Nematicity from mixed S sub(+ or -) + d sub(x2-y2) 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 sub(+ or -) +d sub(x2-y2) SC state is a physically accessible state and may emerge under generic conditions, lowering spontaneously the fourfold rotational symmetry C sub(4) to C sub(2). We provide a detailed study of the mixed S sub(+ or -) +d sub(x2-y2) 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 d sub(x2-y2) 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 sub(3). Our mechanism for nematicity may also be relevant for nonsuperconducting nematic states involving mixed S sub(+ or -) +d sub(x2-y2) spin/charge density waves.