(color online) (a) Raman spectra of Cu_xBi₂Se₃ film samples with x = 0, 0.07, 0.10, and 0.11. (b) The resistivity of Cu_xBi₂Se₃ films with high Cu concentration dependence on temperature. (c) Temperature dependence of resistivity of Cu_0.076Bi₂Se₃ (black square) and after (red circle) relaxation at 2 K for 29 h. (d) Time dependence of resistivity of Cu_0.076Bi₂Se₃ at 2 K over the relaxation process of 29 h. (e) Time dependence of resistivity of Cu_xBi₂Se₃ with various Cu concentrations (x = 0.059, 0.071, 0.076, 0.107) at 2 K. Solid green lines are best fits. Modified from Li et al., Ref. [47]