Vortex State in Na_xCoO_2.yH_2O: p_x\pm ip_y-wave versus d_{x^2-y^2}\pm id_{xy}-wave Pairing

Based on an effective Hamiltonian specified in the triangular lattice with possible $p_x\pm ip_y$- or $d_{x^2-y^2}\pm id_{xy}$-wave pairing, which has close relevance to the newly discovered Na$_{0.35}$CoO$_2$$\cdot y$H$_2$O, the electronic structure of the vortex state is studied by solving the Bogoliubov-de Gennes equations. It is found that $p_x\pm ip_y$-wave is favored for the electron doping as the hopping integral $t<0$. The lowest-lying vortex bound states are found to have respectively zero and positive energies for $p_x\pm ip_y$- and $d_{x^2-y^2}\pm id_{xy}$-wave superconductors, whose vortex structures exhibit the intriguing six-fold symmetry. In the presence of strong on-site repulsion, the antiferromagnetic and ferromagnetic orders are induced around the vortex cores for the former and the latter, respectively, both of which cause the splitting of the LDOS peaks due to the lifting of spin degeneracy. STM and NMR measurements are able to probe the new features of vortex states uncovered in this work.Comment: 4 pages, 4 figures, The slightly shorter version was submitted to PR

Nuclear Spin Relaxation Rate of Disordered px+ipyp_x+ip_y-wave Superconductors

Based on an effective Hamiltonian with the binary alloy disorder model defined in the triangular lattice, the impurity scattering effects on the density of states and especially on the spin-lattice relaxation rate $1/T_1$ of $p_x+ip_y$-wave superconductors are studied by solving numerically the Bogoliubov-de Gennes equations. In the clean limit, the coherence peak of $1/T_1$ is observed as expected. More intriguingly, for strong scattering potential, the temperature dependence of $1/T_1$ exhibits the two different power law behaviors near $T_{\text{c}}$ and at low temperatures, respectively, which is in good agreement with the nuclear quadrupolar resonance measurement.Comment: 4 pages, 3 figure