  Studying the response of materials to strain can elucidate subtle properties
of electronic structure in strongly correlated materials. So far, mostly the
relation between strain and resistivity, the so called elastoresistivity, has
been investigated. The elastocaloric effect is a second rank tensor quantity
describing the relation between entropy and strain. In contrast to the
elastoresistivity, the elastocaloric effect is a thermodynamic quantity.
Experimentally, elastocaloric effect measurements are demanding since the
thermodynamic conditions during the measurement have to be well controlled.
Here we present a technique to measure the elastocaloric effect under quasi
adiabatic conditions. The technique is based on oscillating strain, which
allows for increasing the frequency of the elastocaloric effect above the
thermal relaxation rate of the sample. We apply the technique to Co-doped iron
pnictide superconductors and show that the thermodynamic signatures of second
order phase transitions in the elastocaloric effect closely follow those
observed in calorimetry experiments. In contrast to the heat capacity, the
electronic signatures in the elastocaloric effect are measured against a small
phononic background even at high temperatures, establishing this technique as a
powerful complimentary tool for extracting the entropy landscape proximate to a
continuous phase transition.

Materials Science