Liran Wang

Superconductivity-induced re-entrance of the orthorhombic distortion in Ba1-xKxFe2As2.

Detailed knowledge of the phase diagram and the nature of the competing magnetic and superconducting phases is imperative for a deeper understanding of the physics of iron-based superconductivity. Magnetism in the iron-based superconductors is usually a stripe-type spin-density-wave, which breaks the tetragonal symmetry of the lattice, and is known to compete strongly with superconductivity. Recently, it was found that in some systems an additional spin-density-wave transition occurs, which restores this tetragonal symmetry, however, its interaction with superconductivity remains unclear. Here, using thermodynamic measurements on Ba1−xKxFe2As2 single crystals, we show that the spin-density-wave phase of tetragonal symmetry competes much stronger with superconductivity than the stripe-type spin-density-wave phase, which results in a novel re-entrance of the latter at or slightly below the superconducting transition.

Superconductivity‐enhanced nematicity and “s+d” gap symmetry in Fe(Se1−xSx)

Superconducting iron chalcogenide FeSe has the simplest crystal structure among all the Fe-based superconductors. Unlike other iron pnictides, FeSe exhibits no long range magnetic order accompanying the tetragonal-to-orthorhombic structural distortion, which raises the fundamental question about the role of magnetism and its associated spin fluctuations in mediating both nematicity and superconductivity. The extreme sensitivity of FeSe to external pressure suggests that chemical pressure, induced by substitution of Se by the smaller ion S, could also a be good tuning parameter to further study the coupling between superconductivity and nematicity and to obtain information on both the Fermi-surface changes and the symmetry of the superconducting state. Here we study the thermodynamic properties of Fe(Se

Magnetic moments induce strong phonon renormalization in FeSi

_{1-x}

Dichotomy between in-plane magnetic susceptibility and resistivity anisotropies in extremely strained BaFe 2 As 2

S

Large nematic susceptibility in the double-Q

_{x}

C_{4}

) for 3 compositions,

magnetic phase of Ba

x=0

_{1-x}

, 0.08 and 0.15, using heat-capacity and thermal-expansion measurements. With increasing S content we observe a significant reduction of the tetragonal-to-orthorhombic transition temperature T

Na

_{s}

_{x}

. However, this suppression of T