Discovery of Magnetic Field That Can Flip Heat Flow

Electricity can run between two superconductors even through electrically insulating barriers (yellow). Now researchers have found that a magnetic field (curved arrows) can switch the amount of heat that flows from a hot side (red) to a cold one (blue). Credit: Nature doi:10.1038/nature.2012.12090

Scientists discovered a magnetic field that can control the flow of heat from one body to another. It was first predicted 50 years ago, and its effect could someday lead to a new generation of electronic devices that use heat rather than charge to carry information.

The scientists published their findings in the journal Nature. Physicist Brian Josephson predicted the existence of a tunnel between superconductors separated by a thin layer of an insulator in 1962, a process that is forbidden in classical physics. The Josephson junction was built and used to make superconducting quantum interference devices (SQUIDs) that are sold as ultra-sensitive magnetometers.

In the most recent study, scientists measured the devices’ thermal behavior. They heated one end of a SQUID several micrometers long and monitored the temperature of an electrode connected to it. As researchers varied the magnetic field passing through the loop, the amount of heat flowing through the device changed. The effect was in line with a theory that was put forward previously.

The device worked by partly reversing heat transfer, so that some would flow from the colder body to the warmer one. This is a counter-intuitive process and a device with Josephson junctions imposes a quantum order upon it.

The violation of the second law of thermodynamics, stating that heat will always flow from a hotter body to a colder one, is possible because only part of the total heat flow is subjected to the phase variation. When the heat transferred by single electrons is taking into account, the net flow is still from the hot end to the cold end.

This variation in heat flow can be explained by the phase of the superconductors used. The greatest heat flow occurred when the peaks inside one half of the loop line up with the peaks in the other half. The flow was at its minimum when the peaks met the troughs. The magnetic field shifted those phases relative to each other, modifying the heat flow.

There are applications in solid-state refrigeration, and this research could help to realize tiny but highly efficient heat engines, which could form the basis of “coherent caloritronics”, in which information is carried by heat exchanges instead of electrical ones.

Reference: “The Josephson heat interferometer” by Francesco Giazotto and María José Martínez-Pérez, 19 December 2012, Nature.
DOI: 10.1038/nature11702