2013 年 55 巻 2 号 p. 128-134
Strongly interacting electrons can exhibit novel collective phases, among which the electronic nematic phases are perhaps the most surprising as they spontaneously break rotational symmetry of the underlying crystal lattice. Our highly sensitive magnetic torque measurements under in-plane field rotation in iron-based superconductors BaFe2(As1-xPx)2 provide evidence for the electronic nematic ordering below a characteristic temperature T*. High-resolution crystal-structure analysis by using synchrotron X-ray reveals small but finite orhthorhobic distortionbelow T* and the orthorhombicity shows a big jump at a lower temperature Ts. The results imply an electronic origin of rotational symmetry breaking, whose relation to superconductivityis discussed.