Some materials conduct electricity without any resistance when cooled to very low temperatures. This phenomenon, known as superconductivity, is closely linked to other important material properties. However, as new work by physicist Aline Ramires from the Institute of Solid State Physics at TU Wien now shows: in certain materials, superconductivity does not generate exotic magnetic properties, as was widely assumed. Instead, it merely makes an unusual form of magnetism experimentally observable – so-called altermagnetism.
Back to the future: time-reversal symmetry
How can one tell whether a film is running forwards or backwards? Many physical phenomena make no distinction between forward and reversed time. Magnetism, however, does: if a particle is deflected to the right in a magnetic field, then running time backwards would make it appear to be deflected to the left. In such cases, physicists speak of broken time reversal symmetry. This symmetry breaking can also occur in a subtle, quantum-mechanical way, for example in the quantum states of particles in certain materials. Almost always, it is a clear indication that magnetic effects are at work.
This was how a series of experimental results in different materials had been interpreted in the past: exactly below the temperature at which superconductivity sets in, certain exotic phenomena suddenly became visible. The conclusion seemed obvious – the superconductivity itself must be exotic, producing magnetism and breaking time reversal symmetry precisely at the moment it emerges.
"Signs of broken time reversal symmetry were observed, for example, in the material strontium ruthenate (Sr₂RuO₄), but also in certain layered materials," says Aline Ramires. "At first, everything seemed to make sense if one assumed a special, chiral form of superconductivity capable of generating magnetic effects. But the more experimental results accumulated, the more puzzling the picture became."
New findings strongly suggest that superconducting strontium ruthenate should not, in fact, produce such magnetic effects at all. Other experiments even detected magnetic signatures above the critical temperature – that is, in a regime where superconductivity does not yet exist. "It was clear that something did not add up, but no one could explain these strange contradictions," says Ramires.
Altermagnetism – an exotic form of magnetism
Ramires now shows that these puzzles can be traced back to an unusual type of magnetism that has only been identified in recent years and is known as altermagnetism. In conventional ferromagnetism, all participating electrons align their magnetic moments (their spins) in the same direction.
In antiferromagnetism, the opposite happens: neighboring spins point in opposite directions and cancel each other out on large length scales. "In altermagnetism, neighboring spins also point in opposite directions, but the spatial arrangement of one spin species is not exactly equivalent to that of the other," explains Ramires. "As a result, altermagnetic materials behave in fundamentally different ways."
Symmetry is the key
In certain materials, altermagnetism can exist both above and below the superconducting transition temperature and break time-reversal symmetry. However, the characteristic signatures by which condensed-matter physicists normally detect such symmetry breaking can remain hidden if the material possesses specific internal symmetries.
"If the atoms in a material are arranged in a particular symmetric way, certain effects are not visible – for example the Kerr effect, a change in the optical properties of a material that is regarded as a typical signature of broken time-reversal symmetry," says Ramires. "When superconductivity sets in, some of these spatial symmetries can be broken – and this makes the previously hidden effects measurable."
The conclusion is clear: superconductivity is not responsible for creating magnetism in these materials. The materials are magnetic all along – they are altermagnets. But in some cases, the internal symmetry of a material must be broken before the consequences of this altermagnetism can be observed. Where researchers had long suspected a magnetic 'threshold', the magnetism itself does not change at all – only its observable effects suddenly become visible.
