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arXiv:2408.14222 · Annales Henri Poincaré

The Free Energy of Dilute Bose Gases at Low Temperatures Interacting Via Strong Potentials

Søren Fournais, Lukas Junge, Théotime Girardot, L. Morin, M. Olivieri, Arnaud Triay

Published Mar 9, 2026• 0 citations
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Abstract

Abstract We consider a dilute Bose gas in the thermodynamic limit and prove a lower bound on the free energy for low temperatures which is in agreement with the conjecture of Lee–Huang–Yang on the excitation spectrum of the system. Combining techniques of Fournais and Solovej (Invent Math 232(2):863–994, 2023) and Haberberger et al. (The free energy of dilute Bose gases at low temperatures, 2023), we give a simpler and shorter proof resolving the case of strong interactions, including the hard-core potential.

DOI
10.1007/s00023-026-01676-1

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Abstract We consider a dilute Bose gas in the thermodynamic limit and prove a lower bound on the free energy for low temperatures which is in agreement with the conjecture of Lee–Huang–Yang on the excitation spectrum of the system. Combining techniques of Fournais and Solovej (Invent Math 232(2):863–994, 2023) and Haberberger et al. (The free energy of dilute Bose gases at low temperatures, 2023), we give a simpler and shorter proof resolving the case of strong interactions, including the hard-core potential.
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We prove an upper bound on the free energy of a two-dimensional homogeneous Bose gas in the thermodynamic limit. We show that for a2ρ ≪ 1 and βρ ≳ 1, the free energy per unit volume differs from the one of the non-interacting system by at most 4πρ2|lna2ρ|−1(2−[1−βc/β]+2) to leading order, where a is the scattering length of the two-body interaction potential, ρ is the density, β is the inverse temperature, and βc is the inverse Berezinskii–Kosterlitz–Thouless critical temperature for superfluidity. In combination with the corresponding matching lower bound proved by Deuchert et al. [Forum Math. Sigma 8, e20 (2020)], this shows equality in the asymptotic expansion.