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 {ν}=0.65_-0.14+0.05 in zero field to {ν}=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.