Electronic band structure for occupied and unoccupied states of the natural topological superlattice phase ${\mathrm{Sb}}_2\mathrm{Te}$

We present an experimental study describing the effects of surface termination on the electronic structure of the natural topological superlattice phase ${\mathrm{Sb}}_2\mathrm{Te}$. Using scanning angle-resolved photoemission microscopy, we consistently find various nonequivalent regions on the same surface after cleaving various ${\mathrm{Sb}}_2\mathrm{Te}$ single crystals. We were able to identify three distinct terminations characterized by different Sb/Te surface stoichiometric ratios and with clear differences in their band structure. For the dominating Te-rich termination, we also provide a direct observation of the excited electronic states and of their relaxation dynamics by means of time-resolved angle-resolved photoemission spectroscopy. Our results clearly indicate that the surface electronic structure is strongly affected by the bulk properties of the superlattice.

Figure 1

Crystal structure of ${\mathrm{Sb}}_2\mathrm{Te}$ and scanning photoemission microscopy images and spectra taken at a photon energy of 74 eV. (a) Crystal structure of ${\mathrm{Sb}}_2\mathrm{Te}$ showing both ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ QL and ${\mathrm{Sb}}_2$ BLs building blocks [14]; horizontal arrows indicate the different cleaving planes. (b) SPEM image, recorded at photoelectron kinetic energy corresponding to Sb $4d$ core level for the first cleave, revealing two terminations marked in white and gray intensity color scale; the ${\mathrm{S}}_1$ box includes both terminations. (c) Upper and lower panel present higher resolution images of the ${\mathrm{S}}_1$ regions, obtained at two distinct photoelectron kinetic energies corresponding to the Sb $4d$ and Te $4d$ core levels, respectively. The two images present reversed chemical contrast. (d) Sb $4d$ SPEM image for another cleaved surface, clearly revealing a third kind of termination (appearing as blue in the image), contained in the ${\mathrm{S}}_2$ contour defining the third terminated surface. (e) Same as (c) but for the ${\mathrm{S}}_2$ region. (f, g) Photoemission spectra of the Sb $4d$ and Te $4d$ core levels, obtained from points ${\mathrm{P}}_1$, ${\mathrm{P}}_2$, and ${\mathrm{P}}_3$ and representative of the three coexisting terminations.

Figure 2

Occupied electronic band structure of ${\mathrm{Sb}}_2\mathrm{Te}$. (a) Constant energy cuts for the Te-rich termination showing the Fermi surface topography and presenting the high-symmetry direction $\overline{\mathrm{\Gamma }M}–\overline{\mathrm{\Gamma }{M}^{\prime }}$. (b) Intensity maps of photoelectrons, emitted for parallel wave vector along the $\overline{\mathrm{\Gamma }M}–\overline{\mathrm{\Gamma }{M}^{\prime }}$ direction for the three microscopic regions, corresponding to points ${\mathrm{P}}_1$, ${\mathrm{P}}_2$, and ${\mathrm{P}}_3$ in Fig. 1. (c) Energy distribution curves from the same microscopic regions for $k=-0.4{\text{Å}}^{-1}$ (left), $k=-0.3{\text{Å}}^{-1}$ (center), and $k=0.4{\text{Å}}^{-1}$ (right panel).

Figure 3

Time-resolved ARPES sequence acquired after photoexcitation from the pump pulses for ${\mathrm{Sb}}_2\mathrm{Te}$. The color intensity is presented in logarithmic scale to make the signal from the unoccupied electronic states more evident. The intensity maps have been extracted at emission angles near $\overline{\mathrm{\Gamma }}$, every 10 deg, at +0.25 ps pump-probe time delay.

Figure 4

Ultrafast dynamics the photoexcited electronic states. (a) In the upper-left corner, we exhibit a schematic view of the electronic band structure of the Te-rich terminated region along $\overline{\mathrm{\Gamma }M}–\overline{\mathrm{\Gamma }{M}^{\prime }}$. Excess electron (red) and hole (blue) populations are detected from the difference time-resolved ARPES images obtained before and after photoexcitation. Dashed lines indicate the Fermi level. (b) Ultrafast time evolution of the populations in the $k$-E integration windows 1–18 presented at $t$ = 0.10 ps pump-probe delay.