Magnetotransport in the Normal State of La1.85Sr0.15Cu(1-y)Zn(y)O4 Films

We have studied the magnetotransport properties in the normal state for a series of La1.85Sr0.15Cu1-yZnyO4 films with values of y between 0 and 0.12. A variable degree of compressive or tensile strain results from the lattice mismatch between the substrate and the film, and affects the transport properties differently from the influence of the zinc impurities. In particular, the orbital magnetoresistance (OMR) varies with y but is strain-independent. The relations for the resistivity (ρ=ρ0+AT) and the Hall angle (cotΘH=αT2+C), and the proportionality between the OMR and tan2ΘH (Δρ/ρ=ζtan2ΘH) are followed above about 70 K. We have been able to separate the strain and impurity effects by rewriting the last two of these relations as cotΘH/α=T2+C/α and Δρ/ρ=(ζ/α2)(α2tan2ΘH), where each term is strain-independent and depends on y only. We also find that changes in the lattice constants give rise to approximately the same fractional changes in A, C, and α, while ρ0 is, in addition, increased by changes in the microstructure. The OMR is more strongly suppressed by the addition of impurities than tan2ΘH, so that ζ decreases as y increases. We conclude that the relaxation rate that governs the Hall effect is not the same as for the magnetoresistance. We also suggest a correspondence between the transport properties and the opening of the pseudogap at a temperature which changes when the La-Sr ratio changes, but does not change with the addition of the zinc impurities. Several theoretical models seem to be in conflict with our results. Some recent ones may be more compatible, but have not been carried sufficiently far for a detailed comparison.