New Type of Superconductor Identified by Scientists

So far, there have been two types of stories in the history of superconducting materials: S-wave and D-wave.

Now Cornell researchers – led by Brad Ramshaw, assistant professor to Dick & Dale Reis Johnson at the College of Arts and Sciences – have discovered a possible third type: the G-wave.

Her work “Thermodynamic Evidence for a Parameter of the Superconducting Two-Component Order in Sr.₂RuO_4th, ”Posted on September 21, 2020 in Natural physics. The lead author is PhD student Sayak Ghosh, MS ’19.

Electrons in superconductors move together in so-called Cooper pairs. This “pairing” gives superconductors their most famous property – no electrical resistance – because in order to generate resistance, the Cooper pairs have to be broken apart, which costs energy.

In S-wave superconductors – generally conventional materials such as lead, tin, and mercury – the Cooper pairs consist of an upward and a downward-pointing electron, both of which move head-on toward one another, with no net angular momentum. In the past few decades, a new class of exotic materials has shown what is known as D-wave superconductivity, with the Cooper pairs having two quanta of angular momentum.

Physicists have theorized the existence of a third type of superconductor between these two so-called “singlet” states: a p-wave superconductor with an angular momentum quantum and electrons that pair with parallel rather than antiparallel spins. This spin triplet superconductor would be a big breakthrough for Quantum computing because it can be used to create Majorana fermions, a unique particle that is its own antiparticle.

For more than 20 years, strontium ruthenate (Sr2RuO4) has been a leading candidate for a p-wave superconductor, although recent research has begun to poke holes in the idea.

Ramshaw and his team wanted to determine once and for all whether strontium ruthenate was a sought-after p-wave superconductor. Using high-resolution resonant ultrasound spectroscopy, they discovered that the material may be a completely new type of superconductor: the G-wave.

“This experiment really shows the possibility of this new type of superconductor that we had never thought of before,” said Ramshaw. “It really opens up the space of possibilities of what a superconductor can be and how it can manifest itself. If we ever get to grips with the control of superconductors and put them in the technology with the finely tuned control we have with semiconductors, we really want to know how they work and what types and flavors they have. ”

As in previous projects, Ramshaw and Ghosh used resonant ultrasound spectroscopy to investigate the symmetry properties of superconductivity in a crystal made of strontium ruthenate that was grown and precisely cut by employees of the Max Planck Institute for Chemical Solid State Physics in Germany.

However, unlike previous attempts, Ramshaw and Ghosh encountered a significant problem in attempting to conduct the experiment.

“Cooling of the resonant ultrasound to 1 Kelvin (minus 457.87 degrees) Fahrenheit) is difficult, and we had to build a whole new machine to do this, ”said Ghosh.

With their new setup, the Cornell team measured the reaction of the elastic constants of the crystal – essentially the speed of sound in the material – to a large number of sound waves, while the material cooled down at 1.4 Kelvin (minus 457 degrees Fahrenheit) due to its superconducting transition .

“This is by far the most precise ultrasonic resonant spectroscopy data ever recorded at these low temperatures,” said Ramshaw.

Based on the data, they determined that strontium ruthenate is what is known as a two-component superconductor, which means that the way electrons bind together is so complex that it cannot be described by a single number. it also takes a direction.

Previous studies had used nuclear magnetic resonance spectroscopy (NMR) to narrow the possibilities of the type of wave material, strontium ruthenate, and to effectively eliminate the p-wave as an option.

Finding that the material was two-component, Ramshaw’s team not only confirmed these results, but also showed that strontium ruthenate was also not a conventional S- or D-wave superconductor.

“Resonant ultrasound really lets you go in, and even if you can’t identify all of the microscopic details, you can make broad statements about which ones are excluded,” said Ramshaw. “The only things the experiments agree with are these very, very strange things that no one has seen before. One of them is the G-wave, which means angular momentum 4. Nobody ever thought that there would be a G-wave superconductor. ”

Now researchers can use the technique to study other materials to see if they are potential p-wave candidates.

However, the work on strontium ruthenate is still ongoing.

“This material has been very well studied in many different contexts, and not just because of its superconductivity,” said Ramshaw. “We understand what kind of metal it is, why it is a metal, how it behaves when you change the temperature, how it behaves when you change the magnetic field. So you should be able to construct a theory as to why it becomes a superconductor better here than anywhere else. ”

Reference: “Thermodynamic proof for a parameter of the superconducting two-component order in Sr.₂RuO_4th“By Sayak Ghosh, Arkady Shekhter, F. Jerzembeck, N. Kikugawa, Dmitry A. Sokolov, Manuel Brando, AP Mackenzie, Clifford W. Hicks and BJ Ramshaw, September 21, 2020, Natural physics.
DOI: 10.1038 / s41567-020-1032-4

Co-authors are researchers from the Max Planck Institute for Chemical Physics of Solids; the National High Magnetic Field Laboratory in Florida State University;; and the National Institute for Materials Science in Tsukuba, Japan.

Cornell research was supported by the Department of Energy’s Office of Basic Energy Sciences and the Cornell Center for Materials Research, supported by the National Science Foundation’s Materials Research Science and Engineering Center program.