Spontaneous Lattice Distortion in the Spin-Triplet Superconductor Cu_{x}Bi_{2}Se_{3}

The doped topological insulator Cu_{x}Bi_{2}Se_{3} has attracted considerable attention as a new platform for studying novel properties of spin-triplet and topological superconductivity. In this work, we performed synchrotron x-ray diffraction measurements on Cu_{x}Bi_{2}Se_{3} (0.24≤x≤0.46) to investigate the coupling between the superconducting order parameter and crystal lattice. In the crystals in which the vector order parameter (d vector) is tilted from the crystal high-symmetry directions as evidenced by nematic diamagnetic susceptibility, we find a sizable lattice distortion (∼100  ppm) associated with the onset of superconductivity. In contrast, in crystals with the d vector aligned along the high-symmetry directions, we find no appreciable change in lattice constant. Together with a pronounced vestigial behavior of the distortion, the results are clear evidence for an odd-parity E_{u} order parameter that couples with trigonal lattice. Furthermore, in the crystal with x=0.46 where diamagnetic susceptibility is isotropic in the plane, no lattice distortion accompanying the superconducting transition is found, which is in line with a chiral superconducting state in the highly doped region. Our work shows that lattice distortion can be a powerful diagnosing quantity for nematic superconductivity with two-component order parameter.