O. Chmaissem

Effect of Re substitution on the defect structure, and superconducting properties of (Hg1−xRex)Ba2Can−1CunO2n+2+δ (n = 2, 3, 4)

We have investigated the structure, and superconducting properties of a series of Re substituted HgBa{sub 2}Ca{sub n-1}Cu{sub n}O{sub 2n+2+{delta}} (n = 2, 3, 4) samples by neutron powder diffraction, AC susceptibility, and magnetization measurements. Re atoms substitute at the Hg site and alter the defect structure of the host layer by pulling in four new oxygen atoms at (x x 0) to form an octahedron around Re. Re substitution makes it easier to synthesize the different members of the series; in particular (Hg,Re)-1234 which forms readily with Re concentrations x {ge} 0.2. {Tc} values of the as-prepared samples are 108, 132, and 122 K for the n = 2, 3 and 4 members, respectively. Magnetization measurements show a small enhancement of H{sub irr} with increasing Re-content at low temperatures. No significant enhancement at higher temperatures (T> 70 K) is observed.

Multiple defects in overdoped Tl2Ba2CuO6+δ: effects on structure and superconductivity

Abstract It is shown that the superconducting and structural properties of Tl 2 Ba 2 CuO 6+ δ depend on at least two defects. The Tl/Cu ratio used during synthesis determines the concentration of a defect that is likely to be Cu substitution on the Tl site, based on previously published work that proves the existence of such a defect. The structure exhibits a smaller orthorhombic strain, or tetragonal symmetry, for high concentrations of this defect. T c tends to be lowered by this defect. An oxygen interstitial defect in the Tlue5f8O double layer has a more pronounced effect on T c , allowing the compound to be overdoped as the concentration of this defect increases. Features in the diffraction data also suggest the existence of a third defect which may involve the filling of oxygen vacancies at an oxygen site in the structure during early stages of oxygenation.

Structure and superconductivity in Cr-substituted HgBa2CuO4+δ

Chromium-substituted Hg{sub 1-x}Ba{sub 2}CuO{sub 4+{delta}} compounds have been synthesized. Cr can be incorporated into the structure with a solubility limit around x {approx} 0.27. Cr substitutes at the Hg site and is tetrahedrally coordinated to four oxygen atoms. Two of these are apical oxygen atoms that have moved to new positions (O2) and two are in the Hg/Cr plane (O3) at a (x x 0) site. Additional oxygen atoms (O4) in the Hg/Cr plane at (1/2 1/2 0) affect the carrier concentration, as for HgBa{sub 2}CuO{sub 4+{delta}}. In the as-synthesized samples, {Tc} decreases with increasing Cr content. Attempts to adjust the carrier concentration by post annealing in oxygen at low temperature destroy superconductivity because the samples are metastable under ambient conditions.

Relaxation effects in the transition temperature of superconducting HgBa{sub 2}CuO{sub 4+{delta}}

In previous studies on a number of underdoped and overdoped high-temperature superconductors, including YBa{sub 2}Cu{sub 3}O{sub 7{minus}y} and Tl{sub 2}Ba{sub 2}CuO{sub 6+{delta}}, the transition temperature T{sub c} has been found to change with time in a manner which depends on the sample{close_quote}s detailed temperature and pressure history. This relaxation behavior in T{sub c} is believed to originate from rearrangements within the oxygen sublattice. In the present high-pressure studies on HgBa{sub 2}CuO{sub 4+{delta}} to 0.8 GPa we find clear evidence for weak relaxation effects in strongly underdoped and overdoped samples (T{sub c}{approx_equal}40{endash}50thinspK) with an activation energy E{sub A}(1thinspbar){approx_equal}0.8{endash}0.9thinspeV. For overdoped HgBa{sub 2}CuO{sub 4+{delta}} hE{sub A} increases under pressure more rapidly than previously observed for YBa{sub 2}Cu{sub 3}O{sub 6.41}, yielding an activation volume of +11{plus_minus}5thinspcm{sup 3}; the dependence of T{sub c} on pressure is markedly nonlinear, an anomalous result for high-T{sub c} superconductors in the present pressure range, giving evidence for a change in the electronic and/or structural properties near 0.4 GPa. {copyright} {ital 1999} {ital The American Physical Society}

Defects that control the properties of Tl- and Hg-based superconductors

Defects that affect T{sub c} in Tl{sub 2}Ba{sub 2}CuO{sub 6+{delta}} and HgBa{sub 2}CuO{sub 4+{delta}} and HgBa{sub 2}CuO{sub 4+{delta}} have been characterized by neutron powder diffraction. In Tl{sub 2}Ba{sub 2}CuO{sub 6+{delta}}, the dominant defect is interstitial oxygen between the two Tl-O planes, but Cu substitution on the Tl site also affects properties and there is evidence for a second oxygen defect for compositions in the vicinity of maximum {Tc}. In HgBa{sub 2}CuO{sub 4+{delta}}, there are two competing oxygen defects in the Hg layer. The relative concentrations of these defects switches upon passing from the underdoped region, through the maximum {Tc}, to the overdoped region. This remarkable behavior could result from a change in the topology of the Fermi surface upon passing through the van Hove singularity.

Evidence for two competing defects in HgBa2CuO4+δ

Abstract The defect structure of HgBa 2 CuO 4+δ has been investigated by neutron powder diffraction. By varying the P(O 2 )s over a wide range during annealing, the oxygen content can be modified to both underdope and overdope the compound. Our data give evidence for the presence of at least two defects that contribute to doping. The competition between the two defects results in a “plateau” in the region of maximum T c . This plateau can be explained by a change in the defect chemistry that results in a constant T c (at T c max ) for different amounts of total defect oxygen. This could be interpreted as being due to an electronic topological when the van Hove singularity is crossed.

Structure and a Bond-Valence-Sum Study of the 1-2-3 Superconductors (CaxLa1-x)(Ba1.75La0.25+x)Cu3Oy and YBa2Cu3Oy

We present a study of the crystal structure of the 1-2-3 superconductor [Ca(x)La(1-x)][Ba

The Structure and a Bond-Valence-Sum Study of the 1-2-3 Superconductors CLBLCO and YBCO

(1.75-x)La(0.25+x)]Cu(3)O(y) (``CLBLCO). Because of the presence of both La and Ba ions in the ``Ba layer, the local symmetry of YBa(2)Cu(3)O(y) (``YBCO) is lost. One can no longer assume that an ``apical oxygen atom always lies strictly on the line joining CuI and CuII ions, and a Pauling bond-valence calculation is very useful as an adjunct to the Rietveld refinement of the positions of the La and Ba ions. When a Rietveld analysis is performed allowing for the possibility that the apical oxygen atoms may be slightly displaced along the a or b direction, the bond valence sums for both La and Ba are close to the natural oxidation states of these ions. nWe have also used the bond-valence-sum method to study the mean oxidation states of the CuI and CuII ions in both CLBLCO and YBCO. Our results for YBCO differ from some previously published results. Our main conclusion is that, while the method may be useful in finding the charge concentration on the CuII plane, it definitely does not yield the concentration of mobile holes, nor the concentration of superconducting charge carriers.

Structural features that optimize high temperature superconductivity

We present a study of the crystal structure of the 1-2-3 superconductor [Ca(x)La(1-x)][Ba

Increase of the superconducting T{sub c}, irreversibility fields, and critical currents in tetragonal YBaSrCu{sub 3-x}Mo{sub x}O{sub 7+d}

(1.75-x)La(0.25+x)]Cu(3)O(y) (``CLBLCO). Because of the presence of both La and Ba ions in the ``Ba layer, the local symmetry of YBa(2)Cu(3)O(y) (``YBCO) is lost. One can no longer assume that an ``apical oxygen atom always lies strictly on the line joining CuI and CuII ions, and a Pauling bond-valence calculation is very useful as an adjunct to the Rietveld refinement of the positions of the La and Ba ions. When a Rietveld analysis is performed allowing for the possibility that the apical oxygen atoms may be slightly displaced along the a or b direction, the bond valence sums for both La and Ba are close to the natural oxidation states of these ions. nWe have also used the bond-valence-sum method to study the mean oxidation states of the CuI and CuII ions in both CLBLCO and YBCO. Our results for YBCO differ from some previously published results. Our main conclusion is that, while the method may be useful in finding the charge concentration on the CuII plane, it definitely does not yield the concentration of mobile holes, nor the concentration of superconducting charge carriers.