The fabrication of Cu_x(CuI)_0.002Bi₂Te_2.7Se_0.3 crystals and its superconductivity at pressures

We report a successful observation of pressure-induced superconductivity in the crystal Cu_x(CuI)_0.002Bi₂Te_2.7Se_0.3. The crystal (CuI)_0.002Bi₂Te_2.7Se_0.3was made by the so-called Bridgman method with the corresponding stoichiometric; the followed thermal treatment was employed to intercalate Cu to form Cu_x(CuI)_0.002Bi₂Te_2.7Se_0.3 crystals. The temperature-dependent electrical resistance was measured from 0.8 through 36 GPa. Superconductivity was detected at 3.8 GPa with T_c = 2 K. Upon further pressure increase, it reached maximum T_c = 9.3 K at 15 GPa. Over 15 GPa, the superconducting transition temperature decreased as pressure increased. Furthermore, the Hall effect measurements indicated the carrier concentration dramatically increased about five orders from ~ 10¹⁷ cm^-3 at 4.2 GPa to ~ 10²² cm^-3 at 15 GPa. To study the details of lattice structure migration at high pressures, the high-pressure structure investigations with synchrotron radiation were collected at various pressures from 1.97 to 44.99 GPa. Two structural phase transitions were observed at the pressures of 13.9 and 25.3 GPa, respectively at room temperature. Meanwhile, the results of pressure-dependent strain analysis on the (101) plane reveal a transition similar to the carrier concentration and superconductivity T_c at pressures lower than 13.9 GPa. This suggests that the hidden electronic phase transition may be induced by anisotropic strain in the Cu_x(CuI)_0.002Bi₂Te_2.7Se_0.3 in the low-pressure regime.