Resonant detection of electronic nematicity in correlated electron systems

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In normal metals, the electronic states are governed merely by their interaction with the atomic lattice, and as a result the electronic system inherits its symmetries from the hosting lattice. However, if the interactions between the electrons themselves are sufficiently strong, this simple paradigm must not hold true anymore and intriguing strongly correlated states of lower symmetry than the parent crystal may arise: An electronic nematic state breaks a rotational symmetry while the translational symmetry of the crystal remains intact, named in analogy to the nematic phase in liquid crystals. A hallmark of such states is a pronounced scattering time anisotropy between symmetry related directions. Small external lattice distortions influence the preferred direction of the nematic electronic system, yielding a substantial dependence of the resistance on uniaxial strain. Electronic nematicity has been argued to be of importance for the strong electron correlations in both copper- and iron-based high-temperature superconductors. Nematicity is a rather rare phenomenon, observed only in a handful of systems, and its potential appearance in both known high-Tc classes naturally leads to a fundamental question: Do nematic fluctuations contribute to Cooper pairing, or is nematicity a competing phenomenon irrelevant or even disadvantageous for high-Tc superconductivity? We intend to quantitatively measure the nematic susceptibility in a wide range of pnictide superconductors and correlate it with the transition temperature Tc. Many classes of iron-arsenides are inaccessible to traditional measurement techniques due to the small size of available single crystals. We propose to develop a highly-sensitive resonant nematicity detection method based on Focused Ion Beam (FIB) carved single crystal microresonators. By carving 100-10 micrometer long mechanical oscillators out of pnictide single crystals, we exploit the compression and expansion of the material during a bending mode cycle to directly measure the nematic susceptibility with higher precision and accuracy compared to existing methods. At the same time, the resonance frequency of the cantilever devices will provide a direct measure of the elastic constants of the system. This would enable us to correlate the nematic susceptibility with a thermodynamic property of the ionic lattice, which is a key comparison to identify the degree of the nematic contribution to the symmetry breaking. This project is aimed to provide a comprehensive comparison of nematicity in the various structural classes of pnictide superconductors, and investigate its role in superconductivity. Once this new technique has been developed, it will be a versatile tool to research nematic phenomena in a variety of other materials beyond iron-based superconductors.