Studies on Shubnikov-de Haas oscillations and magnetic properties of cobalt-doped Bi1.9Co0.05 Sb0.05Se3 topological single crystals

Magnetotransport and magnetic properties of pristine Bi_1.9Sb_0.1Se₃ (BSS) and Co-doped Bi_1.9Co_0.05Sb_0.05Se₃ (BCSS) topological insulator (TI) single crystals are reported. Both the samples show metallic behavior, but the overall resistivity decreases upon Co doping. Temperature and field dependent magnetization, M (H) and M (T) measurements on BSS exhibit diamagnetic nature from 2.8 K to 300 K. Whereas at T ≤ 10 K, Co-doped sample shows a paramagnetic behavior in low field range and a diamagnetic behavior at H ≥ ±3.5 T. The analysis of M (H) and M (T) data reveals that Co²⁺ and Co³⁺ might coexist in the diamagnetic matrix. Hall measurements indicate that the carriers are n-type and its density increases by one order of magnitude upon Cobalt doping. The Hall mobility decreases in BCSS, and its temperature dependence shows an increasing behavior as the temperature decreases in both the samples. The Shubnikov-de Haas (SdH) oscillations have been analyzed using Lifshitz-Kosovich (LK) equation. The frequency of SdH oscillation drastically increases for BCSS pointing to the fact that the Fermi energy (E_F) is shifted upward and the mobility of surface state electrons decreases. The substituted Co atoms act as paramagnetic entities and are found to be strong electron donors as well as strong scattering centers. The Berry curvature β derived from LK fit increases from 0.56 to 0.8 upon Co doping and shows the contribution of non-Dirac like bands to the SdH oscillations.