to that at high T in YBa2Cu3O6.1, La2CuO4, Sr2CuO2Cl2 and La2-xMxCuO4 (MSr, Ba). © 1992
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Phase separation in a high-pressure-oxygenated La2CuO4+ delta crystal: Evidence from anisotropic electronic transport and magnetic susceptibility.
We report magnetic susceptibility, resistivity, and thermoelectric power measurements performed on a high-pressure-oxygenated La2CuO4+ single crystal. Distinct anomalies are present in all three quantities between 200 and 280 K that are attributed to a phase separation involving oxygen diffusion. Hysteresis present in the data near the phase-separation temperature indicates that the process is weakly first order. We suggest that this transition may be driven, in part, by interactions between excess oxygen ions and copper spin magnetism. © 1990 The American Physical Society
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Phase separation in a high-pressure-oxygenated La2CuO4+ delta crystal: Evidence from anisotropic electronic transport and magnetic susceptibility.
We report magnetic susceptibility, resistivity, and thermoelectric power measurements performed on a high-pressure-oxygenated La2CuO4+ single crystal. Distinct anomalies are present in all three quantities between 200 and 280 K that are attributed to a phase separation involving oxygen diffusion. Hysteresis present in the data near the phase-separation temperature indicates that the process is weakly first order. We suggest that this transition may be driven, in part, by interactions between excess oxygen ions and copper spin magnetism. © 1990 The American Physical Society
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Heat capacity (150-300 K) and anisotropic magnetic susceptibility (5-300 K) of single-crystal La2CuO4+x
Heat capacity measurements from 150 to 300 K were carried out on a single crystal of La2CuO4+x synthesized by subjecting an La2CuO4 crystal to 3 kbar oxygen pressure at 575 °C. The data reveal three small (about 1%) anomalies at temperatures (T) of 206, 222 and 259 K. The first two are tentatively attributed to CuO inclusions in the crystal. The third is observed on warming, but not on cooling, and is attributed to the previously documented first-order transition from the orthorhombically distorted K2NiF4 structure to a low T mixture of nearly stoichiometric La2CuO4 and oxygen-rich superconducting La2CuO4+y (y>x). The size of the anomaly at 259 K is about one-seventh of that observed previously for a single crystal of La2CuO4 at the second-order tetragonal-to-orthorhombic phase transition temperature of about 530 K. Magnetization measurements from 5 to 300 K and from 50 G to 50 kG are also reported for the La2CuO4+x crystal. The normal state magnetic susceptibility χ(T) is quite anisotropic, with χ(T) for H perpendicular to the CuO2 layers (χc) in good agreement with previous data on a different, but similarly prepared, crystal. The anisotropy in χ is nearly independent of T from 40 to 300 K and the magnitude of χc-χab per CuO2 layer is very similar to that at high T in YBa2Cu3O6.1, La2CuO4, Sr2CuO2Cl2 and La2-xMxCuO4 (MSr, Ba). © 1992
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Bulk superconductivity above 30 K in T-phase compounds.
High-pressure (3 kbar) oxygenation of (La, Sm, Sr)2CuO4 compounds having the T* structure leads to bulk superconducting transitions above 30 K and the appearance of a diamagnetic response at temperatures as high as 1/437 K. With increasing oxygenation pressure, the electrical resistivity and thermoelectric power progress from hoppinglike to more nearly metalliclike transport. The similarity in Tc and transport character with those of T-phase La2-xSrxCuO4, despite significant structural differences, further constrains possible mechanisms for superconductivity in the cuprates. © 1989 The American Physical Society
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Interstitial doping and oxygen exchange in superconducting La2CuO4+δ
The oxygen doping of lanthanum cuprate to generate superconductiving La2CuO4+δ (0<δ≦0.032) has been studied by high-pressure, isotopic-oxygen enrichment and thermal desorption mass spectroscopy (TDMS). Isotopic data show that the additional oxygen incorporated under high pressure readily exchanges with ionic lattice oxygen during enrichment at 860 K. The thermal release of the excess oxygen from superconducting crystals above ∼ 350 K is not bulk diffusion limited. An alternate explanation for the observed rapid O2(g) bursts is proposed. © 1991
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The crystal structure of superconducting La2CuO4.032 by neutron diffraction
The crystal structure of superconducting (37.5 K) La CuO has been refined from single-crystal neutron-diffraction data at room temperature and 15 K. At both temperatures it exhibits the orthorhombic symmetry Cmca. The extra oxygen O4 atoms occupy the special positions ( 1 4y 1 4 with y=0.243 at 15 K). They are located between two successive LaO layers and are surrounded by distorted cubes built up of two interpenetrated tetrahedra, one comprising four La atoms and the other four O1 atoms (the apical oxygen atoms of the CuO octahedra). The O4 insertion causes a 0.75 Å displacement of 4.8% of the O1 atoms towards new O3 positions (x=0.030(5), y=0.182(2), z=0.100(5)). From the refined values of the occupancy factors at 15 K, it is deduced that for each extra O4 three O1 are displaced to O3 with one short O3-O4 distance of 1.64(3) Å. This value indicates the formation of a strong O-O covalent bond of peroxide type with a formal 2-valence. Since the La and Cu sublattices have been found to be fully occupied and the doping does not change the oxygen charge, the La, Cu and O sublattices have the formal valences 6+, 2+, and 8-, respectively. However, the increase in La coordination and the consequent La-O distance readjustment indicate, when compared to the undoped compound structure, that a charge transfer occurs in La CuO , with the excess positive charge going either to the La or to the O sublattice. In the latter case it would correspond to the formation of holes in the O 2p band. © 1989. 2 4.032 6 2 4.03
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