Pushing the Extra Cold Frontiers of Superconducting Science

Measuring the properties of superconducting materials in magnetic fields at tempeartures close to absolute zeros is considered difficult but necessary by many researchers, helping them understand quantum properties.

How cold? Lower than 0.05K, or -272°C, according to physicists at the US Department of Energy’s Ames Laboratory who specialize in developing instrumentation that measures such things. “For many modern (quantum) materials, to properly study the fine details of their quantum mechanical behavior you need to be cool. Cooler than was formerly thought possible,” says Ruslan Prozorov, an Ames physicist.

Prozorov and his research team have developed a method to measure the magnetic properties of superconducting and magnetic materials that exhibit unusual quantum behavior at very low temperatures in high magnetic fields. This method is being used to study quantum critical behavior, mechanisms of superconductivity, magnetic frustration and phase transitions in materials, many of which were first fabricated at Ames.

The team did so by placing a tunnel diode resonator—an instrument that makes precise radio-frequency measurements of magnetic properties—in a dilution refrigerator, a cryogenic device that is able to cool samples down to milli-Kelvin temperature range. While this had been achieved before, previous researchers did not have the ability to apply large static magnetic fields, a crucial step for studying quantum materials.

Prozorov’s group worked to overcome the technical difficulties of maintaining high-resolution magnetic measurements, while at the same time achieving ultra-cold temperatures down to 0.05 K and in magnetic fields up to 14 tesla. A similar circuit has already been used in a very high magnetic field (60 T) when the team performed the experiments at Los Alamos National Laboratory.

“When we first installed the dilution refrigerator, the joke was that my lab had the coldest temperatures in Iowa,” says Prozorov, who conducts his research where Midwestern winters are no laughing matter. “But we were not doing this just for fun, to see how cold we could go. Many unusual quantum properties of materials can only be uncovered at these extremely low temperatures.”

The group studied pairing symmetry in several unconventional superconductors, mapped a very complex phase diagram in a system with field–induced quantum critical behavior, and recently uncovered very unusual properties of a spin-ice system, “none of which would be possible without this setup,” says Prozorov.