An alloy of palladium and tellurium with a small amount of gold added behaves like a wire that loses its resistance when cooled. At ambient pressure, the three samples with different gold contents had superconducting transition temperatures of 2.7, 4.1, and 4.6 kelvin — colder than a freezer, roughly like deep space.
The scientists squeezed these samples in a special chamber, as if clamping them in a vise, and watched what changed. X-rays showed that the crystal lattice, the orderly pattern of atoms, does not break down even at a pressure of 8 gigapascals, which is like placing eight hundred elephants on every square centimeter. All three compositions compressed in the same way.
Heat capacity measurements revealed that superconductivity in these materials is type-II: magnetic field penetrates them in individual vortex filaments, like wisps of smoke in still air. For the composition with the least gold, the behavior resembles the classic BCS model, where electrons pair up under weak interaction. For the other two compositions, the coupling between electrons is stronger, and this changes the properties.
When pressure was raised to 2.5 gigapascals, the transition temperature for the compositions with 25% and 35% gold first rose slightly — to 4.2 and 4.7 kelvin, respectively, and then began to fall. For the composition with 15% gold, the temperature only dropped. At the same time, the overall shape of the temperature-magnetic field diagram barely changed, meaning the superconducting properties remained robust to compression.
It seems that adding gold changes not only the distance between atoms but also how readily electrons form pairs that carry current without loss. Comparison with pure PdTe2 helps reveal how chemical composition controls superconductivity.
A temperature of 4 kelvin is about minus 269 degrees Celsius, colder than the average temperature on Pluto.