Effects of vortices and impurities on the nuclear spin relaxation rate in iron-based superconductors

Abstract

The effects of magnetic vortices and nonmagnetic impurities on the low-energy quasiparticle excitations and the spin-lattice relaxation rate are examined in the iron-based superconductors for the ${s}_{\ifmmode\pm\else\textpm\fi{}}$-, $s$-, and $d$-wave pairing symmetries. The main effect of the vortices is to enhance the quasiparticle excitations and the spin-lattice relaxation rate for all symmetries, which leads to a ${T}^{3}$ dependence of the relaxation rate followed by a nearly linear $T$ dependence at lower temperatures. This enhancement can only be seen for the ${s}_{\ifmmode\pm\else\textpm\fi{}}$- and $d$-wave symmetries in the presence of nonmagnetic impurities. These results suggest that the ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave and $d$-wave pairing states behave similarly in response to the magnetic field and nonmagnetic impurities, therefore, it may be impossible to distinguish them on the basis of the measurements of spin-lattice relaxation rates when a magnetic field and/or impurity scatterings are present.