= K, Rb, Cs)*

We present elastoresistance measurements of the iron-based superconductors AFe₂As₂ (A = K, Rb, Cs) in the A_1g, B_1g and B_2g symmetry channels. These materials have strong electronic correlations and show a crossover between a low-temperature coherent heavy Fermi liquid and a high-temperature regime of reduced quasiparticle coherence [1,2]. Neutralizing the large thermal expansion of these materials by means of a piezoelectric strain cell is found to be essential to obtaining intrinsic results. All three materials have a significant in-plane symmetric A_1g response, consistent with the coherence-incoherence crossover.

[1] F. Hardy et al., Phys. Rev. B 94, 205113 (2016)
[2] F. Hardy et al., Phys. Rev. Lett. 111, 027002 (2013)
[3] P. Wiecki et al., Phys. Rev. Lett. accepted (2020)