Possible lattice and charge order in Cu_xBi₂Te₂Se

Abstract

Metal intercalation into layered topological insulator materials such as the binary chalcogenide Bi 2 X 3 (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 intercalationdriven 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 Bi 2 Te 2 Se. We report the discovery, in Cu 0.3 Bi 2 Te 2 Se, of a periodic lattice distortion (PLD) at room temperature, together with a charge density wave (CDW) transition around T d =220 K. We also report, for the first time, a complete study of the Cu x Bi 2 Te 2 Se 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 A g 1 2 and E g 2 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 T d =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.

Introduction of Sr into Bi2Se3 thin films by molecular beam epitaxy

“…This result comes in contrast with recent Raman measurements on Cu-intercalated Bi2Te2Se crystals where a robust preservation of the linewidth was observed, even up to high Cu concentrations. [22] Qualitatively, the broadening that we observe is similar to what has been reported in other alloys including (Al,Ga)As [23] and transition metal dichalcogenides [24] [25] where substitutional disorder is known to occur. is the Bi site, given the comparable valence of the two atoms (Bi 3+ and Sr 2+ ) yielding an acceptor type defect level.…”

Composition and Illumination-Driven Phase Segregation in Hybrid Perovskite Cs2AgBi(Br1–xIx)6

“…At x < 0.07, the linear trends for T 2g and A 1g (including x = 0.07) mode shifts can be fitted linearly (as shown in Figure a,b), which are evidence that iodine has been doped into the parent compound accompanied by a linear change of the lattice parameters. The A 1g mode softening as a function of the compositional doping parameter has been previously reported in other compounds, where the softening of A 1g is an indicator of lattice expansion. , The opposite shifting directions of T 2g and A 1g might be originated from the different vibrational mechanisms of these two modes . A uniform FWHM from T 2g and A 1g (as shown in Figure c,d) can also be observed at x < 0.07, which further proves that the low I concentration doped samples are relatively in the lattice order.…”

“…The A 1g mode softening as a function of the compositional doping parameter has been previously reported in other compounds, where the softening of A 1g is an indicator of lattice expansion. 34,35 The opposite shifting directions of T 2g and A 1g might be originated from the different vibrational mechanisms of these two modes. 29 A uniform FWHM from T 2g and A 1g (as shown in Figure 5c,d) can also be observed at x < 0.07, which further proves that the low I concentration doped samples are relatively in the lattice order.…”

Phonon softening and higher-order anharmonic effect in the superconducting topological insulator Sr x Bi2Se3

“…Recently, similar anomalies have also been reported in Bi 2 Se 3 [39], and Sb 2 Te 3 [40], and it is proposed that the thermallyactivated formation of the stacking faults is responsible for the anomalies in the optical and acoustic phonons, as well as the optical constants. Recently, charge density wave ordering at around 140 K for Bi 2 Se 3 and around 220 K for Cu x Bi 2 Te 2 Se was reported [41,42]. Also, the magnetic susceptibility study of a superconducting Cu x Bi 2 Se 3 (T c ∼ 4.2 K) crystal revealed a charge density instability at ∼96 K [43], somehow lower than that of pure Bi 2 Se 3 .…”