Metal-to-insulator crossover and pseudogap in single-layer Bi2+xSr2-xCu1+yO6+δ single crystals in high magnetic fields -: art. no. 184524

The in-plane rho(ab)(H) and the out-of-plane rho(c)(H) magneto-transport in magnetic fields up to 28 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 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 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 rho(c)(H)/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 rho(c)(H) persists even for magnetic fields above the pseudogap closing field H-pg. The method suggested by [Shibauchi Phys. Rev. Lett. 86, 5763 (2001)] for evaluating H-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.