Nuclear magnetic resonance investigation ofstrain-tuned iron-based superconductors

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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.