Critical macroscopic quantum effects in quantum nucleation

It is shown that a characteristic feature of quantum nucleation viewed as a decay process of metastable states is that the theoretical temperature dependence of the nucleation rate indicates a sharp first-order phase transition of the crossover from the quantum tunneling regime to the thermal hopping regime. We, furthermore, demontrate that the experimentally observed temperature dependence of the phase separation of supersaturated $\chem{{}^{3}He}$-$\chem{{}^{4}He}$ liquid mixtures at low temperatures can be understood by the first-order phase transition of quantum nucleation