Insulator-to-Metal Crossover in the Normal State ofLa2−xSrxCuO4Near Optimum Doping

A 61-T pulsed magnetic field suppresses superconductivity in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ single crystals and reveals an insulator-to-metal (IM) crossover for both in-plane resistivity ${\rho }_{\mathrm{ab}}$ and $c$-axis resistivity ${\rho }_c$ at a Sr concentration near optimum doping ( $x\simeq 0.16$). The IM transition is unusual in that all underdoped samples ( $x<\mathrm{}0.16\mathrm{}$) show low-temperature insulating behavior, even in samples with linear- $T$ ${\rho }_{\mathrm{ab}}$ above $T_c$ and apparently large $k_{F}l$.