A crystal lattice of strontium ruthenate reacts to various sound waves sent from resonant ultrasound spectroscopy while the material cools down. The highlighted area suggests that this material could be a new type of superconductor. (Image Credit: Cornell University)

 

Until now, only two types of superconducting materials have been identified: s-wave and d-wave. Researchers at Cornell University have recently discovered a new and possible third type of superconductor: g-wave. The team published their findings in Nature Physics on September 21, 2020.

Superconductors are a particular type of material that provides no electrical resistance while conducting, as we all know. This is caused by electrons in superconductors that move together in Cooper pairs, requiring energy to be broken up in order to generate resistance.

 

Bardeen–Cooper–Schrieffer pairs in s-wave superconductors, which contain lead, tin and mercury, are comprised of one electron pointing up and another pointing down. Both of these electrons move towards each other, generating no net angular momentum. Meanwhile, the Cooper pairs in d-wave superconductors have two quants of angular momentum.

 

For years, physicists have theorized that another third type of superconductor with one quanta of angular momentum could exist: p-wave. They also speculated that the electrons pair with parallel instead of antiparallel spins. This superconductor would be a breakthrough in quantum computing since it could be used to produce Majorana fermions, a particle that behaves as its own antiparticle.

 

One potential material for a p-wave superconductor was strontium ruthenate. Recent research has shed light on this idea. Using high-resolution resonant ultrasound spectroscopy, the researchers set out to conclude if strontium ruthenate is a p-wave superconductor. However, they discovered that a completely new type of superconductor, g-wave, exists in the material.  

 

“This experiment really shows the possibility of this new type of superconductor that we had never thought about before,” Brad Ramshaw, the Dick & Dale Reis Johnson Assistant Professor in the College of Arts and Sciences, said, “It really opens up the space of possibilities for what a superconductor can be and how it can manifest itself. If we’re ever going to get a handle on controlling superconductors and using them in technology with the kind of fine-tuned control we have with semiconductors, we really want to know how they work and what varieties and flavors they come in.”

 

While working on previous projects, the team used resonant ultrasound spectroscopy to observe the symmetry properties of the superconductivity in a crystal of strontium ruthenate. However, they came across a big problem while performing the experiment. “Cooling down resonant ultrasound to 1 kelvin (minus 457.87 degrees Fahrenheit) is difficult, and we had to build a completely new apparatus to achieve this,” Sayak Ghosh, doctoral student, said.

 

By using the new setup, the team was able to measure the response of the crystal’s elastic constants to varying sound waves while the material cooled via its superconducting conversion at 1.4 kelvin (-457 degrees Fahrenheit).

Using this data, the team determined that strontium ruthenate is a two-component superconductor. This means that the way electrons bind together is too complex to be described by one number, so it needs a direction too.

 

Nuclear magnetic resonance (NMR) spectroscopy was used in the past in order to figure out what type of wave material strontium ruthenate could be. In the process, the team determined that it was not a p-wave superconductor. Since they confirmed that it was a two-compound material, the team also discovered that strontium ruthenate wasn’t an s- or d-wave superconductor, either.

 

The team can use this approach to study other materials to find out if they could be p-wave candidates. However, they still need to do some work on strontium ruthenate.

 

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