Successive continuous phase transitions in spin-orbit coupled metal Cd₂Re₂O₇

A spin-orbit coupled metal Cd₂Re₂O₇ is known to show two structural phase transitions at T_s1 approx. = 200 K and T_s2 approx. = 115 K, which are likely of an electronic origin. The magnetic torque curves have been measured as a function of the field angle near T_s2, which are decomposed into two terms, major twofold and minor fourfold symmetry terms. A steep but continuous change of the twofold term, associated with no significant hysteresis, is found below T_s2. The result shows a second order transition at T_s2 although it is prohibited from the view point of the crystal symmetry. The torque curve analyses propose possible successive transitions and point group symmetries below T_s2, which include an inverse symmetry breaking, a transition from a low to high symmetry phase with decreasing temperature. (author)