  We report observation of the nonlinear Meissner effect (NLME) in Nb films by
measuring the resonance frequency of a planar superconducting cavity as a
function of the magnitude and the orientation of a parallel magnetic field. Use
of low power rf probing in films thinner than the London penetration depth,
significantly increases the field for the vortex penetration onset and enables
NLME detection under true equilibrium conditions. The data agree very well with
calculations based on the Usadel equations. We propose to use NLME angular
spectroscopy to probe unconventional pairing symmetries in superconductors.
