Effects of Cu intercalation in Single Crystals of Cu_xBi₂Te₂Se

Intercalation of Cu in Topological Insulators such as Bi₂Se₃ and Bi₂Te₃ is known to yield superconductivity. The fundamental origin of superconductivity in these systems, whose parent compounds have non-trivial topology, remains under intense scrutiny. Bi₂Te₂Se is believed to be a topological insulator with structure similar to Bi₂Se₃ and Bi₂Te₃, but with much higher bulk resistivity. Here, we report structure-property relationships of Cu-intercalation in single crystals of Cu_xBi₂Te₂Se (0<x<0.5) using X-ray Diffraction, Raman Spectroscopy and X-ray Photoemission Spectroscopy. For x<0.2, Cu intercalates in Bi₂Te₂Se as Cu¹⁺. Increasing x results an increase in c-axis length along with a softening of A₁g₂ and Eg₂ Raman modes. We observe a double-mode behavior in both A₁g₂ and Eg₂ modes corresponding to the outer atoms of Bi and Te⁽¹⁾/Se⁽¹⁾. Thus, Bi-Te⁽¹⁾ and Bi-Se⁽¹⁾ vibrations are decoupled from each other. For higher x, we observe a triple mode behavior in both A₁g₂ and Eg₂ modes. We ascribe this to additional Cu atoms partially substituting at Bi sites and creating additional modes such as Cu-Se⁽¹⁾ and Cu-Te⁽¹⁾.