Nuclear magnetic resonance investigation ofstrain-tuned iron-based superconductors

研究凝聚态系统的响应作为一个精确可控的实验调谐参数（如温度、磁场、静水压力）的函数，通常是发展系统行为微观理论的最好方法。我们的合作者最近开发了一种基于压电的新方法，该方法可以在适合进行核磁共振（NMR）的几何形状的单晶样品上施加可变原位单轴压力。我们的目标是利用这种强大的技术组合来研究应变调谐条件下的铁基超导体。铁基超导体是一类强相关电子系统，表现出非常规的超导性、磁性、轨道有序和电子向列态。从基础研究的角度来看，大量丰富的基态使得这些材料非常有趣，然而，驱动这些材料物理学的复杂相互作用仍然没有得到很好的理解。我们对BaFe2As2的初步研究表明，这些实验是可行的，并对这种材料的性质产生了重要的见解。具体来说，75As四极性核磁共振谱的响应可以在微观上探测到该体系的电子向列磁化率，并且自旋晶格弛豫速率对该体系中自旋涨落的应变依赖性非常敏感。磁有序态的核磁共振对条纹反铁磁性产生的内部超精细场敏感，对转变温度附近的应变调谐也敏感。在DFG的支持下，我们的目标是完成正在进行的BaFe2As2的应变调谐NMR测量，并在密切相关的系统CaFe2As2上进行类似的测量。比较这些系统对单轴压力的响应，有望深入了解驱动基态的微观物理。CaFe2As2也被认为对静水压力非常敏感，它在比BaFe2As2小十倍的压力下诱导超导性。最后，我们还计划研究其他适合的对单轴压力敏感的铁基超导体体系，包括LiFeAs， nafea, CsFe2As2， RbFe2As2和掺杂的LaOFeAs。与BaFe2As2类似，111s和c122在结构/向列转变以上均表现出破碎的四方对称，表明它们对应变敏感。CsFe2As2和RbFe2As2密切相关，先前的测量表明，通过应变调谐可以达到一个近似的量子临界点。最后，我们的合作者最近才得到尺寸适合单轴压力实验的1111s单晶。我们提出的这些材料的单轴压力相图的比较将大大推进铁基超导体的研究领域。

Studying the response of a condensed matter system as a function of a precisely controllable experimental tuning parameter (e.g. temperature, magnetic field, hydrostatic pressure) is often the best way to develop a microscopic theory of the behavior of the system. A novel piezoelectric-based method of applying variable in situ uniaxial pressure to single crystal samples in a geometry suitable for performing nuclear magnetic resonance (NMR) was recently developed by our collaborators. We aim to use this powerful combination of techniques to study the iron-based superconductors under strain-tuned conditions. The iron-based superconductors are a class of strongly correlated electron systems that exhibit unconventional superconductivity, magnetism, orbital order, and an electronic-nematic state. This plethora of rich ground states makes these materials very interesting from a basic research perspective, however the complex interactions driving the physics of these materials is still not well understood. Our preliminary work on BaFe2As2 demonstrates that these experiments are feasible and yield important insight into nature of this material. Specifically, the response of the 75As quadrupolar NMR spectra can microscopically probe the electronic-nematic susceptibility, and the spin-lattice relaxation rate of is highly sensitive to the strain dependence of the spin fluctuations in this system. NMR in the magnetically ordered state is sensitive to the internal hyperfine field produced by the stripe-antiferromagnetism, which is also sensitive to strain-tuning near the transition temperature.With support from the DFG we aim to complete ongoing strain-tuned NMR measurements of BaFe2As2 and perform similar measurements on the closely related system CaFe2As2. Comparison of the response of these systems to uniaxial pressure promises to yield insight into the microscopic physics driving the ground state. CaFe2As2 is also known to be highly sensitive to hydrostatic pressure, which induces superconductivity at pressures ten times smaller than in BaFe2As2. Finally, we also plan to investigate other suitable iron-based superconductor systems that are sensitive to uniaxial pressure including LiFeAs, NaFeAs, CsFe2As2, RbFe2As2, and doped LaOFeAs. Similar to BaFe2As2, the 111s and Cs 122 all exhibit broken tetragonal symmetry above the structural/nematic transition, indicating that they are sensitive to strain. CsFe2As2 and RbFe2As2 are closely related and previous measurements indicate a proximate quantum critical point that may be accesible via strain-tuning. Finally, single crystals of the 1111s have only recently become available from our collaborators with dimensions suitable for uniaxial pressure experiments. Our proposed comparison of the uniaxial pressure phase diagrams of these materials will significantly advance the field of study of the iron-based superconductors.