Until now, superconducting materials have been of two types: s-wave and d-wave. Now Cornell researchers led by Brad Ramshaw, assistant professor of Dick & Dale Reis Johnson at the College of Arts and Sciences, have discovered a possible third type, the g-wave. Their article was published in the journal Nature Physics.
Electrons in superconductors move together in what are called Cooper pairs. This “pairing” gives superconductors their most famous property, the absence of electrical resistance. Cooper pairs must be broken to create resistance, and this requires energy.
In s-wave superconductors — usually, materials such as lead, tin, and mercury — Cooper pairs are made up of one electron pointing upward and the other downward, both of which move face to face with no net angular momentum. In recent decades, a new class of exotic materials has demonstrated the so-called d-wave superconductivity, in which Cooper pairs have two quanta of angular momentum.
Physicists have put forward the theory of the existence of a third type of superconductor between these two so-called “singlet” states a p-wave superconductor with one quantum of angular momentum and pairing of electrons with parallel rather than antiparallel spins. This spin-triplet superconductor will be a major breakthrough in quantum computing as it can be used to create Majorana fermions – a unique particle that is itself an antiparticle.
For over 20 years, strontium ruthenate (Sr2RuO4) has been one of the leading candidates for the role of a p-wave superconductor.
Ramshaw and his team set out to determine once and for all whether strontium ruthenate is the much-needed p-wave superconductor. Using high-resolution resonant ultrasonic spectroscopy, they discovered that this material is potentially a completely new type of superconductor: the g-wave.
As in previous projects, Ramshaw and Ghosh used resonant ultrasonic spectroscopy to study the symmetry properties of superconductivity in a strontium ruthenate crystal grown by staff at the Max Planck Institute for Solid State Chemical Physics in Germany.
However, unlike previous attempts, Ramshaw and Ghosh faced a major challenge in conducting the experiment.
“Cooling resonant ultrasound down to 1 kelvin (minus -272.15 Celsius) is difficult, and for that, we had to build a completely new apparatus,” explains Ghosh.
With the new setup, Cornell’s team measured the response of a crystal’s elastic constants – essentially the speed of sound in a material – to various sound waves when the material is superconducted to cool down to 1.4 Kelvin (-271.75°C).
“This is by far the most accurate resonance ultrasonic spectroscopy data ever obtained at such low temperatures,” Ramshaw said.
Based on the data, scientists have determined that strontium ruthenate is what is called a two-component superconductor. This means that the way electrons are bound is so complex that it cannot be described by a single number; he needs direction too.
Previous studies have used nuclear magnetic resonance (NMR) spectroscopy to narrow down the possibilities of what kind of wave material strontium ruthenate might be, effectively eliminating the p-wave as an option. By determining that the material was two-component, Ramshaw’s team not only confirmed these findings but also showed that strontium ruthenate is not an ordinary s- or d-wave superconductor.
Researchers can now use this technique to study other materials to see if they are potential candidates for the p-wave.