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.${65}_{\mathrm{-}0.14}^{+0.05\mathrm{}}$ in zero field to ν=1.$0_{\mathrm{-}0.20}^{+0.10\mathrm{}}$ 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.