Vortex origin of tricritical point in Ginzburg-Landau theory

Motivated by recent experimental progress in the critical regime of high-T_c superconductors, we show how the tricritical point in a superconductor can be derived from the Ginzburg-Landau theory as a consequence of vortex fluctuations. Our derivation explains why usual renormalization group arguments always produce a first-order transition, in contrast to experimental evidence and Monte Carlo simulations.