Magnetic-Field Induced Superconductor-Insulator Transition in the ${\mathrm{La}}_{2-\mathit{x}}{\mathrm{Sr}}_{\mathit{x}}{\mathrm{CuO}}_4$ System

The magnetoresistance of underdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{CuO}$ films with $x=0.048\mathrm{}$ and $0.051$ is studied in magnetic fields up to 8.5 T and at temperatures down to 30 mK. The results indicate that the magnetic-field induced superconductor-insulator (SI) transition is qualitatively different from the reentrant transition described for indium oxide and interpreted as leading to a “bosonic insulator” phase. The ground state in the absence of superconductivity appears to be insulating. The transitions become narrower below 1 K and show that the resistive critical field diverges approaching zero temperature, as has also been observed in other high- $T_c$ systems.