Field-induced quasiparticle excitations in novel type II superconductors

We show that the field dependence of the magnetic penetration depth (λ), for which muon spin rotation (μSR) 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, λ 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 λ against an external magnetic field. We review recent results on the field dependence of λ obtained from the application of μSR to novel superconductors that exhibit unconventional characters associated with the anisotropic order parameter. PACS numbers: 74.25.Ha, 74.70.-b, 76.75.+i Field-induced quasiparticle excitations in novel type II superconductors 2