Anomalous changes of electric quadrupole order at low temperatures in the spin-orbit coupled metal Cd2Re2O7

Magnetic torque measurements in the spin-orbit coupled metal Cd2Re2O7 have been performed to investigate unique multipole orders associated with inversion symmetry breaking. The torque curves as a function of field angle are decomposed into two terms, a fourfold symmetry term (tau(4) proportional to H-4), corresponding to an electric quadrupole order, and a twofold symmetry term (tau(2) proportional to H-2), likely corresponding to an electric toroidal quadrupole order at low temperatures. Below 20 K, a strong enhancement of the fourfold symmetry term following a large phase change is observed toward the superconducting transition whereas no such behavior is evident in the twofold symmetry term. The results show anomalous changes of the electric quadrupole order at the low temperatures.