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" A ⁸⁹Y NMR study of substitution for copper in YBa₂Cu₃O₇₋δ and YBa₂Cu4₄O₈ "
Gençten, Azmi
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Latin Dissertation
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828377
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TLets332555
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| Main Entry
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Gençten, Azmi
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| Title & Author
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A ⁸⁹Y NMR study of substitution for copper in YBa₂Cu₃O₇₋δ and YBa₂Cu4₄O₈\ Gençten, Azmi
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| College
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University of Warwick
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| Date
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1992
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1992
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Thesis (Ph.D.)
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| Abstract
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89Y MAS NMR has been used to study the effects of substitution for Cu in YBa2(Cul_xMx)3O7 (M=V,Fe,Co,Ni,Zn,Al and Ga) and YBa2(Cu1,_yMy)4O8 (M=Zn and Fe). Substituted YBa2(Cu1_xMx)3O7 and YBa2(Cu1-yMy)4O8 samples have been characterized by means of XRD and A.C. susceptibility measurements. For YBa2(Cu1_xMx)3O7, a different behaviour is observed for the substitutions (Fe,Co,Al and Ga) which go into the chain site and change the structure to tetragonal, compared to the other substitutions (Ni and Zn). For chain site substitutions the room temperature shift increases linearly with concentration and the relaxation rate decreases whilst the shifts become more positive with decreasing temperature. For these chain site substitutions, changes in 89Y chemical shift have provided evidence for the existence of a true O-T transition. For these substitutions the shift and relaxation appear to obey a Korringa relation and the reduction in Tc is correlated with the change in shift indicating that suppression of Tc is due to the changes in N(EF). A similar temperature dependence of shift for chain site substituted samples with similar Tcs suggests that the Tc is related to the spin susceptibility in the Cu02 planes. Plane site substitutions have only a small effect on the shift, whilst increasing the relaxation rate at room temperature. The increased relaxation rate indicates that some form of magnetic scattering is produced by these substitutions and this combined with the small changes in shift and with the correlation between the reduction in Tc and the change in relaxation rate indicates that magnetic effects rather than hole doping are the cause of the reduction in Tc. In addition, MAS NMR data for 51V and 27A1 in V and A1 substituted YBa2Cu3O7 are presented. Both the 89Y behaviour and 51V data suggest that vanadium substitutes on the chain sites even though the structure remains orthorhombic. In the case of Fe and Zn substitution for Cu in YBa2(Cu1_yMy)4O8 a rapid decrease in Tc has been obtained. For Fe, as the dopant concentration increases, a nearly Korringa like behaviour has been observed, at room temperature, with an increase in shift and a decrease in relaxation rate. Zn substitution caused only a very small change in shift but an increase in relaxation rate. As was found for the plane site substituted YBa2(Cu1_xMx)3O7, this rapid increase in relaxation rate is again an indication for the existence of fluctuating magnetic fields. The correlation between T1-1 and Tc with Zn substitution suggests that magnetic effects are responsible for the rapid decrease in Tc. 89Y NMR data for YBa2Cu307 doped with Zn and Fe combined with that for YBa2Cu307 doped with Zn and Fe have suggested that Zn occupies the Cu(2) site and Fe substitutes into both Cu(l) and Cu(2) sites.
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QC Physics
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University of Warwick
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