该研究团队通过证明交流弹性热效应的直流偏置应变依赖性，将后者分解为对称（旋转对称性保留）和反对称（旋转对称性破缺）应变通道。他们以四方f电子金属间化合物DyB2C2为例，该化合物的反铁电四极序打破了局部四重旋转对称性，而整体保持四方形。研究人员利用弹性热系数的奇点和跃迁时的跳跃来捕捉其四极和磁相变的应变演变，并发现这些演变遵循修正的Ehrenfest关系。在反铁四极（倾斜的反铁磁）相中，反对称应变以双二次（线性二次）方式耦合到底层序参数，这分别归因于保留（破缺）的全局四方对称性。
弹热应变迟滞和光学双折射现象进一步证明了磁相中四方对称性的破缺。此外，在交错四极序中，观察到的弹性热响应反映了熵随反对称应变的二次增长，类似于磁场通过促进伪自旋翻转在反铁磁序中所起的作用。
这项研究结果表明，交流弹性热效应是一种紧凑而敏锐的热力学探针，可以研究电子自由度和自由能应变之间的耦合。这一发现对于研究和理解更广泛种类的量子材料中各种有序相的对称性具有潜力，有助于进一步推动新材料和新器件的开发和应用。
据悉，绝热弹热效应测量给定系统随应变的温度变化，并提供了在温度-应变空间中探测熵景观的热力学方法。
附：英文原文
Title: Elastocaloric signatures of symmetric and antisymmetric strain-tuning of quadrupolar and magnetic phases in DyB2C2
Author: Ye, Linda, Sun, Yue, Sunko, Veronika, Rodriguez-Nieva, Joaquin F., Ikeda, Matthias S., Worasaran, Thanapat, Sorensen, Matthew E., Bachmann, Maja D., Orenstein, Joseph, Fisher, Ian R.
Issue&Volume: 2023-8-22
Abstract: The adiabatic elastocaloric effect measures the temperature change of a given system with strain and provides a thermodynamic probe of the entropic landscape in the temperature-strain space. Here, we demonstrate that the DC bias strain-dependence of AC elastocaloric effect allows decomposition of the latter into symmetric (rotation-symmetry-preserving) and antisymmetric (rotation-symmetry-breaking) strain channels, using a tetragonal f-electron intermetallic DyB2C2—whose antiferroquadrupolar order breaks local fourfold rotational symmetries while globally remaining tetragonal—as a showcase example. We capture the strain evolution of its quadrupolar and magnetic phase transitions using both singularities in the elastocaloric coefficient and its jumps at the transitions, and the latter we show follows a modified Ehrenfest relation. We find that antisymmetric strain couples to the underlying order parameter in a biquadratic (linear-quadratic) manner in the antiferroquadrupolar (canted antiferromagnetic) phase, which are attributed to a preserved (broken) global tetragonal symmetry, respectively. The broken tetragonal symmetry in the magnetic phase is further evidenced by elastocaloric strain-hysteresis and optical birefringence. Additionally, within the staggered quadrupolar order, the observed elastocaloric response reflects a quadratic increase of entropy with antisymmetric strain, analogous to the role magnetic field plays for Ising antiferromagnetic orders by promoting pseudospin flips. Our results demonstrate AC elastocaloric effect as a compact and incisive thermodynamic probe into the coupling between electronic degrees of freedom and strain in free energy, which holds the potential for investigating and understanding the symmetry of a wide variety of ordered phases in broader classes of quantum materials.
DOI: 10.1073/pnas.2302800120
Source: https://www.pnas.org/doi/10.1073/pnas.2302800120