Superconductivity in a Crystal That Changes Type express
A superconductor is a material that, when strongly cooled, loses its electrical resistance: current flows through it without losses. Scientists took a layered crystal of PdTe2, similar to a stack of sheets of paper, and cooled it to temperatures below 1.8 kelvin (that's about -271°C). Under these conditions, PdTe2 becomes superconducting.
Previously, it was believed that PdTe2 is a type-I superconductor: the magnetic field is completely expelled from it, like water from a sponge when squeezed. But in new experiments with mosaic crystals (composed of many slightly rotated pieces), something different was found. The magnetic field penetrates inside the material in the form of thin filaments — vortices. This is a hallmark of a type-II superconductor. The transition from type I to type II is likely caused by disorder in the crystal — small defects and boundaries between pieces.
Measurements of the magnetic field penetration depth showed that the superconducting gap (the energy required to break a pair of electrons) is the same in all directions. This is the so-called s-wave symmetry, like a perfectly round sphere, without protrusions. This type of superconductivity is often found in simple metals, but here it coexists with the unusual topology of the electronic states of PdTe2.
This discovery makes PdTe2 a convenient platform for studying how superconductivity, topology, and surface effects interact in a single material. Possibly, by varying disorder, it will be possible to create materials with switchable superconductivity for future quantum devices.
PdTe2 is a superconductor, but its critical temperature is only 1.8 K. This is colder than the temperature of the cosmic microwave background radiation (2.7 K), which is left over from the Big Bang.