New Superconductors near Broken Rotational Symmetry Instabilities

High temperature superconductivity emerges in the vicinity of rotational symmetry-breaking instabilities in the iron-based and copper-based materials. It has been argued that the superconducting paring might be enhanced by the quantum critical fluctuations with a nematic character. In this four-year research program, we have made significant progress towards the understanding and the application of this idea. We discovered a new mechanism to control superconductivity by suppressing the nematic fluctuations with an anisotropic strain. We also developed a new experimental platform, the elasto x-ray diffraction, to study the electron lattice coupling of the broken rotational symmetry phase. The elastox-ray diffraction revealed a striking transport-structural correspondence of the nematic state in the iron pnictide superconductors. It also uncovered an unusual coupling between the Eu and Fe moments in the Eu based iron pnictides. During the four-year report period, we have total 5 publications.