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Abstract: 
Many intriguing unconventional superconductors display a phase diagram containing strongly correlated states, which can be tuned by external parameters, such as chemical doping or pressure. In recent years, uniaxial pressure, which can break the symmetry of the crystal lattice, has shown the capabilities of tuning the electronic structures of Sr₂RuO₄ across a Van Hove singularity (VHS) [1, 2]. By performing high precision ac-elastocaloric effect (ECE) [3] and ac-heat capacity (HC) [4] measurements under uniaxial pressure along the [100] direction, we mapped out the phase diagram of Sr₂RuO₄ in detail and narrowed down possible superconducting (SC) order parameters. We summarize our main findings, as many unconventional superconductors, Sr₂RuO₄ has a SC dome in proximity to a magnetic phase. Both ECE and HC exhibit no signature of a second transition below Tc. Besides, we observe a strong reversal of the ECE around the VHS upon entering the SC state. Together with a model calculation, these results strongly suggest a node-less gap opening at the VHS and, thus, put a strong constraint on possible SC order parameters.

[1] A. Steppke et al. Science 355, eaaf9398 (2017).          
[2] V. Sunko et al. npj Quant. Mat. 4, 2397–4648 (2019).
[3] Y.-S. Li et al. Nature 607, 276–280 (2022).
[4] Y.-S. Li et al. Proc. Natl. Acad. Sci. USA 118, e2020492118 (2021).