Energy-pressure relation for low-dimensional gases

TL;DR

This paper investigates the energy-pressure relation in low-dimensional gases, quantifying deviations from scale invariance through the internal energy shift, with detailed analysis of 1D and 2D models including Lieb-Liniger and anyonic gases.

Contribution

It introduces the internal energy shift as a measure of deviation from scale invariance and analyzes its behavior in low-dimensional quantum gases, providing explicit results for specific models.

Findings

Internal energy shift is positive and vanishes at zero and infinite coupling.

In 1D, the energy shift peaks at finite temperature and saturates at high temperature.

In 2D, soft-core boundary conditions induce a non-zero internal energy shift.

Abstract

A particularly simple relation of proportionality between internal energy and pressure holds for scale invariant thermodynamic systems, including classical and quantum Bose and Fermi ideal gases. One can quantify the deviation from such a relation by introducing the internal energy shift as the difference between the internal energy of the system and the corresponding value for scale invariant gases. We discuss general thermodynamic properties associated to the scale invariance, provide criteria for which the internal energy shift density is a bounded function of temperature. We then study the internal energy shift and deviations from the energy-pressure proportionality in low dimensional models of gases interpolating between the ideal Bose and the ideal Fermi gases, focusing on the Lieb-Liniger model in 1d and on the anyonic gas in 2d. In 1d the internal energy shift is determined from the thermodynamic Bethe ansatz integral equations and an explicit relation for it is given at high temperature. Our results show that the internal energy shift is positive, it vanishes in the two limits of zero and infinite coupling (respectively the ideal Bose and the Tonks Girardeau gas) and it has a maximum at a finite, temperature depending, value of the coupling. Remarkably, at fixed coupling the energy shift density saturates to a finite value for infinite temperature. In 2d we consider systems of Abelian anyons and non Abelian Chern-Simons particles: as it can be seen also directly from a study of the virial coefficients, in the usually considered hard-core limit the internal energy shift vanishes and the energy is just proportional to the pressure, with the proportionality constant being simply the area of the system. Soft-core boundary conditions at coincident points for the two-body wavefunction introduce a length scale, and induce a non-vanishing internal energy shift.