A tiny sandwich of oxides, grown layer by layer, becomes perfectly conducting at about minus 200 degrees Celsius. This is not a magic trick but a new chapter in the search for high-temperature superconductors. The material with the formula La₂PrNi₂O₇ on a neodymium aluminate substrate is a member of the Ruddlesden-Popper nickelates, structural relatives of the famous cuprates where superconductivity was discovered almost forty years ago.
The main intrigue has always been hidden in the 'pseudogap'—a mysterious state where electrons already sense future pairing but do not yet flow without resistance. It is like fog over a river: you can see there is water, but you cannot discern its depth or current. In this new nickelate heterostructure 'sandwich', there was no such fog. Angle-resolved photoemission spectroscopy, a method that kicks electrons out with light and records their energy and direction, revealed a clear picture: upon cooling below the critical temperature, a superconducting gap (an energy gap that prevents electrons from scattering) emerges, and sharp peaks grow at its edges — a sign of electron pair formation.
The electronic structure turned out to be multi-band: the Fermi surface (the boundary separating occupied and unoccupied electronic states) consists of three types of pockets — alpha, beta, and gamma. Interestingly, the gamma pocket was present in all films, regardless of whether they became superconducting or not. So the secret lies not in it alone, but in the subtle interplay of all three 'pockets' and possibly in how the crystal lattice is strained due to lattice mismatch with the substrate.
Thus, the layered nickel-oxygen sandwich revealed more about itself than expected. It showed that high-temperature superconductivity can exist without the pseudogap 'fog' and gave researchers a clean thermodynamic signal. Now, by varying the composition and strain of the films, one can deliberately search for conditions under which electron pairs run without resistance at even higher temperatures.
The heat capacity jump at the superconducting transition is like when water freezes into ice: its ability to absorb heat changes abruptly, signaling a new order. In nickelates, such a jump was seen directly for the first time, without interference.
In Peter Watts's novel 'Blindsight', alien life forms use room-temperature superconductivity for their consciousness — research on nickelates without a pseudogap brings us closer to understanding how this exotic quantum matter works.