Magnetic-field induced superconductor-insulator transition in the La_2−xSr_xCuO₄ system

The magnetoresistance of underdoped Magnetic-field induced superconductor-insulator transition in the La_2−xSr_xCuO films with x = 0.048 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.