Field-induced quasiparticle excitations in novel type II superconductors

We show that the field dependence of the magnetic penetration depth ($\lambda$), for which muon spin rotation ($\mu$SR) is an excellent microscopic probe, provides useful information on the degree of anisotropy for the superconducting order parameter. In type II superconductors associated with anisotropic order parameters, $\lambda$ 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 $\lambda$ against an external magnetic field. We review recent results on the field dependence of $\lambda$ obtained from the application of $\mu$SR to novel superconductors that exhibit unconventional characters associated with the anisotropic order parameter.