Near field propulsion forces from nonreciprocal media

TL;DR

This paper demonstrates that nonreciprocal materials enable a heat engine that converts thermal radiation into lateral Casimir forces, with efficiency limited by thermodynamic principles and operating velocities depending on the regime.

Contribution

It establishes the necessity of nonreciprocal media for lateral Casimir forces and extends Onsager symmetry to these materials, analyzing the limits of engine efficiency and operational velocities.

Findings

Lateral Casimir forces require nonreciprocal materials.

Engine efficiency is limited to the Carnot limit by thermodynamics.

Operational velocities depend on the regime and material properties.

Abstract

Arguments based on symmetry and thermodynamics may suggest the existence of a ratchet-like lateral Casimir force between two plates at different temperatures and with broken inversion symmetry. We find that this is not sufficient, and at least one plate must be made of nonreciprocal material. This setup operates as a heat engine by transforming heat radiation into mechanical force. Although the ratio of the lateral force to heat transfer in the near field regime diverges inversely with the plates separation, $d$, an Onsager symmetry, which we extend to nonreciprocal plates, limits the engine efficiency to the Carnot value $\eta_c$. The optimal velocity of operation in the far field is of the order of $c\eta_c$, where $c$ is the speed of light. In the near field regime, this velocity can be reduced to the order of $\bar\omega d \eta_c$, where $\bar\omega$ is a typical material frequency.