Spin-Triplet Pairing State of Sr2RuO4 in the c-Axis Magnetic Field

We investigate the spin-triplet superconducting state of Sr2RuO4 in the magnetic field along the c-axis on the basis of the four-component Ginzburg-Landau (GL) model with a weak spin-orbit coupling. We consider superconducting states described by the d-vector parallel to the ab-plane (d parallel to ab), and find that three spin-triplet pairing states are stabilized in the magnetic field-temperature (H-T) phase diagram. Although a helical state is stable at low magnetic fields, a chiral II state is stabilized at high magnetic fields. A non-unitary spin-triplet pairing state appears near the transition temperature owing to the coupling of magnetic field and chirality. We elucidate synergistic and/or competing roles of the magnetic field, chirality, and spin-orbit coupling. It is shown that a fractional vortex lattice is stabilized in the chiral II phase owing to the spin-orbit coupling.