Scanning tunneling microscopy of the charge density wave in $1T-{\mathrm{TiSe}}_2$ in the presence of single atom defects

We present a detailed low-temperature scanning tunneling microscopy (STM) study of the commensurate charge density wave (CDW) in $1T-{\mathrm{TiSe}}_2$ in the presence of single atom defects. We find no significant modification of the CDW lattice in single crystals with native defect concentrations where some bulk probes already measure substantial reductions in the CDW phase transition signature. A systematic analysis of STM micrographs combined with density functional theory modeling of atomic defect patterns indicate that the observed CDW modulation lies in the Se surface layer. The defect patterns clearly show there are no $2H$-polytype inclusions in the CDW phase, as previously found at room temperature [A. N. Titov et al., Phys. Solid State 53, 1073 (2011)]. They further provide an alternative explanation for the chiral Friedel oscillations recently reported in this compound [J. Ishioka et al., Phys. Rev. B 84, 245125 (2011)].

Figure 1

Simultaneously measured empty-state [(a) $V_{\text{bias}}=0.15\mathrm{V}]$ and filled-state [(b) $V_{\text{bias}}=-0.15\mathrm{V}]$ STM micrographs of a $1T-{\mathrm{TiSe}}_2$ single crystal grown at 650 ${}^{\circ }\mathrm{C}$. Image size: 22.2 $\text{nm}\times 11.4\mathrm{nm},I_{\text{t}}=0.2\mathrm{nA},T=4.7\mathrm{K}$. Native defects are labeled A, B, C, and D.

Figure 2

(a) Ball-and-stick model of the $1T-{\mathrm{TiSe}}_2$ lattice showing the positions of the native defects A–D. (b) Model representation of the inequivalent 1/4 (solid symbols) and 3/4 (open symbols) lattice sites for defects A–D in the commensurate CDW phase.

Figure 3

High-resolution STM micrographs centered on Se vacancies [(a), (b)], iodine [(d), (e)] and oxygen [(g), (h)] substitutions at 3/4 sites [(a), (d), (g)] and 1/4 sites [(b), (e), (h)] with corresponding DFT simulations calculated without including the CDW [(c), (f), (i)]. (j) Linear features around defects A and B observed by STM on $1T-{\mathrm{TiSe}}_2$ grown ($11.5\times 11.5{\mathrm{nm}}^2, V_{\text{bias}}=+150\mathrm{mV}, I_{\text{t}}=0.2\mathrm{nA}$). All images are taken on crystals grown at 650 ${}^{\circ }\mathrm{C}$, except for image (j) recorded on a sample grown at 575 ${}^{\circ }\mathrm{C}$ to have a reduced number of intercalated Ti.

Figure 4

High-resolution STM micrographs centered on intercalated Ti at 3/4 sites [(a), (e)] and 1/4 sites [(c), (g)] with corresponding DFT simulations in the presence of the CDW [(b), (d), (f), (h)]. (a)–(d) $V_{\text{bias}}=-150\mathrm{mV}$ and (e)–(h) $V_{\text{bias}}=+150\mathrm{mV}, I_{\text{t}}=0.2\mathrm{nA}$.