Expectation Values of Conserved Charges in Integrable Quantum Field Theories out of Thermal Equilibrium

TL;DR

This paper computes expectation values of conserved charges in (1+1)-dimensional integrable quantum field theories out of equilibrium, extending known results to higher-spin charges and providing exact formulas, including for free fermions.

Contribution

It extends the calculation of conserved charge averages in non-equilibrium states to higher spins and offers exact analytic expressions, broadening the understanding of integrable quantum field theories.

Findings

Averages of conserved charges scale as T^{s+1} in high-temperature limits.

Exact analytic expression for the proportionality coefficient in free fermion models.

Generalization of Stefan-Boltzmann law to conserved charges in integrable QFTs.

Abstract

In this work we present a computation of the averages of conserved charge densities and currents of (1+1)-dimensional Integrable Quantum Field Theories in Generalised Gibbs Ensembles. Our approach is based on the quasi-particle description provided by the Thermodynamic Bethe Ansatz combined with the principles of Generalised Hydrodynamics, and we focus on Non-Equilibrium Steady State averages. When considering the ultraviolet (i.e. high temperature) limit of such averages, we recover the famous result by Bernard and Doyon (2012) for the energy current and density in Conformal Field Theories, and we extend it to conserved quantities with spin $s > 1$. We show that their averages are proportional to $T_L^{s+1}\pm T_R^{s+1}$, with $T_L$, $T_R$ the temperatures of two asymptotic thermal reservoirs. The same power law is obtained when considering some non-thermal generalised Gibbs states. In Conformal Field Theory, the power law is a consequence of the transformation properties of conserved charge operators, while the proportionality coefficient depends on the spin of the operator and on the central charge of the theory. We present an exact analytic expression for this coefficient in the case of a massive free fermion. At equilibrium, proportionality of spin-$s$ density averages to $T^{s+1}$ can be thought of as a generalisation of Stefan-Boltzmann's law, which states that the energy per unit surface area radiated by a black body scales as $T^4$.