Field-induced quasiparticle excitations in novel type II superconductors

We show that the field dependence of the magnetic penetration depth (lgr), for which muon spin rotation (mgrSR) is an excellent microscopic probe, provides useful information on the degree of anisotropy of the superconducting order parameter. In type II superconductors associated with anisotropic order parameters, lgr is sensitive to the quasiparticle excitation induced by the Doppler shift due to a supercurrent around magnetic vortices. The presence of such low energy excitations manifests itself in the non-zero slope of lgr against an external magnetic field. We review recent results on the field dependence of lgr obtained from the application of mgrSR to novel superconductors that exhibit unconventional characters associated with the anisotropic order parameter.