Temperature dependence of instantons in QCD

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We investigate the temperature dependence of the instanton contents of gluon fields, using unquenched lattice QCD and the cooling method. The instanton size parameter deduced from the correlation function decreases from 0.44fm below the phase-transition temperature Tc (≈ 150MeV) to 0.33fm at 1.3 Tc. The instanton charge distribution is Poissonian above Tc, but it deviates from the convoluted Poisson at low temperature. The topological susceptibility decreases rapidly below Tc, showing the apparent restoration of the U(1)ₐ symmetry already at T ≈ Tc.