Determination of the dynamic Young’s modulus of quantum materials in piezoactuator-driven uniaxial pressure cells using a low-frequency AC method

We report on a new technique for measuring the dynamic Young’s modulus, E, of quantum materials at low temperatures as a function of static tuning strain, ϵ, in piezoactuator-driven pressure cells. In addition to a static tuning of stress and strain, we apply a small-amplitude, finite-frequency AC (1 Hz ≲ ω ≲ 1000 Hz) uniaxial stress, σ_ac, to the sample and measure the resulting AC strain, ϵ_ac, using a capacitive sensor to obtain the associated modulus E. We demonstrate the performance of the new technique through proof-of-principle experiments on the unconventional superconductor Sr₂RuO₄, which is known for its rich temperature–strain phase diagram. In particular, we show that the magnitude of E, measured using this AC technique at low frequencies, exhibits a pronounced nonlinear elasticity, which is in very good agreement with previous Young’s modulus measurements on Sr₂RuO₄ under [1 0 0] strain using a DC method [Noad et al., Science 382, 447–450 (2023)]. By combining the new AC Young’s modulus measurements with AC elastocaloric measurements in a single measurement, we demonstrate that these AC techniques are powerful in detecting small anomalies in the elastic properties of quantum materials. Finally, using the case of Sr₂RuO₄ as an example, we demonstrate how the imaginary component of the modulus can provide additional information about the nature of ordered phases.