Abstract
Based on the two-dimensional lattice fermion model, we discuss transitions between different pairing states. Each phase is labeled by an integer which is a topological number and characterized by vortices of the Bloch wave function. The transitions between phases with different integers obey a selection rule. Even without a magnetic field, edge states necessarily exist in the superconductor if the topological number is nonzero. They reflect the topological character of the bulk. Transitions driven by randomness are also discussed numerically.
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