Structure of hot dense matter at subnuclear densities is investigated by quantum molecular dynamics (QMD) simulations. We analyze nucleon distributions and nuclear shapes using two-point correlation functions and Minkowski functionals to determine the phase-separation line and to classify the phase of nuclear matter in terms of the nuclear structure. Obtained phase diagrams show that the density of the phase boundaries between the different nuclear structures decreases with increasing temperature due to the thermal expansion of nuclear matter region. The critical temperature for the phase separation is ≳6 MeV for the proton fraction x=0.5 and ≳5 MeV for x=0.3. Our result suggests the existence of "spongelike" phases with negative Euler characteristic in addition to the simple "pasta" phases in supernova cores until T ≲ 3 MeV.
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