Notes on heat engines and negative temperatures

TL;DR

This paper demonstrates that a Carnot cycle between positive and negative temperature baths can achieve 100% efficiency, challenging the idea that negative temperatures are equivalent to absolute temperatures, using a spin in a magnetic field as an example.

Contribution

It reveals that negative canonical temperatures cannot be equated with absolute temperatures by analyzing Carnot cycles involving negative temperature baths.

Findings

Carnot cycle efficiency can reach unity with negative temperature baths.

Negative temperatures are not equivalent to absolute temperatures.

Spin systems can illustrate negative temperature thermodynamics.

Abstract

We show that a Carnot cycle operating between a positive canonical-temperature bath and a negative canonical-temperature bath has efficiency equal to unity. It follows that a negative canonical-temperature cannot be identified with an absolute temperature. We illustrate this with a spin in a varying magnetic field.