STRAIN DRIVEN DYNAMICS OF PHASE TRANSITIONS IN OXIDE ANTIFERROMAGNETS

相变是指材料性质的变化，通常由温度变化引起。普遍存在的水的相变就是一个很好的例子，随着温度的升高，水从冰变成液态水再变成蒸汽。气压显著地改变了水的温度转变，任何在高海拔地区烤过蛋糕的人都很清楚这一点。该项目的研究部分将通过将非常快的（超声波）声波聚焦在材料上，研究快速变化的压力对技术上重要的磁性材料相变的影响。通过以每秒高达100亿次的速度改变压力，研究人员将探测材料的性质是如何变化的。研究的基本问题是材料的反应速度有多快，反应会是什么，以及为什么特定材料会有这样的反应。教育部分是由pi在从学前教育，K-12教育到本科教育的各级推广和教育方面的丰富经验提供的。将根据某一年龄组的需要拟订关于阶段转变的新活动，以配合这项研究项目。技术摘要：本研究活动将研究具有复杂结构和有序纠缠的材料（即磁电反铁磁Cr2O3和反铁磁Mott绝缘体NdNiO3）中应变驱动相变的动力学。在这两种情况下，反铁磁有序与另一种转变的出现是一致的，Cr2O3的表面边界磁化和NdNiO3的金属绝缘体转变。聚焦表面声波换能器将产生与固定外延应变相当的应变，并允许在单个样品上控制可变应变，以及在高频下驱动应变的能力。这种精确控制晶格激发的方法是独特的，因为它能够在GHz频率下驱动材料在相变中来回移动，导致序参量的大变化，而不是小的扰动变化。这种方法可以扩展到薄膜材料的各种应变敏感排序，并将回答两个基本问题。首先，它将量化外应变对单个薄膜样品相变温度的影响，消除与薄膜生长相关的不确定性。其次，它将测量这些应变驱动相变发生的时间尺度。教育目标将通过扩大pi的投资组合，包括与该项目相关的主题，扩展这一前沿科学的范围。这些pi将与包括K-12学生和教师、科学咖啡馆、科学俱乐部和老年公民团体在内的已经广泛开展的外展和教育项目分享他们的知识、技能和兴奋。

Non-technical abstractPhase transitions refer to changes in a material's properties, often driven by temperature changes. The ubiquitous phase transition of water, which transforms from ice to liquid water to steam as the temperature increases, serves as an excellent example. Pressure significantly alters the temperature of water's transitions, as anyone who has baked a cake at high altitude well knows. The research component of this project will investigate the effects of rapidly changing pressure on phase transitions of technologically important magnetic materials by focusing a very fast (ultrasonic) sound wave on the material. By varying the pressure at rates of up to 10 billion times a second, the investigators will probe how the material's properties vary. The fundamental questions being probed are how quickly can the material respond, what will the response be and why does a particular material respond the way it does. The educational component is informed by the PIs' extensive experience in outreach and education at all levels ranging from preschool, K-12 and undergraduate education. New activities on phase transitions, tailored to the needs of a particular age group, will be developed to tie in with this research project. Technical AbstractThe research activity will investigate the dynamics of strain driven phase transitions in materials that show intricate entanglement between structure and ordering, viz. the magnetoelectric antiferromagnet Cr2O3 and the antiferromagnetic Mott insulator NdNiO3. In both cases the antiferromagnetic ordering is coincident with the appearance of another transition, surface boundary magnetization for Cr2O3 and a metal-insulator transition for NdNiO3. Focused surface acoustic wave transducers will generate strains that are comparable in magnitude to fixed, epitaxial strains and allow for controlled variable strain on a single sample, as well as the ability to drive strain at high frequencies. This approach of precisely controlled lattice excitation is unique in its the ability to drive materials back and forth across the phase transition at GHz frequencies, resulting in large changes in the order parameter, rather than small perturbative changes. This approach can be extended to a wide variety of strain sensitive ordering in thin film materials and will answer two fundamental questions. First, it will quantify the effects of external strain on the phase transition temperature of a single thin film sample, eliminating the uncertainties associated with thin film growth. Second, it will measure the temporal scale over which these strain driven phase transitions occur. The educational objectives will extend the reach of this cutting edge science by expanding the PIs' portfolios to include topics that are relevant to this project. The PIs will share their knowledge, skills and excitement with the already extensive ongoing outreach and education projects that include K-12 students and teachers, science cafes, science clubs, and senior citizen groups.