Search for magnetic field induced gap in a high-Tc superconductor

Break junctions made of the optimally doped high-temperature superconductor Bi2Sr2Ca2CuO8 with Tc of 90 K has been investigated in magnetic fields up to 12 T, at temperatures from 4.2 K to Tc. The junction resistance varied between 1 and 300 kΩ. The differential conductance at low biases did not exhibit a significant magnetic field dependence, indicating that a magnetic field induced gap (Science, 277 (1997) 83), if exists, must be smaller than 0.25 meV.