Ikeda M S, Straquadine J A W, Hristov A T, Worasaran T, Palmstrom J C, Sorensen M, Walmsley P, Fisher I R
Geballe Laboratory for Advanced Materials, Stanford University, 476 Lomita Mall, Stanford, California 94305, USA.
Rev Sci Instrum. 2019 Aug;90(8):083902. doi: 10.1063/1.5099924.
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.
研究材料对应变的响应能够阐明强关联材料中电子结构的微妙特性。在此，我们聚焦于弹性热系数，它构成了一个二阶张量量，描述了熵与应变之间的关系。与更为人熟知的弹性电阻率不同，弹性热效应是一个热力学量。在实验上，弹性热效应测量颇具挑战性，因为测量过程中的热力学条件必须得到很好的控制。在这项工作中，我们提出了一种在准绝热条件下测量弹性热效应的技术。该技术基于振荡应变，这使得弹性热效应的频率能够提高到高于样品的热弛豫速率。我们将该技术应用于钴掺杂铁基超导体，并表明弹性热效应中二阶相变的热力学特征与量热实验中观察到的特征紧密相符。与热容量不同，弹性热效应测量即使在高温下也能在小的声子背景下测量电子特征，此外还能给出有关所涉及序参量对称性的信息。这确立了该技术作为一种强大的补充工具，用于提取接近连续相变时作为应变函数的熵图景。
