#cuprates

Leiden physicist Milan Allan and his group have discovered an apparent paradox within a material that has zero electrical resistance. They report trapped charges, although charges should, in theory, keep flowing in the absence of resistance. The discovery could provide a missing piece of one of the big puzzles in physics today—high-temperature superconductivity. The results are published in Nature Physics.

A material can either be insulating or conductive. In an insulator, an extra electron will get trapped. Thus, no electric current flows in insulators. In a conductor, extra electrons will immediately flow. The more conductive the material is, the faster the electrons will flow.

The research group of Leiden physicist Milan Allan was therefore surprised to discover charge trapping in a material with zero resistance. Charge trapping is supposed to be a telltale sign of an insulator. Together with Leiden theoretical physicist Jan Zaanen, Allan’s group found that the phenomenon could unravel a longstanding mystery about charge transport in a family of materials called cuprates. These poorly understood materials have no resistance, even at relatively high temperatures, and are therefore labeled high-temperature superconductors. The mechanism behind those is one of the big mysteries in physics today.

The results cap 15 years of detective work aimed at understanding how these materials transition into a superconducting state where they can conduct electricity with no loss.

Researchers were ecstatic 35 years ago when a novel and exciting new class of superconducting materials was found.

These copper oxides or cuprates, like other superconductors, conducted electricity without resistance or loss when chilled below a specific degree – but at substantially larger temperatures than scientists had expected. This increased the possibility of having them to work at temperatures near to room temperature for perfectly efficient power lines and other uses.

Research quickly confirmed that they demonstrated two additional classic characteristics of the transition to a superconducting state.  The material expelled magnetic fields as superconductivity occurred, allowing a magnet placed on a chunk of the material to hover above the surface. And during the transition, their heat capacity – the amount of heat required to raise their temperature by a certain amount – displayed a notable abnormality.