Topologically nontrivial bismuth(111) thin films

Abstract

Using high-resolution angle-resolved photoemission spectroscopy (ARPES), the topological property of the three-dimensional Bi(111) films grown on the Bi2Te3(111) substrate were studied. Very different from the bulk Bi, we found another surface band near the point besides the two well-known surface bands on the 30 nm films. With this new surface band, the bulk valence band and the bulk conduction band can be connected by the surface states in the Bi(111)/Bi2Te3 films. Our band mapping revealed odd number of Fermi crossings of the surface bands, which provided new experimental evidences that Bi(111)/Bi2Te3 films of a certain thickness can be topologically nontrivial in three dimension.

Figure 1. Sketch of energy bands of bulk Bi(111).

(a) LDA calculations and (b) ARPES results. Red bands are spin splitting surface states, S1 and S2. Black arrows indicate the spin polarization. Blue and green bands are bulk valence band and bulk conduction band. Dashed line in (b) indicates that no bulk conduction band has been detected in ARPES. The agreement between LDA and ARPES near the is poor.

Figure 2. Characterization of Bi(111) film’s surface.

(a) RHEED pattern and (b) STM topography of 30 nm Bi(111) films on Bi₂Te₃. Insert is the height line profile along the black curve. (c) In-plane lattice constant of Bi films as a function of the thickness. The high voltage of RHEED is 30kV. For the STM image, the bias voltage is 1.42 V, the tunneling current is 162 pA.

Figure 3. ARPES spectra near point.

(a) ARPES spectra of 30 nm Bi(111)/Bi₂Te₃ film and (b) ARPES spectra of 20 nm Bi(111)/Bi₂Te₃ film along -- near point. The incident photon energy hν = 30 eV. Green dash lines mark bulk valence band. (c) Black and red lines are EDCs of 30nm Bi at point and of Au. An energy gap was observed. (d) Black and and red lines are EDCs of 20 nm Bi at point and of Au. There is no energy gap.

Figure 4. ARPES spectra near point.

(a–f) ARPES spectra and corresponding EDCs of 30 nm Bi(111)/Bi₂Te₃ film near point along -- (incident photon energy hν = 42, 46, 44, 40, 38 and 36 eV). Green and blue dashed lines are the guides for the S1, S2 and S3 bands. (g) MDCs near Fermi level extracted from (a). Greed dotted lines mark MDC peaks of S1. (h) SDI plot of ARPES spectra (hν = 30 eV) near point along -- direction. Green dashed line shows the possible dispersion of S1 band. (i) High resolution EDCs along - (hν = 30 eV). Green and blue dotted lines mark EDC peaks of S1 and S3 bands. (j) The corresponding ARPES spectra of (i). (k) Four EDCs from (j). The momentum positions are indicted by the red lines in (j) (k = 0.43 \AA⁻¹, 0.56 \AA⁻¹, 0.60 \AA⁻¹, 0.70 \AA⁻¹). (l) EDCs at point as a function of the incident photon energy.

Figure 5. ARPES spectra from point to point.

Green dashed lines mark the surface states. The white arrows mark the Fermi crossing positions of the surface bands. Odd number of crossings was observed.