Lowering of the Kinetic Energy in Interacting Quantum Systems

TL;DR

This paper demonstrates that at nonzero temperature, interactions in quantum systems can lower the kinetic energy below noninteracting levels, confirmed by simulations of electron gas and dense hydrogen, but not observed in liquid He-3.

Contribution

It reveals that interactions can reduce kinetic energy at finite temperature, contrasting with the ground state behavior, supported by theoretical expansion and numerical simulations.

Findings

Interactions lower kinetic energy at nonzero temperature

Simulations show increased low momentum states in electron gas and dense hydrogen

No such effect observed in liquid He-3

Abstract

Interactions never lower the ground state kinetic energy of a quantum system. However, at nonzero temperature, where the system occupies a thermal distribution of states, interactions can reduce the kinetic energy below the noninteracting value. This can be demonstrated from a first order weak coupling expansion. Simulations (both variational and restricted path integral Monte Carlo) of the electron gas model and dense hydrogen confirm this and show that in contrast to the ground state case, at nonzero temperature the population of low momentum states can be increased relative to the free Fermi distribution. This effect is not seen in simulations of liquid He-3.