Heat capacity of periodically driven two-level systems

TL;DR

This paper introduces a definition of heat capacity for steady periodically driven systems, analyzing how it reveals kinetic information and behaves under various conditions, extending classical thermodynamic concepts to nonequilibrium settings.

Contribution

It provides a novel formulation of heat capacity for driven systems and explores its dependence on kinetic factors, frequency, and amplitude, extending equilibrium thermodynamics.

Findings

Heat capacity exhibits a Schottky peak as a function of temperature.

It depends on transition rates and kinetic barriers in nonequilibrium systems.

Vanishes at absolute zero, consistent with an extended Nernst postulate.

Abstract

We define the heat capacity for steady periodically driven systems and as an example we compute it for dissipative two-level systems where the energy gap is time-modulated. There, as a function of ambient temperature, the Schottky peak remains the dominant feature. Yet, in contrast with equilibrium, the quasistatic thermal response of a nonequilibrium system also reveals kinetic information present in the transition rates; e.g., the heat capacity depends on the time-symmetric reactivities and changes by the presence of a kinetic barrier. It still vanishes though at absolute zero, in accord with an extended Nernst heat postulate, but at a different rate from the equilibrium case. More generally, we discuss the dependence on driving frequency and amplitude.