Superconductivity induced by pressure-induced anisotropic strain in Cu_x(CuI)_0.002Bi₂Te_2.7Se_0.3 crystal

The topological insulators Bi₂Te₃, Bi₂Se₃, and Sb₂Te₃ are found to present superconductivity (SC) at high pressure, however, the revealing of SC in Cu intercalated Bi₂Se₃ implies that they may share the same origin. Indeed, the presence of onset superconducting transition found at about 4 K at 6.6 GPa supports this speculation. To find the mechanism of causing SC, the copper intercalated crystal Cu_x(CuI)_0.002Bi₂Te_2.7Se_0.3 is fabricated to study the SC at high pressures. In this work, the temperature-dependent electrical resistance was measured by a homemade cryogenic electrical measurement system with temperatures down to 1.3 K and a magnetic field up to 9 Tesla at pressures of 0.8 through 36 GPa. The pressure-dependent XRD patterns are also gathered at pressures from 2 through 45 GPa. The first SC-like transition was detected at 3.8 GPa with T_c = 2 K. Upon further pressure increase; they reached maximum T_c = 9.3 K at 15 GPa. Interestingly, however, the XRD results suggest no obvious lattice phase transition until 13.9 GPa. Furthermore, the carrier concentration studied by Hall measurements is also indicating a dramatically up-turn in the magnitude of the power from ~ 10¹⁷ cm^-3 to ~ 10²² cm^-3 in the range of ~3 GPa through 15 GPa. The pressure-dependent carrier concentration, resistivity, and superconductivity T_c strongly imply an electronic phase transition embedded in the ambient pressure crystal phase. The strain analysis on (101) and (015) planes suggest that the hidden electronic phase transition may be induced by the anisotropic strain in the Cu_x(CuI)_0.002Bi₂Te_2.7Se_0.3.