Who Was There First? When Superconductivity Makes Hidden Magnetism Visible

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What if superconductivity is not the cause of magnetic effects at all, but brings their long-hidden magnetism to light? A publication shows how so-called old magnets become visible precisely because superconductivity is forced to break its symmetries.

In solid-state physics, superconductivity has been considered a prime example of a highly symmetrical quantum state for decades. Electrons form Cooper pairs, electrical resistances disappear and many disturbances that are relevant in the normal state lose their significance. However, it is precisely this "order" that makes superconductivity a sensitive detector for something that has often been overlooked: a new form of magnetism, known as "alter magnetism".

A recent paper by Aline Ramires in "Physical Review Research" shows that superconductivity is not just a passive state, but can make actively hidden magnetic orders visible. The article is entitled "From pure to mixed: Altermagnets as intrinsic symmetry-breaking indicators" and combines group theory, superconductivity and real experimental puzzles into a surprisingly coherent picture.

What Distinguishes Altermagnetism from Ferromagnetism and Antiferromagnetism

Classical ferromagnetism is easy to recognize. All spins point in the same direction, creating a macroscopic magnetic field. Antiferromagnetism is more subtle because neighboring spins compensate each other, but the magnetic order remains locally unique.

Age magnetism is in between and at the same time outside this scheme. In an ageing magnet there is no net magnetic field as in the ferromagnet and also no simple spin alternation as in the antiferromagnet. Nevertheless, the electronic energy bands are spin-split, as if a magnetic field were present. Magnetism is therefore "perceptible" for the electrons, but almost invisible for many measuring methods. It is precisely this property that makes alter magnetism difficult to grasp experimentally. And this is precisely where the idea of the paper comes in.

Superconductivity as a Symmetry Detector

The central idea of the publication is that if a material is age-magnetic, this hidden order forces the onset of superconductivity to break certain spatial or temporal symmetries. These symmetry breaks are measurable.

In other words: It is not superconductivity that creates new magnetic effects. Rather, it reveals that a magnetic order was already present. Superconductivity thus becomes a diagnostic tool for old magnetism.

The Mystery Surrounding Sr₂RuO₄

The material Sr₂RuO₄ has kept researchers busy for over twenty years. Various experiments have produced contradictory results. The so-called Kerr effect indicated a broken time reversal symmetry. μSR measurements in the volume did not show this effect. Scanning tunneling microscopy showed two non-equivalent surface sites that could not be explained theoretically.

The publication proposes an elegant solution. The surface of Sr₂RuO₄ can be age-magnetic, while the volume does not exhibit this order. If superconductivity occurs on this surface, it must reduce spatial symmetries due to the existing age-magnetic structure. These reductions also correspond to the experimentally observed effects. This means that surface and volume measurements can be consistently explained for the first time.

The Strange Effects in 4Hb-TaS₂

The behavior of the 4Hb-TaS₂ layer material appeared even more puzzling. A spontaneous vortex phase without an external magnetic field, a so-called magnetic memory effect and ferromagnetic-looking signatures were observed there. At the same time, no real ferromagnetism could be detected.

Here too, the age-magnetic perspective provides a conclusive explanation. The material is already in an age-magnetic state. When superconductivity sets in, this order forces the system to break its spatial symmetries. The resulting effects act like ferromagnetism, but are merely a consequence of the symmetry reduction and not the cause. The apparently ferromagnetic phenomena are therefore secondary effects of a hidden old magnet.

Time-Reversal Symmetry Breaking Reinterpreted

A particularly far-reaching conclusion of the publication concerns the so-called time-reversal symmetry breaking at the onset of superconductivity. Such observations have so far often been interpreted as an indication that superconductivity itself has an unusual, possibly magnetic character.

The new interpretation reverses this view. If the time reversal symmetry appears to be broken during the transition to the superconducting state, this may be an indication that the material was already age-magnetic beforehand. Superconductivity only makes this hidden order visible. Time reversal symmetry is used to analyze whether a physical process looks the same when time is run backwards.

The Role of Group Theory

The figures and tables in the publication show in detail how age-magnetic, ferromagnetic and mixed orders can be formally described using irreducible representations of the respective point groups. If the symmetry of a system is reduced by surfaces, stresses or lattice defects, "pure" alter-magnetic states can be transformed into mixed states containing ferromagnetic components. This formal structure provides the mathematical proof for the transitions observed in the experiment.

Why This Work Is So Important

The paper combines three previously separate areas: superconductivity, magnetism and symmetry analysis. Instead of trying to detect old magnetism directly, it is possible to investigate how superconductivity behaves in a material. If it shows unusual symmetry breaks, the cause may not lie in the superconductivity, but in a hidden age-magnetic order. This makes superconductivity a kind of probe for one of the currently most exciting classes of quantum materials. (mr)