  We show that thermal and magnetic contributions to the breakdown of
superconductivity in radiofrequency (RF) fields can be separated by applying
two RF modes simultaneously to a superconducting surface. We develop a simple
model that illustrates how mode-mixing RF data can be related to properties of
the superconductor. Within our model the data can be described by a single
parameter, which can be derived either from RF or thermometry data. Our RF and
thermometry data are in good agreement with the model. We propose to use
mode-mixing technique to decouple thermal and magnetic effects on RF breakdown
of superconductors.
