Linear and nonlinear ac response in the superconducting mixed state

The response of the vortex system in the mixed state of type-II superconductors to ac fields is discussed from a unique macroscopic point of view. Linear and nonlinear response are derived as two opposite limits of the same diffusionlike equation for magnetic flux. ac susceptibility experiments carried out in the limit of strong nonlinearity are entirely equivalent to magnetic-relaxation or field-ramp experiments. The magnitude and frequency dependence of the nonlinearity threshold, or amplitude of the ac field where one crosses from essentially linear to strongly nonlinear response, are found and discussed for both the vortex-liquid and vortex-glass states.