Magnetic and structural properties near the Lifshitz point in Fe _1+xTe

We construct a phase diagram of the parent compound Fe_1+xTe as a function of interstitial iron x in terms of the electronic, structural, and magnetic properties. For a concentration of x<10%, Fe_1+xTe undergoes a "semimetal" to metal transition at approximately 70 K that is also first-order and coincident with a structural transition from a tetragonal to a monoclinic unit cell. For x≈14%, Fe_1+xTe undergoes a second-order phase transition at approximately 58 K corresponding to a semimetal to semimetal transition along with a structural orthorhombic distortion. At a critical concentration of x≈11%, Fe_1+xTe undergoes two transitions: the higher-temperature one is a second-order transition to an orthorhombic phase with incommensurate magnetic ordering and temperature-dependent propagation vector, while the lower-temperature one corresponds to nucleation of a monoclinic phase with a nearly commensurate magnetic wave vector. While both structural and magnetic transitions display similar critical behavior for x<10% and near the critical concentration of x≈11%, samples with large interstitial iron concentrations show a marked deviation between the critical response indicating a decoupling of the order parameters. Analysis of temperature dependent inelastic neutron data reveals incommensurate magnetic fluctuations throughout the Fe_1+xTe phase diagram are directly connected to the "semiconductor"-like resistivity above T_N and implicates scattering from spin fluctuations as the primary reason for the semiconducting or poor metallic properties. The results suggest that doping driven Fermi surface nesting maybe the origin of the gapless and incommensurate spin response at large interstitial concentrations.