A reference system for unconventional superconductivity

Superconductivity is a quantum state of matter that could significantly revolutionize technologies such as electricity distribution and computers. While it is present in specialized fields such as medical imaging, superconductivity’s widespread use is limited by the need to cool down materials to cryogenic temperatures close to -273.15^o C in order to reveal their quantum properties. The technical challenges and costs associated with this make superconductivity in everyday life difficult to achieve.

Superconductors are classified in two categories. Conventional superconductors are ubiquitous in metals cooled down to temperatures that are sufficiently low. Our understanding of conventional superconductors was achieved in 1957 with the Bardeen-Cooper-Schrieffer theory, for which the researchers were awarded the Nobel Prize in Physics in 1972.

Unconventional types of superconductors are far more mysterious. The physics that allow their superconducting phase to emerge are extremely complex and are the subject of debate in the scientific community. Our understanding and explanation of this phenomenon, could by the same token lead to the discovery of materials that have the potential to completely revolutionize technology.

Within unconventional superconductors is the sub-category of cuprates, a class of copper-based materials that can exhibit superconducting properties at temperatures far above those of conventional superconductor materials.

In 1994, a material with the exact same structure as cuprates but with ruthenium rather than copper was discovered to be superconducting. Although its superconducting temperature is very low, this material has a distinct advantage: it can be synthesized in a very pure way. Its resemblance to cuprates and its purity have thus warranted several studies. Its properties are now well catalogued under almost every condition. The infatuation over this material has only grown as it was initially predicted to be a useful candidate to help build the quantum computer.

This material, Sr2RuO4, also known as strontium ruthenate, should therefore be easily understood and represent an idealized case of a superconducting material whose physics would be perfectly understood. Some researchers even claim that it would be fruitless to search for a theory for other unconventional superconductors without first explaining the superconductivity of this material. To make matters more complicated, the differences between experiments on this material are so at odds that even the most fundamental characteristic of the superconducting state, the symmetry of the order parameter, remains a mystery.

In this paper, the symmetry of the superconducting order parameter of strontium ruthenate is determined using numerical simulations. Since superconductivity is a phenomenon that is very sensitive to material details, a first-principle approach is used to obtain a minimally-biased description of the electrons that form the said quantum state. Based on this description, superconductivity has been studied by assuming the most promising mechanism in this context: the theory of charge and spin fluctuations.

The results contradict initial expectations, in particular the potential use of this material for quantum computing. Among these results, two new families of quantum states have been achieved, which have been discussed very little in the context of this material. This opens up new possibilities that are very interesting from a fundamental point of view. The most likely type of superconductivity in this approach is the same as the one observed in cuprates. This result is aligned with previous experiments of Professor Taillefer’s group and with recent experiments in tunneling spectroscopy.

About the main author: Olivier Gingras

Olivier is a PhD student in physics in Michel Côté’s group at the Physics Department of the Université de Montréal. He is co-supervised by Professor André-Marie Tremblay of the Université de Sherbrooke and by research professional Reza Nourafkan, both members of l’Institut quantique. Olivier began his graduate studies in January 2015 and is scheduled to complete in the fall of 2020. He received an NSERC scholarship for his master’s studies and a FRQ-NT scholarship for his doctoral studies. More recently, he was the recipient of the prestigious UdeM “Excellence Hydro-Québec” scholarship worth $20,000.