Possible lattice and charge order in CuxBi2Te2Se

Abstract

Metal intercalation into layered topological insulator materials such as the binary chalcogenide Bi2X3 (X = Te or Se) has yielded novel two-dimensional (2D) electron-gas physics, phase transitions to superconductivity, as well as interesting magnetic ground states. Of recent interest is the intercalation-driven interplay between lattice distortions, density wave ordering, and the emergence of new phenomena in the vicinity of instabilities induced by intercalation. Here, we examine the effects of Cu-intercalation on the ternary chalcogenide Bi2Te2Se. We report the discovery, in Cu0.3Bi2Te2Se, of a periodic lattice distortion (PLD) at room temperature, together with a charge density wave (CDW) transition around Td = 220 K. We also report, for the first time, a complete study of the CuxBi2Te2Se system, and the effect of Cu-intercalation on crystal structure, phonon structure, and electronic properties for 0.0 ≤ x ≤ 0.5. Our electron diffraction studies reveal strong Bragg spots at reciprocal lattice positions forbidden by ABC stacking, possibly resulting from stacking faults, or a superlattice. The c-axis lattice parameter varies monotonically with x for 0 < x < 0.2, but drops precipitously for higher x. Similarly, Raman phonon modes and soften monotonically for 0 < x < 0.2 but harden sharply for x > 0.2. This indicates that Cu likely intercalates up to x ∼ 0.2, followed by partial site-substitutions at higher values. The resulting strain makes the 0.2 ≤ x ≤ 0.3 region susceptible to instabilities and distortions. Our results point toward the presence of an incommensurate CDW above Td = 220 K. This work strengthens prevalent thought that intercalation contributes significantly to instabilities in the lattice and charge degrees of freedom in layered chalcogenides. Further work is required to uncover additional density wave transitions, and possible ground states such as superconductivity.