Spiral vortex in anisotropic superconductors

In uniaxial superconductors with mass ratio ε=m M < ε_c, m and M being the effective masses perpendicular and parallel to the symmetry axis respectively and ε{lunate}_c being a critical value less than 1 2, the usual straight vortex parallel to the symmetry axis is unstable even when an external field is applied parallel to the uniaxial axis. We propose that the stable vortex in this case is a spiral one propagating along the symmetry axis. The amplitude and pitch of the vortex depend on the anisotropic mass ration. The spiral vortex is very flexible for small ε. Unusual properties of the vortices observed in cuprate oxide superconductors may be related to the unusual nature of the spiral vortices.