Elastoresistive and elastocaloric anomalies in iron pnictides

Electronic nematicity is a prominent feature in Fe-based superconductors, but is also present in at least some cuprates, and heavy fermion superconductors. Being a possible common thread for unconventional superconductivity, the role of nematic fluctuations for superconductivity needs to be assessed. This calls for new techniques that allow for tuning nematicity continuously towards quantum criticality. In the first part of this talk I will discuss how to disentangle the response of nematicity to strain components of different symmetry, and will show that both symmetric (A_1g) and antisymmetric (B_1g) strain are suitable means to tune the critical temperature of the nematic phase transition in Fe based superconductors. In the second part of the talk, I will outline recent experimental advances exploring the thermoelastic properties of these materials. In particular, the finite response of nematicity to strain causes anomalies in the elastocaloric effect as well as in the elastoresistivity which can be measured via an AC technique. These anomalies are proportional to the corresponding heat capacity anomalies and are understood to be a direct consequence of the strain dependence of the nematic and the antiferromagnetic transition temperatures. A similar mechanism should more generally be expected for any phase transition tunable by strain.