A strongly robust type II Weyl fermion semimetal state in Ta3S2

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Science Advances  24 Jun 2016:
Vol. 2, no. 6, e1600295
DOI: 10.1126/sciadv.1600295
  • Fig. 1 Methodology to design robust Weyl semimetals with well-separated Weyl nodes.

    (A and B) The previous way of looking for Weyl semimetal states. Without SOC, the conduction and valence bands show some nodal crossings (which are not Weyl nodes). The inclusion of SOC splits each nodal point into a pair of Weyl nodes of opposite chiralities. In this way, the separation between the Weyl nodes is entirely determined by the SOC strength of the compound. For example, the first and the only Weyl semimetal in experiments, TaAs, belongs to this type (17, 19). In the absence of SOC, the conduction and valence bands cross each other to form nodal lines. The inclusion of SOC gaps out the nodal lines and gives rise to 12 pairs of Weyl nodes (19). (C and D) Our new methodology to design robust Weyl semimetals with well-separated Weyl nodes. We propose to look for compounds that are already Weyl semimetals without SOC. The inclusion of SOC will split each Weyl node into two nodes of the same chirality. In this way, SOC becomes irrelevant. The separation between the Weyl nodes of opposite chiralities is determined by the magnitude of the band inversion, which is not limited by the SOC strength and can be very large.

  • Fig. 2 Crystal and band structure of Ta3S 2.

    (A) Side view of the crystal structure for one unit cell of Ta3S2. The dark gray and yellow balls represent the Ta and S atoms, respectively. (B) Same as (A) but for the top view. Ta3S2 crystallizes in a base-centered orthorhombic structure with space group Abm2 (# 39) with lattice constants a = 5.6051 Å, b = 7.4783 Å, and c = 17.222 Å. As can be seen from the crystal lattice structure, Ta3S2 lacks inversion symmetry. (C) The primitive first BZ of Ta3S2 showing a base-centered orthorhombic structure and the location of relevant high-symmetry points. Representing the (Embedded Image) surface is a blue rectangular plane. The yellow dashed lines represent the projection of high-symmetry points on the (Embedded Image) surface. (D) First-principles electronic band structure calculation for Ta3S 2 in the absence of SOC. Bulk band crossings observed along the Γ-X-Z-X1 direction line near the Fermi level are a part of a line node on the ky = 0 plane. (E) Same as (D) but with the addition of SOC. After the incorporation of SOC, the band structure is fully gapped along all high-symmetry directions. Zooming into the red box along the Γ-Y direction shows a minimum gap size between the conduction band minimum and the valence band maximum of ≈ 7 meV, which is 96.5% smaller than the 200-meV gap observed along the same direction in (D). (F and G) Zooming in near the Fermi level for (D) and (E), respectively. Upon the inclusion of SOC, we clearly observe that the band structure becomes fully gapped along all high-symmetry directions.

  • Fig. 3 Weyl nodes in Ta3S2.

    (A) The primitive first BZ of Ta3S2 showing the k-space location of the Weyl points in the absence of SOC. The black and white circles represent Weyl points with opposite chirality. On the kx = 0 plane, represented in tan, we observe two pairs of Weyl nodes with chiral charge ± 1. Only one pair of Weyl nodes is an irreducible representation. The line node on the ky = 0 plane that corresponds to the crossing along the Γ-X-Z-X1 direction is shown in blue. (B) Same as (A) but after the incorporation of SOC. Each Weyl node splits into two spinful Weyl nodes of the same chirality and moves into the bulk. Therefore, we observe eight Weyl nodes in a single BZ. (C) The energy dispersion along the three momentum space directions of the Weyl node in the absence of SOC. (D) Same as (C) but with the addition of SOC. From this series of energy dispersion cuts along the three momentum space directions around the Weyl point, its type II character is revealed (that is, observe along ky that both crossing bands have the same sign of velocity along one momentum direction). Furthermore, observe that the Weyl nodes also move deeper in binding energy, ≈ −10 meV.

  • Fig. 4 Fermi arcs in Ta3S2.

    (A) Surface state spectrum for the (Embedded Image) surface of Ta3S2 at constant energy. The surface states are shown in yellow. A finite amount of projected bulk Fermi surfaces is observed because of the type II character of the Weyl nodes. The projected Weyl nodes are denoted by black and white circles. (B) Energy dispersion cut along Cut1 (red dashed line) for a pair of projected Weyl nodes around Embedded Image in (A). As a guide, the red dashed line represents the energy level of the Weyl node (E W1 ≈ −10 meV). The left panel shows two surface states (blue curves) emerging from box 1 and three entering box 2. The top-rightmost panel is zoomed into box 1 and shows a touching point, which defines the Weyl node as type II and two Fermi arc surface states that are labeled α and β. The β Fermi arc surface state terminates onto the Weyl node. The bottom-rightmost panel is similar to the top-rightmost panel. Here, we observe three Fermi arc surface states, α, β, and γ. Again, the β Fermi arc surface state is observed to terminate onto the Weyl node. (C) Same as (A) but with the projected bulk bands represented by shaded areas. The Fermi arc surface states connecting pairs of projected Weyl nodes are distinguishable from the remaining trivial surface states. (D) Zoom-in of the surface states around the projected Weyl nodes near Embedded Image. Shown, among other things, are eight projected Weyl nodes. Single Fermi arc surface states, β, are shown to connect two individual pairs of Weyl nodes, which is consistent with the predicted ± 1 chiral charge for each Weyl node. The α and γ Fermi arc surface states described in (B) are also labeled. (E) Cartoon illustration of (D) showing a possible Fermi arc linking scheme. The exact connectivity pattern of Fermi arcs with Weyl nodes depends on the surface conditions.

  • Fig. 5 Topological metal-to-insulator transition and tunabilities.

    (A) Energy dispersion cut along ky for an irreducible pair of Weyl nodes as the b lattice constant is tuned to various values. At b′ = b (left panel), we observe a pair of type II Weyl nodes. At b′ = 1.03b (middle panel), the two Weyl nodes approach each other and their separation decreases by about half. Another important feature shown is the VHSs, which are labeled with yellow circles. At b′ ≥ 1.04b (right panel), the two Weyl nodes annihilate each other and the band structure becomes fully gapped. (B) Calculation of the Wilson loop of the Wannier function centered in the kz = 0 plane and in the kz = π plane is performed to confirm the strong topological insulator (STI) state. Red dashed lines are arbitrary reference positions. The left panel shows one band crossing the red dashed line, which corresponds to Z2 invariant of 1 for the kz = 0 plane. The right panel shows no band crossing the red dashed line, which corresponds to Z2 invariant of 0 for the kz=π plane. Z2 = 1 confirms the STI state in Ta3S2. (C) An illustration of the topological metal-to-insulator transition for the Weyl nodes around the Embedded Image pocket on the (Embedded Image) surface. Fermi arc surface states are shown in yellow. Because b′ is tuned, the Weyl nodes reach the critical point at b′ = 1.037 and the system becomes fully gapped for b′ > 1.037 with a single topological surface state with the Fermi surface enclosing the Kramers’ point Embedded Image, resulting in a topological insulator state. Furthermore, because b′ is tuned from its original value, the VHSs arising from the Weyl cone reach the Fermi level when b′ = 1.01b and b′ = 1.025b. The Weyl nodes reach the Fermi level when b′ = 1.013b.

  • Fig. 6 New type of critical point.

    (A) Illustration of the critical point for TaAs as two opposite-facing parabolas are driven into each other. (B) Similar to (A) but for MoxW1−xTe2, which shows two opposite-facing parabolas that are tilted away from being vertical. (C) Ta3S2 shows two parabolas that face the same direction. This new type of critical point leads to a saddle point in the band structure and gives rise to a VHS. (D) The momentum space locations of the newly emerged band touchings resulting from driving the system’s conduction and valence bands toward each other by changing SOC λ into the critical point λcritical = 1.027 λ, which are represented by the green dots. (E) The saddle point behavior is clearly demonstrated from the band dispersions for λcritical = 1.027 λ by noting that the touching point for the conduction band along the kx and ky directions is at the energy minimum, whereas the touching point along the kz direction is at the energy maximum. (F) The Fermi surfaces corresponding to energy levels 1, 2, and 3 along the kx direction in the far-left panel in (E). (G) The saddle point band structure brings about the VHS, which generates a maxima in the DOS (N, blue solid line) and a divergence in the first derivative of the DOS (N′, red dashed line) at the energy of the VHS. a.u., arbitrary units. (H) Illustrations of the conduction and valence bands along the kz direction as the system is driven into its critical points by either decreasing the c to ccritical lattice constant or increasing the SOC strength λ to λcritical.

Supplementary Materials

  • Supplementary Materials

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    • Supplementary Text
    • fig. S1. Fermi arc surface states and Weyl nodes on the (‾202) surface without SOC.
    • fig. S2. Distribution of Weyl nodes in Ta3S2.

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