Nematicity from mixed S +or-+dx 2-y 2 states in iron-based superconductors

We propose a novel mechanism for nematicity potentially relevant in some iron-based superconductors (SCs). We demonstrate that the mixed S +or-+dx 2-y 2 SC state is a physically accessible state and may emerge under generic conditions, lowering spontaneously the fourfold rotational symmetry C 4 to C 2. We provide a detailed study of the mixed S +or-+dx 2-y 2 nematic state including its behavior under a Zeeman field. As a fingerprint of this state, we predict at low temperatures a first-order Zeeman-field-induced transition from the mixed nematic SC phase to the pure dx 2-y 2 SC phase. The Zeeman field-temperature phase diagram for a nodeless mixed nematic state exhibits three distinct SC phases and a tetracritical point, remarkably reminiscent of the one observed in UPt 3. Our mechanism for nematicity may also be relevant for nonsuperconducting nematic states involving mixed S +or-+dx 2-y 2 spin/charge density waves.