Cosmic strings (or vortex strings) are topological defects arising when U(1) symmetry is spontaneously broken in quantum field theories. In three-dimensional relativistic systems, loops of cosmic strings are known to be unstable because of their trivial topological charges. In this talk, I will show that a new stable solution of classical EOMs exists in the form of a knot (or link) consisting of two kinds of cosmic string loops. The key mechanism is the Chern-Simons coupling between the "axion" and gauge field, which gives electric charges to the knot.
