Quantum mystery that puzzled physicists for decades finally solved

Physicists from the California Institute of Technology and Yale University have learned to precisely calculate the Kondo effect — a phenomenon in which magnetic impurities cause the resistance of a metal to increase upon cooling. The new approach takes into account the real structure of the material rather than just its simplified model. The work has been published in the journal Science.

It would seem straightforward: the colder the metal, the better it conducts electricity. Thermal motion subsides, and electrons glide more freely through the crystal lattice. But as soon as a negligible fraction of magnetic atoms is added, the familiar logic breaks down. At a certain temperature, the resistance stops decreasing and unexpectedly begins to rise.

The mechanism behind this paradox has long been known to physicists: conduction electrons "cluster around" the magnetic impurity, screening its magnetic moment. However, predicting at exactly what temperature the effect will manifest in a specific material has until now been extremely difficult — doing so requires accounting for the coordinated behavior of a vast number of electrons.

The researchers found a solution by turning to quantum chemistry methods originally developed for describing molecules. Using these methods, they were able to reconstruct in detail the electronic structure of an impurity atom inside the metal and, on that basis, determine the characteristics of the resulting quantum state.

The approach was tested on seven transition metals embedded in a copper matrix. The result is impressive: for most of the studied impurities, the prediction accuracy turned out to be far superior to previous model-based calculations — in some cases, the difference reached approximately two orders of magnitude. The theoretical values were compared with experimental data, yielding a convincing match.

So far, the method has been tested on relatively simple systems. However, in the future it has the potential to pave the way for studying far more complex quantum materials, including high-temperature superconductors. The main takeaway of the work is the ability to derive the properties of a specific material directly from fundamental laws of physics, starting from its actual chemical composition.