title: Proximity of iron pnictide superconductors to a quantum tricritical point.
abstract: In several materials, unconventional superconductivity appears nearby a quantum phase transition where long-range magnetic order vanishes as a function of a control parameter like charge doping, pressure or magnetic field. The nature of the quantum phase transition is of key relevance, because continuous transitions are expected to favour superconductivity, due to strong fluctuations. Discontinuous transitions, on the other hand, are not expected to have a similar role. Here we determine the nature of the magnetic quantum phase transition, which occurs as a function of doping, in the iron-based superconductor LaFeAsO(1-x)F(x). We use constrained density functional calculations that provide ab initio coefficients for a Landau order parameter analysis. The outcome is intriguing, as this material turns out to be remarkably close to a quantum tricritical point, where the transition changes from continuous to discontinuous, and several susceptibilities diverge simultaneously. We discuss the consequences for superconductivity and the phase diagram.
authors.affiliation: Dipartimento di Fisica, Università di Roma 'La Sapienza', P. Aldo Moro 2, Roma 00185, Italy. gianluca.giovannetti@gmail.com
authors.forename: Gianluca
authors.initials: G
authors.lastname: Giovannetti
authors.forename: Carmine
authors.initials: C
authors.lastname: Ortix
authors.forename: Martijn
authors.initials: M
authors.lastname: Marsman
authors.forename: Massimo
authors.initials: M
authors.lastname: Capone
authors.forename: Jeroen
authors.initials: J
authors.lastname: van den Brink
authors.forename: José
authors.initials: J
authors.lastname: Lorenzana
mesh_terms: D001152:Arsenicals / Q000737:chemistry*; D004553:Electric Conductivity; D007501:Iron / Q000737:chemistry*; D007811:Lanthanum / Q000737:chemistry*; D008280:Magnetics; D011312:Pressure; D011789:Quantum Theory; D013696:Temperature
chemical_list: D001152:Arsenicals; D007811:Lanthanum; D007501:Iron