AC elastocaloric effect as a probe for thermodynamic signatures of continuous phase transitions

Abstract

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.

Hydrostatic and Uniaxial Pressure Tuning of Iron‐Based Superconductors: Insights into Superconductivity, Magnetism, Nematicity, and Collapsed Tetragonal Transitions

“…Conversely, given the recent successes in tuning uniaxial strain in situ, this has initialized ideas to use ac elastocaloric measurements as a tool to explore specific heat. [ 120 ] The key idea here is, that similar to the oscillating heat in ac calorimetric experiments, an oscillating strain can induce a temperature oscillation, related to the specific heat of the sample and which can be recorded by a thermometer. Proof‐of‐principle tests of this idea were presented for the iron‐pnictide BaFe 2 As 2 .…”

“…Proof‐of‐principle tests of this idea were presented for the iron‐pnictide BaFe 2 As 2 . [ 120 ] The extension to use the ac elastocaloric technique for measurements of the specific heat at finite offset strains is underway. [ 121 ] In addition, thermal expansion measurements can also be performed under uniaxial pressure.…”

“…The tunability of the nematic and magnetic phase transition by uniaxial pressure has been tested in the example case of an underdoped Ba(FeCo x ) 2 As 2 . [ 120 ] This response was carefully compared to the hydrostatic pressure response. In this way, it was possible to perform a symmetry decomposition of the strains, which are induced by the uniaxial stress.…”

A temperature-modulated dilatometer by using a piezobender-based device

“…This method is similar to temperature-modulated calorimetry (TMC) and other temperature-modulated measurements. [30][31][32] The basic principle is to periodically heat a sample so that the oscillating changes of a sample's length and temperature, i.e., ΔL and ΔT, can be simultaneously determined with the assistance of a lock-in system. High resolutions can thus be achieved because of the lock-in system's capability of detecting small signals.…”

Elastocaloric determination of the phase diagram of Sr$_2$RuO$_4$

“…Although used quite widely in association with materials with huge elastic responses, to the extent that it has been proposed for cooling technologies [16], the direct measurement of the elastocaloric effect has been much less widely employed in the field of unconventional superconductivity or correlated electron physics, partly because the expected signal size is much smaller. In this project we build on recent work that demonstrated the physical insight that could be obtained by using a.c. methods to perform high-resolution measurements of ∆T /∆ε in Fe-based superconductors [17][18][19] to study the elastocaloric effect in Sr 2 RuO 4 . As described in detail in the methods section, we superimpose a small oscillatory component on the background steady strain, and lock into the oscillatory component of the thermal response, which directly measures ∆T /∆ε.…”