EFFECT OF A MAGNETIC-FIELD ON THE CRITICAL CONDUCTIVITY EXPONENT AT THE METAL-INSULATOR-TRANSITION

From measurements of the resistivity down to 70 mK in magnetic fields to 7.5 T of a series of just-metallic, uncompensated p-type Si:B samples, we have established that the critical exponent which characterizes the approach of the zero-temperature conductivity to the insulating phase changes from nu = 0.65(-0.14)+0.05 in zero field to nu = 1.0(-0.20)+0.10 in a magnetic field of 7.5 T. This is the first clear experimental evidence for a change in universality class at the metal-insulator transition caused by the application of a magnetic field.