Frequency-dependent sensitivity of AC elastocaloric effect measurements explored through analytical and numerical models.

We present a comprehensive study of the frequency-dependent sensitivity for measurements of the AC elastocaloric effect by applying both exactly soluble models and numerical methods to the oscillating heat flow problem. These models reproduce the finer details of the thermal transfer functions observed in experiments, considering here representative data for single-crystal Ba(Fe1-xCox)2As2. Based on our results, we propose a set of practical guidelines for experimentalists using this technique. This work establishes a baseline against which the frequency response of the AC elastocaloric technique can be compared and provides intuitive explanations of the detailed structure observed in experiments.

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

Studying the response of materials to strain can elucidate subtle properties of the electronic structure in strongly correlated materials. Here, we focus on the elastocaloric coefficients, forming a second rank tensor quantity describing the relation between entropy and strain. In contrast to the better-known 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. In this work, we present a technique to measure the elastocaloric effect under quasiadiabatic 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 heat capacity, elastocaloric effect measurements allow for the electronic signatures to be measured against a small phononic background even at high temperatures and in addition give information on the symmetry of the involved order parameters. This establishes the technique as a powerful complimentary tool for extracting the entropy landscape as a function of strain proximate to a continuous phase transition.

Kondo-like phonon scattering in thermoelectric clathrates.

Crystalline solids are generally known as excellent heat conductors, amorphous materials or glasses as thermal insulators. It has thus come as a surprise that certain crystal structures defy this paradigm. A prominent example are type-I clathrates and other materials with guest-host structures. They sustain low-energy Einstein-like modes in their phonon spectra, but are also prone to various types of disorder and phonon-electron scattering and thus the mechanism responsible for their ultralow thermal conductivities has remained elusive. Our thermodynamic and transport measurements on various clathrate single crystal series and their comparison with ab initio simulations reveal an all phononic Kondo effect as origin. This insight devises design strategies to further suppress the thermal conductivity of clathrates and other related materials classes, with relevance for thermoelectric waste heat recovery and, more generally, phononic applications. It may also trigger theoretical work on strong correlation effects in phonon systems.