Local electronic structure of a single magnetic impurity in superconductingNaxCoO2∙yH2O

Abstract

The recent discovery of superconductivity in layered ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}∙y{\mathrm{H}}_{2}\mathrm{O}$ has attracted much attention because this ${\mathrm{CoO}}_{2}$-based system shares many similarities with the high-Tc cuprate superconductors and studies of this system may shed light on the superconducting mechanism of the latter. In this work, we study the electronic structure around a magnetic impurity induced, for example, by Ni substitution of the Co in ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}∙y{\mathrm{H}}_{2}\mathrm{O}$. Four possible nodeless pairing symmetries ($s$, ${p}_{1}+i{p}_{2}$, ${d}_{1}+i{d}_{2}$, and $f$ wave) are considered. The local density of states (LDOS) and the subgap impurity states are studied. The sixfold anisotropic symmetry is found for the distribution function of the impurity states. This anisotropic feature disappears when doped away from the Van Hove singularity. For the three non-$s$-wave pairing states, except for the subgap deltalike peaks, the LDOS is characterized by dip/dip/gap/dip (or dip/gap/dip/dip) and dip/gap/dip features when the doping level is near and away from the Van Hove singularity, respectively. The dips' relative positions in the LDOS (or peak's relative position in the DOS), and other related properties are found to be quite distinct for different pairing states, suggesting that these properties can be taken as a probe to determine the intrinsic pairing symmetry by the scanning tunneling microscope experiments on the system.