This work reports the effect of oxygen doping on the anisotropic electrical resistivity and thermoelectric power of single crystal YBa<sub>2</sub>Cu<sub>3</sub>O<sub>x</sub> and nominally 6%Ca-doped YBa<sub>2</sub>Cu<sub>3</sub>O<sub>x</sub>. The a, b and c-axis thermopower of single crystal YBa<sub>2</sub>Cu<sub>3</sub>O<sub>x </sub>has been measured over a wide doping range. The overdoped a-axis thermopower of single crystal YBa<sub>2</sub>Cu<sub>3</sub>O<sub>x </sub>and nominally 6%Ca doped YBCO displays an unusual abrupt change in gradient at about 130K, which is independent of doping. This effect becomes more noticeable as the doping is increased. However it is not observed in any underdoped a-axis thermopower measurements. The a-axis thermopower an be scaled in terms of <i>T</i>*, in a similar way to polycrystalline YBCO. However the overdoped a-axis thermopower will not scale because of the abrupt gradient change and the negative value of the overdoped a-axis thermopower. At high dopings the b-axis single crystal thermopower of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>x </sub>shows markedly different behaviour to the a-axis thermopower, both in magnitude and temperature dependence. This is ascribed to a significant additional contribution from the chain conduction channel. However as the doping is progressively lowered the b-axis thermopower becomes more similar to the a-axis thermopower, reflecting the decreasing effect of the chain contribution on the b-axis thermopower. This can be observed in the in-plane thermopower anisotropy <i>S<sub>a</sub></i>(<i>T</i>)/<i>S<sub>b</sub></i>(<i>T</i>), which tends to unity as the doping decreases. The temperature dependence of the chain thermopower for several dopings has been determined from the measured in-plane thermopower and resistivity components, within the same model where the chains and planes conduct independently (ICPM). This temperature dependence is discussed within the usual Mott diffusion thermopower scenario. The c-axis thermopower has been measured for several dopings and is positive in magnitude and gradient, except at <i>T</i>*, where the gradient abruptly changes sign and the c-axis thermopower then becomes larger as the temperature is decreased.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:597825 |
| Date | January 2004 |
| Creators | Cole, Jody Richard |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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