Universal nonlinear conductivity close to an itinerant-electron quantum critical point

We study the conductivity in itinerant-electron systems near to a magnetic quantum critical point. We show that, for a class of geometries, the universal power-law dependence of resistivity upon temperature may be reflected in a universal nonlinear conductivity; when a strong electric field is applied, the resulting current has a universal power-law dependence upon the applied electric field. For a system with thermal-equilibrium current proportional to T-alpha and dynamical exponent z, we find a nonlinear resistivity proportional to E(z-1)/[z(1+alpha)-1].