Discovery of a strain-stabilised charge density wave in LiFeAs

In many high temperature 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 ($C_4$) to two-fold ($C_2$). In order 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 $C_4$ symmetry. Our experiments uncover a new strain-stabilised nematic phase which exhibits a unidirectional charge density wave (CDW) in LiFeAs, an electronic state which not only breaks rotational symmetry but also reduces translational symmetry through a characteristic long-range stripe-like modulation of the electronic density of states. We follow the evolution of the superconducting gap from the unstrained material with $C_4$ symmetry through the new nematic phase with $C_2$ symmetry and CDW order to a state where superconductivity is completely suppressed.