When electrons in a solid interact strongly, they can form novel states of matter with fascinating technological possibilities and intriguing intellectual challenges. They might realize the macroscopic quantum state with zero electrical resistivity – superconductivity – or spontaneously lose the spherical symmetry of their properties, such that the electrical resistivity becomes different when current flows in different directions – an electronic nematic state. Surprisingly, more and more nematic superconductors, combining both, have recently been discovered. Such observations suggest a fundamental link between nematicity and superconductivity that is not yet understood. It may be that the emergence of superconductivity is favoured or strengthened if a material is already nematic, or superconductivity itself might cause nematicity.
Answering these questions is hindered by the lack of systematic data on the nematicity-superconductivity interaction, due to the absence of routine high-resolution probes of nematicity that are applicable in the superconducting state. To investigate this interaction, we are working on establishing a novel “distortiometry” approach based on measuring a material’s elastic response to anisotropic stress, taking full advantage of established capacitance dilatometry and recent advances in strain-tuning techniques.
We first investigate materials that show superconductivity with a “background” of nematic order. One established example for this is FeSe. Second, we investigate materials whose superconductivity seems to cause nematicity. A much-studied example for this is Sr-Bi2Se3. Our goals are (1) to obtain an exhaustive picture of the relation between unconventional superconductivity and competing nematic phases, (2) to solve the question whether nematicity is really induced by superconductivity in materials such as Sr-Bi2Se3, and (3) provide a broad experimental based of materials and results to determine whether nematicity is an unknown natural stability of superconductors. Thus, we aim to establish whether nematicity and unconventional superconductivity are linked by a universal principle, possibly uncovering a dramatic twist in the long-standing superconductivity research.
