Manipulating the nematic director by magnetic fields in the spin-triplet superconducting state of CuxBi2Se3

Electronic nematicity, a consequence of rotational symmetry breaking, is an emergent phenomenon in various new materials. In order to fully utilize the functions of these materials, ability of tuning them through a knob, the nematic director, is desired. Here we report a successful manipulation of the nematic director, the vector order parameter (d vector), in the spin-triplet superconducting state of ${\mathrm{Cu}}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ by magnetic fields. At $H=0.5\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, the ac susceptibility related to the upper critical field shows a twofold symmetry in the basal plane. At $H=1.5\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, however, the susceptibility shows a sixfold symmetry, which has never been reported before in any superconductor. These results indicate that the $\mathbf{d}$ vector initially pinned to a certain direction is unlocked by a threshold field to respect the trigonal crystal symmetry. We further reveal that the superconducting gap in different crystals converges to ${p}_{x}$ symmetry at high fields, although it differs at low fields.