Density Wave Order in Superconducting Nb-Bi₂Se₃

Topological Insulators such as Bi₂Se₃ and Bi₂Te₃, observed to have hexagonally deformed Fermi surfaces and strong nesting, have been proposed as potential candidates for the existence of density wave order. The presence of such order becomes particularly interesting in the context of superconductivity appearing in systems such as Cu-Bi₂Se₃ and Nb-Bi₂Se₃, and of possible intercalation-induced spontaneous symmetry breaking. So far, from experiments, we have some hints of density wave order, such as from electron diffraction and angle-resolved photoemission spectroscopy in Cu-Bi₂Se₃, and the observation of multiple Fermi surfaces in torque-magnetometry based quantum oscillations in Nb-Bi₂Se₃. However, there are no direct reports of Charge Density Wave order from resistivity. Here we report, for the first time, evidence for the coexistence of superconductivity with CDW order from transport measurements on single crystals of Nb_x-Bi₂Se₃ (0 < x < 0.25). We also report our studies of specific heat, magnetization, and ²⁰⁹Bi NMR. Based on our results, we argue that the Nb d orbital might not be responsible for the observation of multiple Fermi surfaces, and that such order could simply be induced by intercalants (Bi or Nb, or both). Further work is needed in this area.