The HMF model for fermions and bosons

TL;DR

This paper extends the Hamiltonian Mean Field (HMF) model to quantum particles, analyzing how quantum effects like Pauli exclusion and Heisenberg uncertainty influence phase stability and transitions at zero temperature.

Contribution

It introduces a quantum HMF model for fermions and bosons, providing exact thermodynamic limits and analyzing phase transitions and stability in the quantum regime.

Findings

Homogeneous phase becomes stable in quantum regime for both fermions and bosons.

Fermions stabilize due to Pauli exclusion, bosons due to Heisenberg uncertainty.

First order transition for fermions, second order for bosons as Planck constant varies.

Abstract

We study the thermodynamics of quantum particles with long-range interactions at T=0. Specifically, we generalize the Hamiltonian Mean Field (HMF) model to the case of fermions and bosons. In the case of fermions, we consider the Thomas-Fermi approximation that becomes exact in a proper thermodynamic limit. The equilibrium configurations, described by the Fermi (or waterbag) distribution, are equivalent to polytropes with index n=1/2. In the case of bosons, we consider the Hartree approximation that becomes exact in a proper thermodynamic limit. The equilibrium configurations are solutions of the mean field Schr\"odinger equation with a cosine interaction. We show that the homogeneous phase, that is unstable in the classical regime, becomes stable in the quantum regime. This takes place through a first order phase transition for fermions and through a second order phase transition for bosons where the control parameter is the normalized Planck constant. In the case of fermions, the homogeneous phase is stabilized by the Pauli exclusion principle while for bosons the stabilization is due to the Heisenberg uncertainty principle. As a result, the thermodynamic limit is different for fermions and bosons. We point out analogies between the quantum HMF model and the concepts of fermion and boson stars in astrophysics. Finally, as a by-product of our analysis, we obtain new results concerning the Vlasov dynamical stability of the waterbag distribution.