Lack of coupling between superconductivity and orthorhombic distortion in stoichiometric single-crystalline FeSe

The coupling between superconductivity and othorhombic distortion is studied\nin vapor-grown FeSe single crystals using high-resolution thermal-expansion\nmeasurements. In contrast to the Ba122-based (Ba122) superconductors, we find\nthat superconductivity does not reduce the orthorhombicity below Tc. Instead we\nfind that superconductivity couples strongly to the in-plane area, which\nexplains the large hydrostatic pressure effects. We discuss our results in\nlight of the spinnematic scenario and argue that FeSe has many features quite\ndifferent from the typical Fe-based superconductors.\n