Quantum Monte Carlo study of pressure melting in Yukawa systems

The phase diagram of the Yukawa system at finite temperatures is explored by quantum Monte Carlo simulations. The path-integral method uses the primitive approximation for the density matrix of distinguishable particles; exchange effects are not included in our simulations. Previous zero-temperature calculations have found that both ground-state Yukawa fermions and bosons have two melting transitions. As the density increases, the stable phase changes from the low-density fluid phase to the solid phase and then to a high-density fluid phase. The melting transition at high density is referred to as pressure melting. In our work, the solid-fluid transition is estimated from the stability of the initial lattice during a Monte Carlo run. Our quantum simulations have successfully demonstrated pressure melting at finite temperatures in systems with de Boer parameters ${\Lambda }^{\mathrm{*}}$=0.004 and 0.005. This results from the soft-core interaction in the quantum systems and is not seen in our classical Yukawa simulations. No solid is found at ${\Lambda }^{\mathrm{*}}$=0.01. Since previous studies show that there is no ground-state Yukawa solid when the system is sufficiently quantum mechanical, our work suggests that there is no quantum Yukawa solid with ${\Lambda }^{\mathrm{*}}$=0.01 at any temperature and any density.