Domain control by adjusting anisotropic stress in pyrochlore oxide Cd₂Re₂O₇

The 5d pyrochlore oxide Cd₂Re₂O₇ exhibits successive phase transitions from a cubic pyrochlore structure (phase I) to a tetragonal structure without inversion symmetry below T_s1 of ~200 K (phase II) and further to another noncentrosymmetric tetragonal structure below T_s2 of ~120 K (phase III). The two low-temperature phases may be characterized by odd-parity multipolar orders induced by the Fermi liquid instability of the spin–orbit-coupled metal. To control the tetragonal domains generated by the transitions and to obtain a single-domain crystal for the measurements of anisotropic properties, we prepared single crystals with the (0 0 1) surface and applied biaxial and uniaxial stresses along the plane. Polarizing optical microscopy observations revealed that inducing a small strain of approximately 0.05% could flip the twin domains ferroelastically in a reversible fashion at low temperatures, which evidences that the tetragonal deformation switches at T_s2 between c > a for phase II and c < a for phase III. Resistivity measurements using single-domain crystals under uniaxial stress showed that the anisotropy was maximum at around T_s2 and turned over across T_s2: resistivity along the c axis is larger (smaller) than that along the a axis by ~25% for phase II (III) at around T_s2. These large anisotropies probably originate from spin-dependent scattering in the spin-split Fermi surfaces of the cluster electric toroidal quadrupolar phases of Cd₂Re₂O₇