Transport properties of the underdoped

The effect of magnetic field and disorder on the transport properties of the underdoped $\chem{Y_{0.5}Pr_{0.5}Ba_2Cu_3O_{7-\delta}}$ films has been investigated in a broad temperature range in magnetic fields up to 50$\un{T}$. In ultrathin films with strong disorder, where the mean free path l corresponds to $k_\ab{F}l < 4$, a convincing power law resistivity vs. temperature $\rho_{ab}\sim T^{-2 \gamma}$ ($\gamma = 0.31$–0.39) is observed in a very broad temperature range $T \approx 2$–40$\un{K}$. However, in superconducting thicker films with much weaker disorder ($k_\ab{F}l > 14$ in normal state), the high magnetic field, by suppressing superconductivity, recovers the normal-state transport behavior with the $\rho_{ab}\sim\ln{T}$-dependence at low temperature in $\sim 50$ $\un{T}$ field. The two different transport behaviors indicate that the normal-state transport properties of the underdoped cuprates are very sensitive not only to the hole concentration but also to the degree of the disorder