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Resistivity in Y Ba₂Cu₃O₆₃̣₃₃:

University of British Columbia

The resistivity of two high quality YBa₂Cu₃O₆.₃₃₃ single crystals was measured, the first via the Montgomery method, which allows the determination of all three components of the resistivity tensor, and the second via a colinear arrangement of contacts that allows the determination of pa. A comparison of ρ[sub a] between the two crystals allowed the intrinsic behaviour to be separated from the extrinsic behaviour. The intrinsic behaviour of the resistivity was found to be a quadratic temperature dependence, subject to two abrupt and doping independent changes in the fitting parameters, which occurred at 45 K and 206 K (these changes were most visible in the derivatives of the resistivity). A low temperature Ln(1/T) behaviour, reported elsewhere in the literature, was found to be sample dependent, and thus extrinsic. The intrinsic behaviour of the resistivity showed a very weak upturn at low temperatures, but the ground state appears to be essentially metallic. ρ[sub c] was fit to a model having two components added in series, one component being of the form of one over a quadratic expression. This inverse quadratic term contained almost all the doping dependence of the c-axis resistivity, with the remaining term (dominant at low temperatures) proving to be almost doping independent. The derivative of one over the inverse quadratic term showed a remarkable similarity to the derivatives of the intrinsic in-plane resistivities, implying that the inverse quadratic term is also intrinsic behaviour. Most notably, the derivative of the inverse quadratic showed discontinuous changes of slope at 45 K and 206 K again. However, a further two changes of slope, occurring at 172 K and 239 K, were seen as well. It is suggested that all four of these changes in behaviour affect the resistivity along all three principal axes, but that the ones at 172 K and 239 K are too subtle to be seen above the experimental noise in the in-plane data. These changes of behaviour cannot be due to the charge carriers alone or associated with the CuO₂ planes only. It is conjectured that they may be due to subtle structural changes in the unit cell that alter either the charge carrier-phonon coupling in the crystal, or the shape of the Fermi surface. A model due to Rojo and Levin was found to fit both the in-plane and out-of-plane resistivity data very well, provided that the low temperature (possibly extrinsic) contribution to the resistivity is neglected, and the temperature dependence of boson-assisted hopping along the c-axis is taken to be proportional to T². This model is consistent with the metallic ground state suggested by the in-plane data, and further suggests that the ρ[sub a] ~ 1/ρ[sub c] behaviour seen in the data is to be expected ρ[sub c] for underdoped YBa₂Cu₃O₆[sub +x].

Text

2009-12-11

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http://hdl.handle.net/2429/16402

Physics

University of British Columbia

UBCV

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