| 1. |
- Ghorbani, Shaban Reza, et al.
(författare)
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Thermoelectric power and resistivity of Nd1-xCaxBa2Cu3Oy and Nd1-xLaxBa2Cu3Oy
- 2000
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Ingår i: Physica. C, Superconductivity. - 0921-4534 .- 1873-2143. ; 339:4, s. 245-252
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Tidskriftsartikel (refereegranskat)abstract
- The structural and transport properties of Nd1-xCaxBa2Cu3Oy (x = 0.0, 0.03, 0.06, and 0.10) and Nd1-xLax- Ba2Cu3Oy. (x nominally 0.0, 0.05, 0.10, and 0.15) have been studied by X-ray powder diffraction, electrical resistivity, and thermoelectric power measurements. In both series, the a- and b-axis lattice parameters decrease with increasing doping while the c-axis lattice parameter increases. The transport properties suggest improved metallic behaviour with increasing Ca doping in contrast to La doping. An anomalous sharp peak in the thermoelectric power close to T-c was observed, which depended on the doping content. We analysed the thermoelectric power as a function of temperature with a two-band model with an additional linear T term. An excellent agreement between model and data was obtained.
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| 2. |
- Ghorbani, Shaban Reza, et al.
(författare)
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Thermoelectric power and resistivity of Nd1-xPrxBa2Cu3O7-delta
- 2001
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Ingår i: Physica. C, Superconductivity. - 0921-4534 .- 1873-2143. ; 353:1-2, s. 77-84
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Tidskriftsartikel (refereegranskat)abstract
- The structural and transport properties of Nd1-xPrxBa2Cu3O7-delta (x = 0.0, 0.05, 0.10, and 0.15), have been studied by X-ray powder diffraction, electrical resistivity measurements, and thermoelectric power measurements. The decrease of the superconducting critical temperature with doping is described by a parabolic term and a linear term. The c-axis lattice parameter and the cell volume decrease with doping up to x = 0.10. Analyses of the thermoelectric power, critical temperature, c-axis lattice parameter, and the cell volume suggest that the Pr ions have a valence close to +4 at low doping concentration. The room temperature resistivity indicates that hole concentration decreases with increasing Pr doping. The room temperature thermoelectric power is positive for all samples and increases with increasing Pr doping due to a decrease in the carrier concentration. We analyzed the thermoelectric power as a function of temperature with a two-band model with an additional linear T term. An excellent agreement between model and data was obtained. The band width of the narrow band increases and its peak position moves away from the Fermi energy with increasing doping concentration.
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| 3. |
- Lundqvist, Pieter
(författare)
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Structure and physical properties of doped 123-superconductors and search for new superconducting hydrides
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The field of high temperature superconductors (HTSC) isstill controversial. The difficulty to explain the high Tcvalues as well as other anomalous superconductingand normal state properties of the HTSC’s necessitatesfurther studies and search for empirical relations in the largeamount of collected data.The major part of this thesis is devoted to the study ofcharge neutral doping with equal amounts of Ca and Pr/Th in theYBa2Cu3O7-δfamily of high temperature superconductors.The strong effect on Tcin spite of preserved charge concentration hasbeen investigated by neutron diffraction and calculations ofbond valence sums, µSR experiments and measurements of Tcand the upper critical field from electricalresistivity. In the first part of the thesis, some results fromthe literature have been examined with an attempt to summarizetrends and empirical relations. In addition some exploratorywork searching for new superconductors in the hydrides isreported.The valence of Pr in the Y-123 family is anothercontroversial issue. Results from literature are presentedsuggesting a valence change from Pr4+at low Pr content to mostly Pr3+at high Pr content. At low doping levels Cacompensates for the electron doping by Pr and isolates a stronglinear depression of Tc. The linear depression rate increases linearlywith increasing ion radius of the rare earth host whereas theparabolic Tcdependence on hole concentration is not affected.For Ca-Th doping the linear depression rates are even higherand suggested to be due to electronic disorder. For both chargeneutral dopings the structural changes with doping are smallwith an increasing planar copper to apical oxygen (Cu2-O4)distance as the most distinct one.High temperature superconductors, electrical conductivity,structural studies, neutron powder diffraction, X-ray powderdiffraction, muon spin rotation, YBa2Cu3O7-δ, substitution studies, hole concentration,charge neutral doping, disorder.
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