Coexistence of odd-parity and even-parity order parameters in the multipole order phase of the spin-orbit coupled metal Cd_2Re_2O_7

摘要

Magnetic torque measurements have been performed to investigate the phase transitions of the metallic pyrochlore compound Cd_2Re_2O_7, in which a spin-orbit (SO) interaction leads to unique multipole orders associated with an inversion symmetry breaking. We find that the magnetic torque signals with a fourfold symmetry (τ_4 ∝ H~4) as well as a twofold symmetry (τ_2 ∝ H~2) are significantly enhanced at low temperatures below a structural phase transition temperature (～200 K). The analyses of the torque curve symmetries show that the τ_4 term arises from an even-parity order parameter (OP) with the irreducible representation E_g, whereas the τ_2 term from an odd-parity OP with E_u, T_(1u) or T_(2u). The parity mixing of the primary OPs shows a peculiar phase transition in the SO coupled Cd_2Re_2O_7. The coexistence of the two OPs provides important insights into the origin of the multipole orders induced by the SO interaction.