CPM Seminar

Magnetism without tetragonal symmetry-breaking in iron-based superconductors

Rafael Fernandes

University of Minnesota

The proximity of superconductivity to a magnetic instability is a common feature of many unconventional superconductors. As a result, elucidating the origin and the properties of the magnetic state is an important step to advance our understanding of these materials. In most iron-based materials, superconductivity appears upon suppression of a stripe-orthorhombic magnetic state, in which spins align parallel to each other along one in-plane direction and anti-parallel to each other along the other direction. Recent experiments in hole-doped materials, however, revealed a novel magnetic ground state that remains tetragonal and is inconsistent with spin stripes. More interestingly, this novel tetragonal magnetic phase appears close to optimal doping, where the superconducting transition temperature is the highest. In this talk, we will show that this tetragonal magnetic state can be explained as a double-Q order that arises naturally from an itinerant microscopic description of these materials, and is incompatible with localized spins. Two types of double-Q order are discussed, displaying either a non-uniform magnetization (called charge-spin density-wave) or a non-collinear magnetization (called a spin-vortex crystal). Upon increasing the temperature, we show that both tetragonal phases melt in a two-stage process, giving rise to “vestigial” paramagnetic phases that display unusual charge and chiral orders. While fluctuations of these vestigial electronic phases may enhance the superconducting transition temperature, we show that long-range tetragonal magnetic order competes more strongly with superconductivity than the stripe orthorhombic magnetic order.