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In a recent featured review written for Physics Today, Anne de Visser (UvA-IoP) explains how some very special materials can conduct electricity without any resistance – and behave even better when a magnetic field is turned on.

Magnet
This 33 T Bitter magnet at the High Field Magnet Laboratory in Nijmegen, the Netherlands, is used to measure superconductors in very strong magnetic fields. (Image courtesy of Dick van Aalst, Radboud University, The Netherlands.)

Sometimes, materials are able to conduct electricity without any resistance whatsoever: superconductivity. Superconductors could in principle form a solution to many of the world’s energy problems – but unfortunately, most superconducting materials only exhibit their special properties under very special circumstances. The temperature at which these materials operate is far below room temperature, or the applied pressure must be enormous. Moreover, the presence of external influences, such as a magnetic field, often also destroys the superconductivity effect.

Recently, however, a small family of materials has been discovered that bucks this trend: rather than suppressed, superconductivity is actually promoted by the magnetic field. The resistance-less conduction of electricity may even survive in the highest magnetic fields that can be produced in the laboratory.

This unrivalled property, dubbed ‘magnetic field-boosted superconductivity’, is found in superconducting ferromagnets: materials that combine the existence of superconductivity with magnetic properties also present in the magnets that we encounter in everyday life. In his article, de Visser – who discovered one of these very special superconducting materials – explains how the fact that some superconductors improve in a magnetic field calls for an unorthodox explanation in terms of the building blocks that carry the electric current.

Magnetic field-boosted superconductivity, Anne de Visser, Physics Today 73, 11, 44 (2020).