Superconducting Vortex with Antiferromagnetic Core

We show that a superconducting vortex in underdoped high Tc superconductors could have an antiferromagnetic core. This type of vortex configuration arises as a topological solution in the recently constructed SO(5) nonlinear σ model and in Landau-Ginzburg theory with competing antiferromagnetic and superconducting order parameters. Experimental detection of this type of vortex by muon spin resonance and neutron scattering is proposed.