Pressure-Induced New Topological Weyl Semimetal Phase in TaAs

We report a new pressure-induced phase in TaAs with different Weyl fermions than the ambient structure with the aid of theoretical calculations, experimental transport and synchrotron structure investigations up to 53 GPa. We show that TaAs transforms from an ambient $I{4}_{1}md$ phase ($t\text{−}\mathrm{TaAs}$) to a high-pressure hexagonal $P\text{−}6m2$ ($h\text{−}\mathrm{TaAs}$) phase at 14 GPa, along with changes of the electronic state from containing 24 Weyl nodes distributed at two energy levels to possessing 12 Weyl nodes at an isoenergy level, which substantially reduces the interference between the surface and bulk states. The new pressure-induced phase can be reserved upon releasing pressure to ambient condition, which allows one to study the exotic behavior of a single set of Weyl fermions, such as the interplay between surface states and other properties.

Figure 1

(a) Calculated enthalpy relative to that of the $I{4}_{1}md$ ($t\text{−}\mathrm{TaAs}$) phase at pressure up to 50 GPa. (b) Unit cell volume vs pressure. (c) Crystal structures of ambient phase ($t\text{−}\mathrm{TaAs}$) and the two best candidates of high-pressure phases [$P\text{−}6m2$ ($h\text{−}\mathrm{TaAs}$) and $P{2}_1/c$ ($m\text{−}\mathrm{TaAs}$)]. Brown and green balls represent the Ta atom and As atom, respectively.

Figure 2

Calculated electronic band structures of TaAs in the $h\text{−}\mathrm{TaAs}$ phase at 25 GPa. (a) Electronic band structures with spin-orbit coupling. (b) Calculated Fermi surfaces of the $h\text{−}\mathrm{TaAs}$ phase. (c) The Weyl nodes distribution in the first Brillouin zone. The Weyl nodes obtained from scanning in the $k_z=0.027$ plane (parallel to $\mathrm{\Gamma }\text{−}K\text{−}M$) (d) and $\mathrm{\Gamma }\text{−}K\text{−}A$ plane (e), respectively.

Figure 3

Topological properties of TaAs in the $h\text{−}\mathrm{TaAs}$ phase. (a) The surface state of As termination of TaAs in the $h\text{−}\mathrm{TaAs}$ phase. Green dots are the bulk Weyl nodes projected to the 2D surface Brillouin zone. (b) The 3D Brillouin zone and the projected 2D surface Brillouin zone. (c) Evolution of the constant energy contour of two Fermi arcs. It can be seen that the two arcs move to the same direction as the energy shifts. (d) The band structure of the (100) surface along the closed loop ($\overline{\mathit{\Gamma }}\text{−}\overline{M}\text{−}\overline{L}\text{−}\overline{\mathit{\Gamma }}$). The red dots are the surface electronic structure which projected to As termination.

Figure 4

High-pressure synchrotron XRD patterns of TaAs. Structural analysis reveals the irreversible phase transition $I{4}_{1}md$ ($t\text{−}\mathrm{TaAs}$) to $P\text{−}6m2$ ($h\text{−}\mathrm{TaAs}$) in TaAs upon decompression. (a) Lattice constant $a$, (b) lattice constant $c$, and (c) unit cell volume vs pressure. Representative Rietveld refinements at (d) 0.22 and (e) 53.11 GPa upon compression, respectively. (f) The pattern at 0.74 GPa upon decompression can be refined by the $h\text{−}\mathrm{TaAs}$ structure, suggesting that the high-pressure hexagonal phase is reserved.

Figure 5

Temperature dependences of resistance of a TaAs single crystal at various pressures. (a),(b) Sample resistance profiles vs temperature under high pressure. (c) The semiconductor-metal transition temperature $T_{M-I}$ and specific resistance as a function of applied pressure at 1.8 and 300 K, respectively. The $T_{M-I}$ is defined as the hump on the resistance curve. (d) Isothermal magnetoresistance at 4.5 K under representative pressures. The dashed lines denote the linear behavior. The arrows represent the characteristic magnetic field $B^{*}$, below which the magnetoresistance deviates significantly from linearity.