# Periodic thermodynamics of the Rabi model with circular polarization for arbitrary spin quantum numbers

## TL;DR

This paper develops a theoretical framework for the periodic thermodynamics of a driven spin system with arbitrary quantum number, revealing novel nonequilibrium magnetic behaviors and a quasitemperature concept distinct from the bath temperature.

## Contribution

It analytically derives the quasistationary Floquet-state distribution for arbitrarily strong driving and uncovers a formal analogy to thermal paramagnetism in a nonequilibrium setting.

## Key findings

- Quasistationary distribution is Boltzmannian with a quasitemperature.
- The quasitemperature is independent of the spin quantum number.
- Strong driving can induce a transition from paramagnetic to diamagnetic response.

## Abstract

We consider a spin $s$ subjected to both a static and an orthogonally applied oscillating, circularly polarized magnetic field while being coupled to a heat bath, and analytically determine the quasi\-stationary distribution of its Floquet-state occupation probabilities for arbitrarily strong driving. This distribution is shown to be Boltzmannian with a quasitemperature which is different from the temperature of the bath, and independent of the spin quantum number. We discover a remarkable formal analogy between the quasithermal magnetism of the nonequilibrium steady state of a driven ideal paramagnetic material, and the usual thermal paramagnetism. Nonetheless, the response of such a material to the combined fields is predicted to show several unexpected features, even allowing one to turn a paramagnet into a diamagnet under strong driving. Thus, we argue that experimental measurements of this response may provide key paradigms for the emerging field of periodic thermodynamics.