LINEAR AND NONLINEAR AC RESPONSE IN THE SUPERCONDUCTING MIXED-STATE

The response of the vortex system in the mixed state of type-II superc onductors to ac fields is discussed from a unique macroscopic point of view. Linear and nonlinear response are derived as two opposite limit s of the same diffusionlike equation for magnetic flux. ac susceptibil ity experiments carried out in the limit of strong nonlinearity are en tirely equivalent to magnetic-relaxation or field-ramp experiments. Th e magnitude and frequency dependence of the nonlinearity threshold, or amplitude of the ac field where one crosses from essentially linear t o strongly nonlinear response, are found and discussed for both the vo rtex-liquid and vortex-glass states.