Metal-to-insulator crossover and pseudogap in single-layerBi2+xSr2−xCu1+yO6+δsingle crystals in high magnetic fields

Abstract

The in-plane ${\ensuremath{\rho}}_{ab}(H)$ and the out-of-plane ${\ensuremath{\rho}}_{c}(H)$ magneto-transport in magnetic fields up to $28\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ has been investigated in a series of high quality, single crystal, hole-doped La-free Bi2201 cuprates for a wide doping range and over a wide range of temperatures down to $40\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. With decreasing hole concentration going from the overdoped $(p=0.2)$ to the underdoped $(p=0.12)$ regimes, a crossover from a metallic to an insulating behavior of ${\ensuremath{\rho}}_{ab}(T)$ is observed in the low temperature normal state, resulting in a disorder induced metal insulator transition. In the zero temperature limit, the normal state ratio ${\ensuremath{\rho}}_{c}(H)∕{\ensuremath{\rho}}_{ab}(H)$ of the heavily underdoped samples in pure Bi2201 shows an anisotropic 3-D behavior, in striking contrast with that observed in La-doped Bi2201 and LSCO systems. Our data strongly support that the negative out-of-plane magnetoresistance is largely governed by interlayer conduction of quasiparticles in the superconducting state, accompanied by a small contribution of normal state transport associated with the field dependent pseudogap. Both in the optimal and overdoped regimes, the semiconducting behavior of ${\ensuremath{\rho}}_{c}(H)$ persists even for magnetic fields above the pseudogap closing field ${H}_{\mathit{pg}}$. The method suggested by [Shibauchi et al. Phys. Rev. Lett. 86, 5763 (2001)] for evaluating ${H}_{\mathit{pg}}$ is unsuccessful for both under- and overdoped Bi2201 samples. Our findings suggest that the normal state pseudogap is not always a precursor of superconductivity.