Conduction electrons drive giant, nonlinear elastic response in Sr2RuO4
The hardness of a material normally is set by the strength of chemical bonds between electrons of neighbouring atoms, not by freely flowing conduction electrons. Now a team of scientists from MPI-CPfS, Germany, Japan, Korea and the United States has shown that current-carrying electrons can make the lattice much softer than usual in the material Sr2RuO4.
![Young’s modulus of Sr2RuO4 as a function of strain at 4 K. The pronounced dip at around exx = -0.0045 indicates that the lattice becomes softer at that strain. This softening is caused by a pressure-driven change in the conduction electron system. Figure has been adapted from [1].](/3656806/original-1698328532.jpg?t=eyJ3aWR0aCI6MzQxLCJmaWxlX2V4dGVuc2lvbiI6ImpwZyIsIm9ial9pZCI6MzY1NjgwNn0%3D--372231b0c2fe94730bcc39bc7dca5a4cea4b5331 "Young’s modulus of Sr2RuO4 as a function of strain at 4 K. The pronounced dip at around exx = -0.0045 indicates that the lattice becomes softer at that strain. This softening is caused by a pressure-driven change in the conduction electron system. Figure has been adapted from [1].")
![Young’s modulus of Sr2RuO4 as a function of strain at 4 K. The pronounced dip at around exx = -0.0045 indicates that the lattice becomes softer at that strain. This softening is caused by a pressure-driven change in the conduction electron system. Figure has been adapted from [1].](/3656806/original-1698328532.jpg?t=eyJ3aWR0aCI6MjQ2LCJvYmpfaWQiOjM2NTY4MDZ9--1459446937fe8ac5a346e9e68038353c8069ce2b)
The hardness of materials is determined by the strength of the chemical bonds that are formed between the electrons of the neighbouring atoms. For example, the bonds in diamond are very strong, so it is one of the hardest materials known. The bonding is rooted in the laws of quantum mechanics, and the complex compounds that are of most interest in forefront research today are known as ‘quantum materials.’ In many quantum materials, layers of strongly bonded atoms separate layers in which current can flow due to a small subset of the electrons in the material. An intuitive picture is that the strongly bonded layers determine the hardness, giving the current-carrying electrons a rigid atomic background, called a lattice, in which to flow.
The flowing electrons and the lattice know about each other, and if the lattice changes for some reason there will be effects on the current patterns. If the flowing electrons interact strongly with each other, they can spontaneously change current patterns, but if that happens, the effect on the lattice is usually very weak. Indeed, because the lattice bonding is usually so strong, the lattice is often thought of as dominating the current-carrying electrons, and even a very weak change of the lattice is often proposed to be the driver of the current pattern change.
In newly published work [1], a team from our Institute has uncovered clear proof of a case in which a tiny fraction of the current-carrying electrons can dominate all the others and make the lattice much softer. After the experiment was performed in-house, we collaborated with colleagues from Germany, Japan, Korea and the United States to understand the surprising result. An explicit model constructed by the groups of Joerg Schmalian and Markus Garst at the Karlsruhe Institute of Technology was crucial to unravelling the detective story, as were results from complementary experiments published in 2022 [2]. The work gives a new perspective on a decades-old problem, and will, we hope, be influential for future research.
[1] H. M. L. Noad, K. Ishida, Y.-S. Li, E. Gati, V. Stangier, N. Kikugawa, D. A. Sokolov, M. Nicklas, B. Kim, I. I. Mazin, M. Garst, J. Schmalian, A. P. Mackenzie & C. W. Hicks, Giant lattice softening at a Lifshitz transition in Sr2RuO4, Science 382, 447-450 (2023). DOI: https://doi.org/10.1126/science.adf3348
[2] Y.-S. Li, M. Garst, J. Schmalian, S. Ghosh, N. Kikugawa, D.A. Sokolov, C. W. Hicks, F. Jerzembeck, M. S. Ikeda, Z. Hu, B. Ramshaw, A. W. Rost, M. Nicklas & A. P. Mackenzie, Elastocaloric determination of the phase diagram of Sr2RuO4, Nature 607, 276-280 (2022). DOI: https://doi.org/ 10.1038/s41586-022-04820-z
