John Lester Wagner

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 TlO 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.

Fabrication and transport studies on PrBa2(Cu1−xMx)3O7: M=Ga, Zn, and Co

Abstract We have fabricated M-doped PrBa 2 Cu 3 O 7 (Pr123), i.e. PrBa 2 (Cu 1− x M x ) 3 O 7 for M=Zn, Ga, and Co, with x =0.05, 0.10, 0.15, and 0.20. Experimental data indicates single phase samples for the Ga and Co doping up to 20% level. Second phases appear in Zn-doped samples when the doping level reaches 0.15. At 77 K the electrical resistivity of these compounds is several orders of magnitude higher than that of undoped Pr123. We also found no orthorhombic to tetragonal phase transition in the doped samples with all samples remaining in orthorhombic. Their lattices parameters are very close to those of YBa 2 Cu 3 O 7− δ (Y123). For this reason these compounds may serve as improved buffer-layer materials for Y123 superconducting electronic circuits and devices.

Fabrication of and transport studies on PrBa2(Cu0.8T0.2)3O7

Abstract We have fabricated transition metal doped PrBa 2 Cu 3 O 7−δ (Pr123) and found that the electrical resistivity of these compounds is much higher than that of undoped Pr123. Their lattices parameters match very well of those of YBa 2 Cu 3 O 7−δ (Y123). These compounds may serve as improved buffer-layer materials for Y123 superconducting electronic circuits and devices.

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.

Universal Phase Diagrams and “Ideal” High Temperature Superconductors: HgBa2CuO4+δ

The structural and superconducting properties of HgBa2CuO4−δ have been investigated over an extended range of doping (0<δ<0.25). Two series of samples were measured in which oxygen content was varied using different annealing conditions. Both series were found to have non-parabolic doping dependence as well as unusual structural phenomena in the vicinity of maximum Tc. Samples of identical oxygen content were found to have Tc variations as large as 30 K. These differences in superconducting and structural properties are evidence of oxygen ordering present in HgBa2CuO4+δ.

TRANSPORT STUDIES ON NANOMETER THICK YBa2Cu3O7-δ MULTILAYERS USING Y0.4Pr0.6Cu3O7 AND PrBa2(Cu0.8Ga0.2)3O7 AS BUFFER LAYERS

We have fabricated PrBa2[Cu1-xGax]3O7 (PBGCO) ceramic samples for x = 0.05, 0.10, 0.15 and 0.20. The electrical resistivities of these samples are many orders of magnitude higher than that of PrBa2Cu3O7 (Pr123) while the other physical properties remain much the same. PBGCO therefore may be a possible buffer layer for YBa2Cu3O7-δ (Y123) superlattices/multilayers. For this reason YBa2Cu3O7-δ/PrBa2(Cu0.8Ga0.2)3O7 (Y123/PBCGO) and YBa2Cu3O7-δ/Y0.4Pr0.6Ba2Cu3O7 (Y123/YPBCO) superlattices were fabricated and their transport properties were studied. It is found that although PBGCO is better for insulating Y123 films in trilayers, inter-Y123 layer coupling exists between separated Y123 layer.

Structural and superconducting properties of HgBa2CuO4+δ over an extended doping range: nonparabolic doping behavior

The structural and superconducting properties of HgBa2CuO4+(delta ) have been investigated over a large doping range (0 < (delta) < 0.25). Two series of samples were measured in which oxygen contents were varied through different annealing conditions. Both series are found to have non-parabolic doping behavior as well as unusual structural phenomena in the underdoped region and at maximum Tc. Samples of identical oxygen content are found to have variations in Tc large as 30 K. These differences in superconducting and structural properties are evidence of oxygen/charge ordering present in HgBa2CuO4+(delta ). Results are discussed in terms of structural distortions present and possible charge ordering and the formation of filamentary condensates.