Discovery of a strain-stabilized charge density wave in LiFeAs

In a number of high T_C superconductors, small orthorhombic distortions of the lattice structure result in surprisingly large symmetry breaking of the electronic states and macroscopic properties, an effect often referred to as nematicity. This nematicity has been studied extensively on materials with an orthorhombic crystal structure, where the lattice symmetry is already reduced from four-fold (C₄) to two-fold (C₂). To directly study the impact of symmetry breaking lattice distortions on the electronic states, we image at the atomic scale the influence of strain-tuned lattice distortions on the correlated electronic states in the iron-based superconductor LiFeAs, a material which in its ground state is tetragonal, with C₄ symmetry. Our experiments uncover a new strain-stabilized nematic phase that exhibits a unidirectional charge density wave (CDW) in LiFeAs, an electronic state which not only breaks rotational symmetry but also reduces translational symmetry. We follow the evolution of the superconducting gap from the unstrained material with C₄ symmetry through the new nematic phase with C₂ symmetry and CDW order to a state where superconductivity is completely suppressed.