Insulator-to-Metal Crossover in the Normal State of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>La</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo><mml:mi mathvariant="italic">x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>Sr</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="italic">x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>CuO</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>Near Optimum Doping

A 61-T pulsed magnetic field suppresses superconductivity in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ single crystals and reveals an insulator-to-metal (IM) crossover for both in-plane resistivity ${\ensuremath{\rho}}_{\mathrm{ab}}$ and $c$-axis resistivity ${\ensuremath{\rho}}_{c}$ at a Sr concentration near optimum doping ( $x\ensuremath{\simeq}0.16$). The IM transition is unusual in that all underdoped samples ( $x&lt;0.16$) show low-temperature insulating behavior, even in samples with linear- $T$ ${\ensuremath{\rho}}_{\mathrm{ab}}$ above ${T}_{c}$ and apparently large ${k}_{F}l$.

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