Figure: The energy structure of two representative superconducting states considered by Boettcher and Herbut. The red circles mark the points where the superconducting parameter vanishes. Phase (b) has the shortest such lines among all of the possible superconducting phases, and therefore represents the most stable configuration.

New type of superconductivity in topological materials

The motivation This recent study by Simon Fraser University (SFU) researchers was a direct response to an experiment by physicist John-Pierre Paglione's group at the University of Maryland (unpublished preprint at Specifically, the Paglione lab’s research suggested a new type of superconducting phase forming at low temperatures in the material YPtBi (yttrium-platinum-bismuth). This material is one of the family of materials called "half-Heuslers", which are peculiar because in their high temperature phase their electrical properties are between those of normal metals and electrical insulators. They have been under much scrutiny lately for their unusual (i.e. "topologically non-trivial") electronic structure. Although other experiments have indicated an unconventional superconductivity among other members of the family at low temperatures, the particularly clear-cut results of the Paglione experiment caught the attention of SFU’s Igor Herbut, who has worked on other aspects of the physics of these and similar materials.

The discovery – Physicists Boettcher and Herbut at SFU formulated the simplest theoretical model and used it to show that many different superconducting phases are possible, and that these phases actually compete strongly among themselves.  They determined mathematically which phase is preferred under various conditions, including the conditions of Paglione’s experiment. In the process, they discovered certain novel and entirely unexpected mathematical identities pertaining to the determination of the stability of various superconducting phases.

Its significance – The most stable phase they found is quite unusual, with several new features, and may be related to Paglione’s experimental observation. This formulation of the model and the subsequent analysis paves the way for future studies of the superconducting phenomenon in half-Heuslers and related materials. Further, this study has a direct impact on researchers in the field of superconductivity, particularly those who study competition between different superconducting phases. More broadly it impacts research in statistical and condensed matter physics on so-called "nematic" phase transitions, which occur in liquid crystals, a material that is used in computer screens and other modern technological devices.

Herbut and Boettcher are presenting this work at the American Physical Society (March 23rd, Los Angeles), a meeting with around 10,000 attendees, which makes it perhaps the largest gathering of physicists on the planet.  Of note, Herbut was elected recently as a Fellow of the American Physical Society as a result of his outstanding contributions to the theory of Dirac fermions in strongly correlated electron systems.

Read the paper“Unconventional Superconductivity in Luttinger Semimetals: Theory of Complex Tensor Order and the Emergence of the Uniaxial Nematic State” by Boettcher, I; Herbut, IF. PHYSICAL REVIEW LETTERS 120(5): 057002 (2018). DOI: 10.1103/PhysRevLett.120.057002

Website article compiled by Jacqueline Watson with Theresa Kitos